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Contents 516 I. 516 II. 518 III. 518 IV. 521 V. 523 VI. 523 VII. 526 526 References 526 SUMMARY: The relevance of infochemicals in the relationships between organisms is emerging as a fundamental aspect of aquatic ecology. Exchanges of chemical cues are likely to occur not only between organisms of different species, but also between conspecific individuals. Especially intriguing is the investigation of chemical communication in microalgae, because of the relevance of these organisms for global primary production and their key role in trophic webs. Intraspecific communication between algae has been investigated mostly in relation to sexuality and mating. The literature also contains information on other types of intraspecific chemical communication that have not always been explicitly tagged as ways to communicate to conspecifics. However, the proposed role of certain compounds as intraspecific infochemicals appears questionable. In this article, we make use of this plethora of information to describe the various instances of intraspecific chemical communication between conspecific microalgae and to identify the common traits and ecological significance of intraspecific communication. We also discuss the evolutionary implications of intraspecific chemical communication and the mechanisms by which it can be inherited. A special focus is the genetic diversity among conspecific algae, including the possibility that genetic diversity is an absolute requirement for intraspecific chemical communication.

Limitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P-acquisition efficiency) and by improved productivity per unit P taken up (P-use efficiency). This review focuses on improved P-use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P-use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait.

Summary A cost-benefit analysis of transport and of water relations in plants of a number of life forms is presented. For planktophytes, it appears that an increase in size above ∼ 50 μm diameter would restrict growth rate in oligotrophic environments owing to a greater increment of restriction by boundary layer of nutrient uptake than of maximum specific growth rate as organism size increases. For macroscopic aquatic haptophytes (mainly algae), even the most favourable combination of velocity of water flow and plant morphology cannot reduce these restrictions enough to make nutrient diffusion to the plant surface less limiting for growth in oligotrophic environments than is the case for planktophytes. Macroscopic aquatic rhizophytes, with their need for intraplant N- and P- flux from rhizoid or root to shoot, and for reduced C-flux from shoot to rhizoid or root, involves cytoplasmic streaming in giant-celled algae, and transport in the phloem and (probably) the xylem (using root pressure) in vascular plants. It is likely that the energetic running costs of nutrient transport (joules used per mol solute transported over a distance of 1 m) is higher for the giant-celled algae than for the vascular plants. The predominant (in terms of global biomass) terrestrial plants are the rhizophytic, homoiohydric desiccation-intolerant sporophytes of vascular plants. Such plants incur greater penalties, in terms of reduced specific growth rate under resource-saturated or resource-limited conditions, and of reduced resource use efficiency, with increased plant height. This is a result of diversion of resources to producing supporting tissue, and xylem and phloem, in larger amounts per unit biomass than is the case for smaller plants. By virtue of increased size, however, a plant can command a higher incident photon flux density as well as access to a greater depth of soil from which nutrients and water can be extracted. Other major categories of terrestrial rhizophytes are desiccation-tolerant homoiohydric sporophytes of vascular plants, endohydric but poikilo hydric gametophytes of mosses, and ectohydric gametophytes of archegoniates and thalli of many algae and lichens. In the order in which they are listed, these plants have a smaller potential stature (none are more than 2 m high), and thus can command access to fewer resources than can taller plants with deeper root systems in the same community. However, the smaller plants have the potential for higher specific growth rates under resource-saturated and resource-limited conditions, and for higher resource use efficiencies, than do the homoiohydric, desiccation-intolerant plants of larger stature, since they have a smaller diversion of resources to supporting and long-distance transport tissues. The extreme of these trends is found in the haptophytic soil algae with no non-green cells. Quantitation of the differences between life forms requires much more investigation, as do differences within life forms with respect to the handling of water and solute transport. Investigating the possible selective significance of these differences will be an even more challenging task.

Submerged benthic plants (algae and ‘higher plants’) can be classified into two groups on the basis of their attachment to the substrate. ‘Haptophytes’ are attached to the surface of rocks; ‘Rhizophytes’ have rhizoids, roots or rhizomes within the substratum (sand or mud). Haptophytes must obtain all of their nutrients from the bulk water phase, while rhizophytes can obtain some of their nutrients from the sediment. Evidence as to the sites of nutrient uptake on the surface of rhizophytes supports the view that a substantial fraction of phosphorus and nitrogen and, in some cases, inorganic carbon, is obtained from the sediment. This might be expected to make the portion of the rhizophyte which occurs in the bulk water phase more specialized for interception of photosynthetic light and C than the corresponding portion of the haptophyte which also intercepts and absorbs all other mineral nutrients. The morphology of the haptophyte and its frequent occurrence in more rapidly flowing water means that the superficial unstirred layer is less of an impedance to nutrient transport from the bulk medium to the plant surface. This in turn improves the supply of inorganic C, which is reflected in more negative delta 13C values and higher biochemical capacities for photosynthesis per unit plant surface in haptophytes than in rhizophytes. Haptophytes frequently have colourless hairs on their surface, especially in media of low nutrient content or with poor water movement: in many cases, these hairs seem to be involved in nutrient uptake. Despite their different mechanisms for uptake of nutrients, rhizophytes and haptophytes can produce similar biomasses and chlorophyll m−2, although rhizophytes may have rather lower productivities. The role of symbiotic associations (mycorrhizas in vascular plants; microalgae in invertebrates) in uptake of nutrients and photosynthesis is discussed in relation to haptophytic and rhizophytic strategies, as is the evolution of land plants.

Contents I. Introduction II. Phylogeny of diatoms and the habitat of the earliest diatoms III. Origin of the silicon requirement in diatoms IV. The phylogeny and ontogeny of turgor- resistant silicified walls V. Possible selective advantages of silicified walls VI. Conclusions Acknowledgements References Summary The silicified bipartite cell walls of diatoms (Bacillariophyceae) are produced in intracellular compartments by precipitation from supersaturated Si(OH)4 and are then externalized. Fossil evidence of silicification is for marine, probably neritic, centric diatoms from approx. 120 Mya. Regardless of the initial selective significance of silicification, and of other current roles of silicification, the increased density resulting from silicification increases the sinking rate of cells; this can be partly or wholly offset by regulation of the protoplast solute content. Acclimatory and regulatory changes in silicification (relatively slow), and intracellular solute composition (relatively rapid), and intracellular solute composition (relatively rapid) of marine diatoms alter cell density over periods of hours to days. Density changes via changes in resource supply and, probably, parasitism, would move cells into optimal resource supply conditions, and remove parasitized, infective cells from surface populations of uninfected cells. Regulation of sinking rate could have been the first function of external or internal silica if the earliest silicified diatoms were planktonic.

Summary • The importance of N derived from internal remobilization and root uptake to growth in spring is reported here for the perennial grasses Festuca rubra and Agrostis capillaris derived from seminatural grasslands in Scotland, UK. • Plants grown in sand culture, received 15N-enriched nutrient solution during the first year of growth and nutrient solution with N at natural abundance during the subsequent spring and summer when destructive harvests were taken. • Labelled N was recovered in new growth of overwintering tillers and new tillers. Remobilized N contributed 70% and 82% for F. rubra and A. capillaris, respectively, to the total N in new above-ground growth in early spring, declining to 34% and 45%, respectively, by mid June. Species showed similar patterns of remobilization on a new growth biomass basis. The root system did not remobilize N to support new above-ground growth. Labelled N was derived from senescing leaves present on overwintering tillers. Net balances of labelled N suggest that N was translocated between tillers; reproductive tillers acted as sinks, vegetative tillers as the source of N. • Initial growth in spring is largely independent of N uptake from the soil, provided that overwintering leaves are present on the plants.

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol−1 CO2) and double ambient (720 μmol mol−1 CO2) carbon dioxide (CO2) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO2 concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO2 concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO2 concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation (Vcmax) and the light saturated rate of electron transport (Jmax) between the control and elevated CO2 treatment.

Ammonia was necessary for the origin of life. However, NH 3 would not have been a significant component of the neutral or mildly reducing (N 2 , CO 2 , H 2 O) atmosphere which characterized the Hadean earth ( > 3.8 Gyr ago), especially in view of the u.v. lability of NH 2 , the greater u.v. output of the young sun than pertains today, and the absence of an atmospheric u.v. screen. Heterogeneous phase reactions, following atmospheric chemistry, have been proposed as a prebiotic NH 3 source. Lightning, or bolides (meteorites and comets), generated NO x . , and Fe 2+ in the sea could have reduced NO 2 - resulting from the NO x . to NH 4 + ; this disequilibrium NH 4 + level could have supported the production of the nitrogenous organic building blocks of life. NO x . and NH y , thus could both have had important roles in the origin and evolution of life. Burgeoning biota could soon have depleted abiotically generated NH 4 . , and biological N 2 fixation could have evolved in its present Fe-demanding, O 2 -sensitive forms in the Archaean O 2 -free, Fe 2+ -rich environment. Organic matter could have driven biological denitrification reactions based on NO 2 - and NO 3 - generated abiologically using some redox components which evolved in earlier chemolithotrophs and photolithotrophs, regenerating atmospheric NO x . and producing N 2 O. Any atmospheric NH 2 leaking from oceanic biology would have been subject to u.v. breakdown and rain-out. Although O 2 -evolving photosynthesis probably began in the Archean some 3.5 Gyr ago, any O 2 accumulation was local until 2.0 Gyr ago (Proterozoic) due to consumption by oxidation of Fe 2+ and S 2- . However, localized O 2 accumulation before 2.0 Gyr ago could account for the observed early evolution of cytochrome oxidase and the possibility of O 2 -consuming chemolithotrophic and chemoorganotrophic nitrification, with further possibilities of NO x . production. Oxygen accumulation globally from 2.0 Gyr onwards coincided approximately with the evolution of eukaryotes, which contributed phagotrophy to the reactions of the N cycle as well as the nutrification-like aerobic production of NO . by nitric oxide synthetase, while lightning and bolides could now generate NO' from N 2 and O 2 . Evidence for terrestrial ecosystems is found from 1.0 Gyr onwards; NO x . and NH 3 generated by terrestrial biota stands a greater chance of escaping to the atmosphere than do these compounds generated in the sea where recycling within the water body is likely. As CO 2 levels fell and O 2 levels rose, NH 2 cycling in the photorespiratory carbon oxidation cycle might have been evident as early as 1 Gyr ago, although this does not seem to be a major contributor to atmospheric NH 3 today. Embryophyte evolution on land 450 Myr ago, together with symbionts and biophages, increased primary productivity. and N cycling on land, with greater quantitative possibilities for NO x . and NH 3 escape to the atmosphere. The evolution of lignin (and related phenylpropanoids) at least 400 Myr ago, with associated NH 3 recycling in vascular land plant, does not seem (on present evidence) to increase NH 3 loss to the atmosphere significantly. Biomass burning occurred at least 3 50 Myr ago with lightning as the likely ignition source; such burning yielded NO . , NH 3 , N 2 O and N 2 from organic N.

The cluster roots of Grevillea robusta A. Cunn. ex R. Br. are composed of determinate rootlets that stop growing, but remain physiologically active for several months. Their apical organization, both before and after maturation, was studied by light and transmission electron microscopy. Each cell layer forms a dome, with an initial cell at its end. Xylem elements form a complicated triarch array at the base of the rootlet, passing along the rootlet as two files, and then joining at the tip to form a single file, surrounded by six pericycle cells. At the base of the rootlet, shorter xylem cells and thick-walled support cells are visible. A root cap, present in rootlets grown in vermiculite, was eventually displaced by root hair growth. Rootlets grown in Hoagland's solution lacked root caps and were significantly shorter than those grown in vermiculite. Cell fate was analysed in terms of cell position and is discussed in terms of pattern and development.

Light, fluorescence and electron microscopy were employed to follow the development of the endodermis in cluster roots and lateral roots of Grevillea robusta A. Cunn. ex R. Br. Endodermal cells had three different origins: rootlet endodermis arose from the rootlet meristem; endodermis covering the primordium shortly after initiation came from division of parental endodermis; cells at the junction between parent and rootlet endodermis developed from re-differentiated rootlet cortical cells. In the cluster root, the Casparian band formed in three ways, and was not initially present opposite the two sets of single xylem elements in the rootlet stele. A new clearing technique was developed that allowed visualization of xylem, suberized endodermis, Casparian band formation and phenolic compounds. In lateral roots, endodermal differentiation was asynchronous, but was related to position relative to protoxylem poles. However, the observed delay began before these poles had differentiated. At the tip of mature rootlets, which are determinate, the endodermis terminates in a ‘dome’ of cells, with the initial cell differentiating as an endodermal cell. Results are discussed in terms of determinate development in roots and the spatial and temporal contexts within which this development takes place.

summary This work explored whether the natural abundances of carbon and nitrogen isotopes could be used to describe the movement of C and X within wheat plants; we also considered whether isotopic analyses of aphids or their honeydew would substitute for direct analysis of phloem exudate. The δ13C of ears and roots (sinks) most closely matched those of the sugars + organic acids fraction (sources) in both growth stages; phloem δ13C matched that of leaf blade sugars. Xylem exudate δ13C matched no other putative (and measured) source in the ear-forming stage and matched that of whole roots and ears in the grain-filling stage. The δ15N of grain and roots (sinks) resembled that of leaf amino acids (sources) in the ear-forming stage. In the gram-filling stage, ear δ15N continued to resemble that of leaf amino acids, and δ15N of roots most closely resembled that of whole leaves. In the grain-filling stage, phloem δ15N fell between that of leaf blade amino acids and that of whole leaves and was 15X-depleted relative to internal and external NO, -N. In both growth stages, xylem exudate δ15N was less than that of soil NO3−-N and more than that of residual soil N after mineral N extraction. The isotopic values are generally in agreement with data from other approaches, such as isotope labelling; they show NO3−-N reduction in both shoots and roots of wheat and significant N recycling (root-shoot-phloem-root) and C movement. Aphids might serve as a substitute for isotopic analysis of phloem δ15N. having the same value as their food source. Their excreta was 15N-enriched relative to phloem.

SUMMARY We investigated interactions between the effects of elevated atmospheric carbon dioxide concentrations g and soil water availability on root biomass, root length and nutrient uptake by spring wheat {Triticum aestivum cv. Tonic). We grew plants at 350 and 700/^mol mol"^ COg and with frequent and infrequent watering ('wet' and ' dry' treatments, respectively). Water use per plant was 1-25 times greater at 350 than at 700 /^mol COg mol~^, and 1-4 times greater in the 'wet' tban in the 'dry' treatment. Root biomass increased with [COJ and with watering frequency. Elevated [COg] changed tbe vertical distribution of tbe roots, witb a greater stimulation of root growtb in tbe top layers of the soil. These data were confirmed by tbe video data of root lengths in the 'dry' treatment, which showed a delayed root development at depth under elevated [COg]. The apparent amount of N mineralized appeared to be equal for all treatments. Nutrient uptake was affected by [COg] and by watering frequency, and there were interactions between these treatments. Tbese interactions were different for N, K and P, which appeared to be related to differences in nutrient availability and mobility in the soil. Moreover, these interactions changed witb time as tbe root system became larger with [COJ and witb watering frequency, and as fluctuations in soil moisture contents increased. Elevated [COg] affected nutrient uptake in contrasting ways. Potassium uptake appeared to be reduced by the smaller mass flow of water reaching the root surface. However, this might be countered with time by the greater root biomass at elevated [COJ, by tbe greater soil moisture contents at elevated [COg], enabling faster diffusion, or botb. Pbosphorus uptake appeared to be increased by tbe greater root biomass at elevated [CO2]. We conclude that plant nutrient uptake at elevated [COJ is affected by interactions witb water availability, tbough differences between nutrients preclude generalizations of the response.

SUMMARY The 13C/12C ratio (expressed as δ13C) of benrhic photolithotrophs. in the Dighn Water (= Burn) were measured fur comparison with that of the potential inorganic carhun sources. CO2 and HCO3-, in the Burn. The Burn water contains an average of 65.7 mmol m-3 CO2 with δ13C of -14.7% and 1600 mmol m-3 HCO3- with δ13C of -4.%. δ13C values of riparian vegetation were also measured as contributors, after respiration in the soil or the Burn, to the δ13C of inorganic carbon in the Burn. The potential range of differences in 13C/12C between dissolved CO2 and plant organic C is set by the intrinsic 13c/12C discrimination (α value) in CO2 fixation by Rubisco. Main results and conclusions are. as follows, (i) A literature survey suggests that there is no convincing evidence that the α, Values (rate constant for 12CO2 fixation relative to that for 13CO2 fixation by Rubisco in the absence of CO2 transport limitation) for the‘lower plants’in the Burn (diatoms, green and red algae, mosses) are significantly different from the well-established αp values for the flowering plum enzyme. (ii) In confirmation of earlier work, the semi-erect 'streamer’gametophytes of the red alga Lemanea mamillosa and the moss Fontinalis antipyetica have δ13C values which can only be interpreted in terms of diffusive CO2 entry with minimal limitation of photosynthesis by CO- diffusion, (iii) The serui-erect grren alga Cladophora glomerata and the flowering plant Ranunculus penicillatus ssp. pseudofluitons (formerly var. calcareus) are- both able to use HCO3-. Their δ13C values indicate that, if the HCO3- -use system does not (as is likely) discriminate significantly between 13C and 12C, then a substantial fraction of the inorganic C made available to Rubisco must return to the medium, carrying 13C-inorganic C not fixed by Rubisco. (iv) Two sets of δ13C data from different hydrodynamic regimes distance from leading edge of a flat stone; different size of thalli) show that the attainable differences in situ in thickness of the diffusion boundary layer do not alter the fractional limitation of photosynthesis of Cladophora by external diffusion of inorganic C, considered with HCO3 use. (vi) The entrusting red alga Hildenbrandia rivularis has a δ13C value suggestive of CO2 as the inorganic C source, but not entirely ruling nut HCO3-. Marine species of both Hildenbrundia and Cladophora have δ13C values which, even when corrected for source inorganic C δ13C values, are 10%, more positive than the freshwater species. (vii) Mats of pennate diatoms were shown by pH-drift to by able to use HCO3-; the relatively high (i.e. not very negative) δ12C value of these mats could relate to a relatively‘non-leaky’HCO3- aequisition mechanism and/or to limitation by external diffusion (e.g. through the mat).

SUMMARY The growth and nitrogenase activity of stem nodules of Sesbania rostrata Brem were strongly influenced by the humidity surrounding the nodule. Nodules developed under high humidity showed the fastest rates of growth and the highest specific nitrogenase activity (determined by acetylene reduction). Development of stem nodules in both the light and dark was similar and showed a similar lowering of growth and activity with decreasing humidity. Stem nodules developed in the dark were pale and unlike the dark green nodules that developed in the light, they did not have chloroplasts in the nodule cortex. Adventitious root development required darkness and relative humidities approaching 100%. In stem nodules that were grown at high humidity and subsequently exposed to lower humidities, there was only a small loss of nitrogenase activity, and high rates of acetylene reduction were maintained for 6 d after treatments were begun. The area of nodule attachment to the stem or root was measured in nodules of many sizes and found to be consistently greater in stem nodules. This may have implications for the comparative water balance of nodules on these two organs.

eference s SUMMARY Potential advantages of ammonium relativ e to nitrate assimilation (assumin g equal nitrogen supply in the two forms), deduced from known biochemical pathways and the site of nitrate assimilation in vascular land plants, include (i) a greater maximum specific growth rate, (ii) lower costs of photons and (in transpiring plants) water per unit carbon assimilated, and (iii) lower costs of iron, manganese and molybdenu m per unit carbon assimilated per unit time. Actual measurements sho w that the growth rat e and photon cost predictions are often, but not invariably, born e out; while data on the water cos t of growth almos t invariabl y contradic t the prediction by showing a lower water cost from nitrate-supplied than ammonium-supplied plants. Few data seem to be available that test the predictions as to metal costs of growth; some of the predictions are borne out. These possible or realised advantages to the photolithotrop h of using ammonium must be considered in the context of the relative availability of the two sources of combined nitrogen . In the oceans nitrification o f ammonium produce d in mineralization does not compet e well with photolithotrophic assimilation of ammonium in the euphotic zone, so ammonium is the major combined nitrogen source for 'recycled' primary production, whil e ' n e w' production largely uses nitrate produced in deep, dark water. When both nitrogen source s are available to phytoplankton organisms ammonium is almost invariably preferred, wit h complete suppression of nitrate use with as little as 1-2 mmol m^^ ammonium. In terrestrial habitats the nitrification of ammoniu m produced in mineralizatio n is not photoinhibited (as can occur in the surface water of the ocean) but is subject to inhibition by anoxia, low p H and (possibly) plant-produced defence compounds. However, the lower diffusion coefficien t for ammonium than for nitrate in soils means that nitrogen-limited plant growth as a given rate needs a higher mean dissolved ammonium concentration in th e soil than is th e case for nitrate when the soil contains only one of these nitrogen sources an d root distribution and morphology are unaffected by the nitrogen source. With equal mean ammonium and nitrate concentrations, a nitrogen-limited plant supplied solely with ammonium would need a more extensive root/root hair/mycorrhiza system to attain the same nitrogen uptake rate on a per plant basis as would a nitrate-supplied plant, with consequences for resource allocation by, and growth rate of, the ammonium-grown plant. In addition to the larger mean area-based ammonium assimilation rate by photolithotrophs in th e oceans, consideration of the interactions among ammonium diffusion coefficient, ammonium diffusion distance, organism surface area per unit biomass and th e organis m maximum specific growth rate in the ocea n relative to soils can plausibly account for the lower mean ammonium concentrations in the ocean than in soils.

summary Ammonium (used here to signify NH4+ plus NH3) is the immediate inorganic precursor of organic nitrogen in photolithotrophs. In marine habitats ammonium is also the major exogenous nitrogen source, and the same is probably true of terrestrial habitats. In addition to this major role of ammonium as exogenous nitrogen source, it is also quantitatively very important as an endogenous nitrogen source with nitrate or N2 as exogenous nitrogen source, and as a recycled nitrogen compound in photorespiration and in phenylpropanoid synthesis. The shoots of terrestrial plants have higher NH3 compensation partial pressures than most natural soils, and especially higher than that of the sea. However, long-distance transport of gaseous NH3 (e.g. from continents to oceans) is a negligible component of the ‘natural’ global nitrogen cycle. Current concepts of evolution of the atmosphere and biosphere do not involve high partial pressure of NH3 in the atmosphere: any ammonium produced in inorganic or biological processes is removed from the atmosphere by rain-out, or, over larger time-scales, by the action of ultraviolet radiation (especially before the ozone screen came into being) and of hydroxyl radicals (especially after oxygenation of the atmosphere). In addition to posing problems for the origin of life and arguing for an early rather than a late evolution of diazotrophy, a low NH3 partial pressure renders implausible arguments that phenylpropanoid synthesis was restricted early in evolution by the effect of high ammonia in reversing phenylalanine ammonia-lyase.

summary Circulating ionic currents are apparently ubiquitous in growing and differentiating eukaryotes. In many the current is carried by H+, in which case the medium around the apex of growing organs is alkalinized while the medium adjacent to subapical portions is acidified. These two zones may help in, respectively, molybdenum acquisition and reduction of aluminium toxicity, and phosphorus and iron acquisition, by rhizophytic plants. An analysis of the taxonomic distribution of H+-borne circulating current shows that all rhizophytes (terrestrial, freshwater and, probably, marine) except those in the green algal class Ulvophyceae have such currents; the Ulvophyceae have circulating currents carried by Cl−. The Ulvophyceae do not seem to have any alternative means of causing pH zonation in the rhizosphere. Their marine habitat, with the possibility of molybdenum acquisition from the alkaline bulk water phase, may mean that the Ulvophyceae are not disadvantaged by the absence of rhizosphere acid-alkaline zonation as would be a land plant; similarly, they are less likely to suffer aluminium toxicity to the growing zone of rhizoids. Ulvophyceae do not occur as terrestrial plants; this may relate not only to the absence of rhizosphere pH zonation in these organisms but also to mechanical problems related to their coenocytic habit, which in turn results from their inability to produce plasmodesmata.

SUMMARY Iron is involved in many photosynthetic, respiratory and nitrogen assimilation reactions of plants as Fe bound tightly to polypeptides catalysing redox reactions. Manganese is involved as tightly bound Mn in photoreaction II of photosynthesis and in certain superoxide dismutases, while loosely bound Mn2+ is the unique activator of some enzymes, and is an alternative to Mg2+ in activating many enzymes. This paper uses data on the quantitative role of Fe and Mn in catalysts to predict the efficiency with which Fe and Mn are used in C assimilation [mol C assimilated (mol catalytic metal in enzyme)−1 s−1] and the metal cost of C assimilation [mol catalytic metal in enzyme (mol C assimilated)−1 s−1] in photolithotrophic growth in relation to genetic and environmental variables. The genetic variables were the relative content of thylakoid proteins in major taxa (cyanobacteria and red algae, chlorophytes and chromophytes) and smaller-scale taxonomic differences (various subtypes of C4 metabolism, and C3 metabolism, in terrestrial vascular plants). The environmental variables were the range of photon flux densities in which photolithotrophic growth of O2 evolvers can occur, and the inorganic C supply conditions controlling the repression/de-repression of the inorganic C concentrating mechanism in cyanobacteria and microalgae. The results of the computations yield the following conclusions. The largest predicted difference in Fe and Mn costs of photolithotrophic growth is related to changes in the photon flux density for growth. The predicted Fe cost increased 50-fold, and the Mn cost increased 80-fold, at the lowest extreme of photon flux density compared to the highest found naturally. The increase is partly countered by the larger ratio of light-harvesting pigments to thylakoid protein complexes assumed for the cells grown at low photon flux densities, although the extent of the increase in photosynthetic unit size is limited by considerations of efficiency of excitation energy transfer. However, the major influences are the higher pigment content in biomass enabling a larger fraction of incident light to the absorbed, and the sub-maximal specific reaction rates of redox catalysts (whose content is constrained via excitation energy transfer considerations) at very low photon flux densities. A smaller difference, four-fold or less, in Fe and Mn costs of photolithotrophic growth, is predicted by comparing major taxa (cyanobacteria plus red algae; chlorophytes plus chromophytes) with contrasting ratios of thylakoid redox catalysts. Differences in Fe and Mn costs of growth of less than two-fold are predicted when the Fe- and Mn-efficient organisms with CO2− concentrating mechanisms (C4 land plants; algae with active inorganic C influx) and high requirements for ATP relative to NADPH, are compared with organisms relying on CO2 diffusion from air or air-equilibrated solutions and C3 biochemistry. These predictions of variations in Fe and Mn costs of photolithotrophy have implications for the ecology of phototrophs.

SUMMARY Photosynthetic O2 evolution by the upper littoral lichen, Lichina pygmaea (Lightf.) C.Ag., under light-saturated conditions at 5 °C is saturated by the 2 mol m−3 inorganic C found in seawater at pH 8.0. Photosynthesis is not reduced when pH is increased to pH 9.4, and is slightly reduced at pH 10.0, when submersed in seawater with 2 mol m−3 inorganic C. The rate of photosynthesis at pH 10 greatly exceeds the rate of uncatalysed conversion of HCO3−. It is concluded that HCO3− is used in photosynthesis. Since extracellular carbonic anhydrase is present, it is possible that CO2 enters the photobiont (Calothrix) cells even during HCO3 use. pH drift experiments support the notion of HCO3− use. Emersed photosynthesis at 5 °C is more than half-saturated by 35 Pa (normal atmospheric) CO2; the light- and CO2-saturated emersed photosynthetic rate is not significantly different from the light and inorganic C-saturated photosynthetic rate when submersed. Inorganic C diffusion from the thallus surface to the photobiont needs, at least under some conditions, carbonic anhydrase activity which permits HCO3− fluxes to supplement CO2 movement. The CO2 compensation partial pressure at 5 °C is 0.83 Pa, i.e. at the low range of values found for terrestrial cyanobacterial lichens. Dark 14C-inorganic C assimilation when submersed is a small fraction of the dark respiratory rate, consistent with the observed absence of diel CAM-like variation in intracellular titratable acidity. The high value (−11.5%) of δ13C, the low CO2 compensation partial pressure, and the relatively high affinity for inorganic C., are consistent with the operation of an inorganic C concentrating mechanism such as occurs in free-living cyanobacteria and probably occurs in terrestrial cyanobacterial lichens and in most intertidal algae.

SUMMARY An analysis of published data suggests that the N2-fixing symbiotic vascular plants extrude more H+ per unit N fixed than would be expected from data on the same genotypes growing on NH4+ if the plants had the same chemical composition when grown on the two N sources. The H+/N ratio with urea as the N source is similar to that with N2. The higher H+/N ratio and higher organic acid/N ratio with N2 or urea as N source implies higher whole-plant energy and water costs per unit of biomass and, ultimately, inclusive fitness, produced. The rhizosphere acidification resulting from H+ extrusion may serve to change rhizosphere pH to some ‘optimal’ value, and to increase the availability of such limiting resources as P, Mo and Fe which are especially needed in diazotrophy. Data in the literature are consistent with these possibilities in the few cases examined. Within the plant, data on xylem and phloem sap composition in conjunction with shoot composition, of diazotrophically-growing legumes suggest that shoot acid-base homoiostasis can be maintained via the import of appropriate solutes in the xylem and the export of appropriate solutes in the phloem. Acid-base regulation of the nodules in the absence of any H+ exchange with their environment can also probably be explained in terms of the solutes supplied in the phloem and exported in the xylem. This conclusion is based on data in the literature on the composition of stem phloem sap and of xylem sap exuding from detached nodules of diazotrophic vascular plants. These considerations do not exclude the possibility of net H+ efflux from nodules fixing N2 in contact with an aqueous medium. The limited data available are consistent with extrusion of some of the H+ generated in nodules as an alternative to their neutralization by metabolism of organic anions entering in the phloem. Such H+ extrusion by nodules could aid in their acquisition of Fe from the medium, albeit not always at a phase in the life or the nodule when there is a net requirement for Fe.

SUMMARY Many data support the view that, when NH4+ [or N2, NH3, or CO(NH2)2] is the N source for plant cell growth, the excess H+ generated in the synthesis of core metabolites is excreted to the bathing medium (biophysical pH-stat). This paper explores the possibility that a ‘biochemical’ disposal of these excess H+ could occur, thus allowing net NHJ assimilation to take place in the shoot of land plants. A‘biochemical’ H+-neutralizing pH-stat requires that a non-toxic resource be taken into the plant in a form in which metabolism can convert into a non-toxic product with H+ consumption or OH− production. This possibility was explored for reductive metabolism of B(OH)3, Si (OH)4, H2PO4−, H2AsO4−, O2, SO42, SeO42− and HVO42; and for oxidative metabolism of Cr, Br−, I−, Fe2+ and Mn2+. For B(OH)3 and Si(OH)4, reductive metabolism (even if it were thermo-dynamically possible, granted the reductants available to plants) does not involve significant 11+ removal. H2PO4− reduction may be thermodynamically possible, but is quantitatively insignificant as an H+ sink in plants. H2AsO4− reduction is probably a detoxification mechanism, and it is not a significant H+ sink in plants for which quantitative data are available. O2 assimilation (reduction) into -OH, and thence ≡O+, occurs in anthocyanin synthesis, but not to an extent which disposes of much of the excess H+ produced in growth with NHJ as N source. SO42− reduction in excess of that required by primary, core metabolism (i.e. that leading to amino acids, thylakoid sulpholipid and cell wall esters) can generate OH−, but such ‘secondary’ SO42− metabolism is related to osmoregulation and to chemical defence rather than to H+ disposal per se. The quantity of S metabolism which is ‘negotiable’ is not, apparently, adequate to neutralize a substantial fraction of excess H+ formed during growth. SeO4−2 reduction and assimilation performs largely a detoxification and/or chemical defence role, and the quantities involved (even in Se-accumulators) do not generate enough OH− to offset much of the excess H+ formed during growth. Vanadate reduction is quantitatively insignificant as an H+ sink. Oxidation of Cl−, Br− and I− in incorporation into C-halide groups is, in land plants, a quantitatively insignificant process. H+ disposal via HCl volatalization from an aqueous phase of low pH which contains Cl− does not seem to be an important sink for H+ in terrestrial plants. Oxidation of F− to form ≡C-F usually produces CH2F.COO− with no net H+ consumption. Oxidation of Fe2+ and Mn2+ is H+-consuming as long as oxides or hydroxides of Fe3 and Mn4+ are not formed. However, the oxides and hydroxides are often formed so that the oxidation processes consume OH− rather than H+. These quantitative considerations, together with those of the availability of starting materials and the toxicity of end products suggest that the H+-consuming processes, even in combination, probably cannot dispose of all of the H+ generated in growth with NHJ as N source. In general, these H+-consuming reactions seem to be related to synthesis of osmoregulatory and chemical defence compounds, and to detoxification; the products appropriate to these functions generally have high molecular masses per mol H+ consumed in their synthesis, a feature which does not make them ideal parts of a ‘biochemical pH-stat’.

SUMMARY The glucose analogue, 3-O-methyl glucose, is actively transported inwards at the plasmalemma of Hydrodictyon africanum. It is likely that a similar mechanism is operative for glucose influx, although demonstration of its active transport is complicated by an apparent coupling of the rates of glucose entry and metabolism. Active OMG Transport, and the influx and metabolism of glucose, need ATP. This ATP can be supplied, in order of increasing rates of the two processes, by fermentation, cyclic photophosphorylation, oxidative phosphorylation and by photo-phosphorylation plus oxidative phosphorylation.

SUMMARY The capacity of the symplast and the phloem to transport Ca2+ and H* (OH−) is discussed in relation to the concentrations of free and bound Ca2+ and H* in these transport systems, and the general restrictions on the concentrations of free Ca2+ and H* in cytoplasm. It is concluded that the low Ca2+ and H+ transport capacity of symplast and phloem are consequences of the cytoplasmic nature of these transport systems. The significance of the low transport capacity for these two ions relative to the rate at which the plant produces or consumes H+ or can take up Ca2+ and transport it in the xylem is discussed. Other transport characteristics of the phloem can also be related to the cytoplasmic nature of the transport conduit: the nature of the transported organic C and N compounds is partly dependent on the requirement that high concentrations of these solutes can be tolerated by cytoplasmic enzymes, i.e. sucrose, mannitol and amino-acids are ‘compatible solutes'. The properties of the intracellular transport systems (symplast and phloem) are contrasted with those of extracellular transport systems such as plant xylem and animal blood.

SUMMARY The distribution of IAA between the vacuole and the bathing solution in Hydrodictyon africanum is consistent with passive entry of undissociated IAA and passive efflux of both undissociated IAA and of IAA-, with PIAA (permeability coefficient)* about io-3 cms-1 and PIAA- about -6 -1A o-6 cm s . The involvement of IAA- in the efflux results from the inside-negative P.D. between the medium and the vacuole. The cytoplasm is at a higher pH than either the vacuole or the bathing solution used in most experiments; this is maintained by active H+ efflux at the plasmalemma, and active influx at the tonoplast. In this situation the efflux of IAA- is further promoted by the increased concentration of IAA- in the relatively alkaline cytoplasm. Thus the apparent active efflux of IAA from these cells can be explained in terms of passive driving forces of concentration and electrical potential acting on IAA and IAA-, with the distribution of these two species dictated ultimately by PIAA, PIAA-) the pH of the various compartments and the electrical potential difference between them. If PIAA/PIAA- were larger at the apical than at the basal end of coleoptile cells, such an effect could explain polar IAA transport, with metabolic energy being used only to maintain the relative permeabilities to the two species, the pH gradient and the electrical gradient.

SUMMARY The assimilationof ammonium ion in plant cell cytoplasm produces at least one H+ per NH+4; N2 fixation generates 0.1-0.2 H+ per N assimilated; NO-3 assimilation produces almost one OH- per NO-3. H+ or OH- produced in excess of that required to maintain cytoplasmic pH for H+ or OH-, the major process involved is H+ efflux (frequently by active transport) from the cell. IN higher land plants, much of assimilated N occurs as hoot protein; the shoot cells have no direct acess to the H+ and OH- sink of the soil solution. When ammonium ion is the N source it is assimilated into organic-N in the roots. The shoot is supplied with a mixture of amino-acids, amides and organic acids which an be incorporated (with neutral photosynthate) into cell material without damaging pH changes. Similar considerations apply to symbiotic N2 assimilation in root nodules. IN both cases the excess H+ generated in the root cell cytoplasm is exerted is excreted to the soil solution; there is no mechanism whereby photolithotrophic plant can, in the long term, counter intracellular acidity without resort to active H+ efflux to an extracellular sink. When nitrate is reduced in roots, the organic compounds involved in N transportged to the shoot are similar to those used when ammonium or N2 is the N source with similar implications for the regulation of shoot pH. The excess OH- generated in the roots is partly excreted to the soil solution, and partly neutralized by the ‘biochemical pH stat’ which produces strong organic acids from essentially neutral precursors. When nitrate is assimilated solely in shoots, the excess OH- is initially neutralized by the operation of the biochemical pH state. Storage of the inorganic cation-organate in shoot cell vacuoles could lead to turgor and volume regulation problems in these cells. These are avoided when an insoluble salt (calcium oxalate) is the product of the pH stat, or when the cation organate is translocated to the roots where organate breakdown regenerates OH-, whcih is lost to the soil solution. This mixture of biochemical, and long and short distance transport processes, enables cells remote from a large sink for H+ or OH- to produce protein without unfavourable pH changes. These processes related to pH regulation during N assimilation have important consequences for the carbon and energy economy of the plant.

SUMMARY 157INTRODUCTION 158AN.^LYTICAL METHODS 159LIGHT HARVESTING BY MICROALGAE 160RANGE OF PHOTON FLUX DENSITIES ALLOWING GROWTH AND PHOTOSYNTHESIS INPHOTOTHOPHIC MICROALGAE (GENOTYPIC ADAPTATION) 163 Growth 163Photosynthesis 165Photoinhibition 165 PHENOTYPIC ADAPTATION 168 Changes in amounts of pigments 168Interpretation of the effects of pigment changes: models 169Observed changes in P vs I curves 170 ENERGETIC CONSIDERATIONS 174 General 174Reduction of capital costs 175Reduction of maintenance costs 175Energetic costs of changing the photosyothetic apparatus 177S2-S3 decay 177Proton leakage due to passive uniport 178 PHYLOGENETIC ASPECTS OF DIFFERENCES IN LIGHT RESPONSES OF MICROALGALPHOTOSYNTHESIS AND PHOTOLITHOTROPHIC GROWTH 178 Phylogenetic diflerences in photosynthetic structures 178Comparison of tbe photosynthetic characteristics of green algae and otherpbototrophs 180 ECOLOGICAL CONSIDERATIONS 182ACKNOWLEDGEMENTS 185REFERENCES 185 SUMMARY Analysis of data in the literature relating to micrcalgal adaptations to different photon fluxdensities indicates that different algal classes have significantly different ligbt requirennents forgrowth and photosynthesis. Although there is some variability within each class, dinoflagellatesand blue-green algae generally photosynthesize and grow best at low photon flux densities.Diatoms also tend to be able to grow at very low photon flux densities (growth for some specieshas been reported at less than 1 fi.E m"' s~'). Comparison of the photon flux densities at whichphotoinhibition occurs in dinoflagellates and diatoms suggests that the former often experiencephotoinhibition at comparatively low irradiances. In contrast, diatoms often can toleraterelatively high light environments. This tolerance of a large absolute range of photon fluxdensities may, in part, explain why diatoms are often associated with spring blooms. Green algae* New address; Department of Botany, La Trobe University, Bundoora, Victoria, 3083 Australia.0028-646X/83/020157 + 35 S03.00/0 © 1983 The New Phytologist

SUMMARY When ATP is supplied by oxidative phosphorylation alone, cyclic photophosphorylation alone or from both photosynthetic and oxidative phosphorylations, exogenous glucose is metabolized to a similar range of non-volatile products of which a major one is starch. When only fermentative ATP supply can occur, much less starch is produced. Cyclic photophosphorylation alone supports a rather lower rate of starch synthesis from exogenous glucose than does oxidative phosphorylation alone. This is apparently due to rate-limitation by glucose influx, since cyclic ATP production supports a faster rate of starch synthesis than does oxidative ATP production when endogenous substrate is used. Production of CO2 from specifically labelled glucose is consistent with the operation of the EMP, PPP and TCAC in both light and darkness in the presence of oxygen. Fermentation of exogenous glucose is inhibited by light absorbed by photosystem one, probably via cyclic photophosphorylation. Respiratory CO2 production is also inhibited to a small extent by photo-system one, although most of the light inhibition needs photosystem two. This DCMU-reversed inhibition of CO2 production in the light is due to a direct inhibition of decarboxylation reactions as well as to reassimilation of respired CO2. The relatively small randomization of C1 and C6 labelled glucose during assimilation into starch suggests that conversion to triose phosphate may not be essential for transport of hexose across the outer chloroplast membranes.

SUMMARY Iron has many catalytic roles in photosynthesis, respiration and nitrogen assimilation; molybdenum is involved in NO3 and (usually) N2 reduction. This paper concentrates on computing the efficiency of use of Fe and Mo in growth of organisms growing with different mixtures of the major resources photons, carbon and nitrogen. This computed efficiency is defined as mol C assimilated (mol catalytic metal in the organism)-1 s-1. The efficiency is computed from the known in vivo involvement of Fe- and Mo-containing enzymes, the specific reaction rate of these enzymes, the specific rate of these enzymes in vitro, and the growth requirements for the products of the reactions, for organisms using different energy, carbon and nitrogen sources. The predicted Fe use efficiency is greater for chemo-organotrophic than for photolithotrophic growth with a given nitrogen source, and increases in the order N2 NO3- NH4+ for growth with different nitrogen sources in the presence of constant photon and carbon availability. The predicted changes with variations in nitrogen source are greater than those with variations in photon and carbon source. These predicted values may be compared with observed metal use efficiencies, defined as mol C assimilated (mol total metal in the organism)-1 s-1. Relatively few such observed values are available From the literature. For Fe in photolithotrophically growing microalgae the observed values can be as low as 16–17% of the predicted values, i.e. 83–84% of the Fe in the organism is not accounted for by known catalytic uses of Fe if the catalysts are operating at their maximum specific reaction rates. One possibly discordant prediction relates to the Fe and Mo cost of N2 fixation; the computations suggest that in vitro estimates of nitrogenase activity may be less than those sometimes found in vivo. The predictions also have possible implications for resource availability interactions (photons, carbon, nitrogen, Fe, Mo) in natural conditions.

The mobility of dinophytes allows them to migrate dielly, in a vertically stratified environment, between deeper regions where concentrations of nutrients (principally nitrate) are higher but light low, and less deep regions with lower nutrient concentrations but more light. When there is a substantial (several metres) depth range in which dinophytes can grow but, at any given depth, growth is restricted by both nitrate and light availability, vertical migration from a deeper station at night to a less deep station during the day could increase the availability of light and nitrate to a dinophyte cell over a complete light-dark cycle. Cost-benefit analysis shows that the benefits of migration (increased acquisition of photons and nitrate relative to non-migratory cells) could very substantially exceed the costs (the energy and nitrogen required to produce and operate the flagella apparatus and any additional nitrate or photon-harvesting machinery which may be required as a result of migration). It is therefore concluded that, in some habitats, vertical migration could increase the growth rates of dinophytes. The occurrence of such increases under natural conditions, and their selective significance, await further investigation.

Summary Light-stimulated active chloride influx in Chara corallina, like that in other giant algal cells, requires the operation of photo-system two of photosynthesis. Active phosphate influx and glucose influx also require photo-system two for light-stimulation, and are not stimulated by light under anaerobic conditions. This contrasts with the situation in Nitella and Hydrodictyon where influx of phosphate and glucose can be supported by photo-system one, i.e. by cyclic photophosphorylation alone. None of the other metabolic processes in Chara tested in this work (acetate uptake and oxidation, starch synthesis, and passive Rb+ influx), or described elsewhere in the literature, are light-stimulated when photo-system two is inoperative. Possible explanations of this apparent inability of cyclic photophosphorylation to act as a source of metabolic energy in Chara are suggested. Their significance is discussed in relation to hypotheses of the light reactions of photosynthesis and mechanisms of light-dependent solute transport.

Summary Data are presented which show that the photosynthetic and respiratory characteristics of the giant-celled alga Chaetomorpha darwinii are those of a shade plant (denned physiologically in terms of rates of photosynthesis at light and inorganic carbon source saturation, and the irradiance required to saturate photosynthesis and to compensate respiration by photosynthesis). This is in agreement with previous suggestions that giant-celled algae have the characteristics of ‘shade plants’, while small-celled macroalgae can be either ‘sun plants’ or ‘shade plants’. A comparison of giant-celled algae with small-celled macro-algae in terms of ecological strategies has been made, using previously published data on Enteromorpha intestinalis, Chaetomorpha linum (‘fugitive’small-celled), Chaetomorpha darwinii, Hydrodictyon africanum (‘stressed’ giant-celled), Cham corallina (‘canopy-dominant’/‘stressed’ giant celled) and Laminaria digitata (‘canopy-dominant’ small celled), as well as new data on net and mass spectrometric measurements of O2 exchange in L. digitata. It is suggested that the ability of giant-celled algae to compete as ‘canopy dominants’ may be related to cytoplasmic streaming in the ‘absence of many (or any) plasmodesmatal barriers to diffusion providing an efficient means of long-distance transport in a differentiated plant, as an alternative to phloem-type translocation in small-celled ‘canopy dominants’. The significance of the giant-celled habit in ‘stressed’ macroalgae is not clear, although the absence of giant-celled algae from the ‘fugitive’ category may be related to their limited capacity for rapid growth relative to small-celled algae.

SUMMARY Freshwater algae have an intracellular osmolarity in excess of that of the medium. The tendency for water to enter the cells is resisted, in cells with a mechanically functional cell wall, by the ability of the wall to withstand a turgor pressure equivalent to the intracellular osmolarity; in cells lacking such a mechanically functional wall the water which enters is expelled by a contractile vacuole (or its functional equivalent). A comparison of the energy costs of cell volume regulation by these two mechanisms has been carried out making energy costings based on the minimum thermodynamic requirement and on plausible mechanisms for the two processes (wall synthesis and contractile vacuole function). This analysis yielded the following conclusions. (1) Wall synthesis is a ‘growth’ process, while contractile vacuole operation is a’maintenance’process; other things being equal, a cell wall is a preferable mechanism of volume regulation in non-growing cells. (2) For a 5 μm radius cell with a specific growth rate of 8 × 10−6 s−1 and a 10 μm radius cell with a rate of 4 × 10−6 s−1 the energy input rate for contractile vacuole operation is considerably lower than for wall synthesis on the basis of minimum thermodynamic requirement; the plausible mechanism approach makes the two energy costs quite similar. Decreased growth rates favours the wall over the contractile vacuole mechanism. (3) Increased cell size (10 μm radius rather than 5 μm) has similar effects on the energy costs of both volume regulation mechanisms, provided a (cell organic weight)−0.32 dependence of intrinsic growth rate is incorporated into the computations. (4) Increased intracellular osmolarity (at constant extracellular osmolarity) causes a directly proportional increase in energy input rate for the cell wall mechanism, and a more than proportional increase in energy cost for the contractile vacuole mechanism. (5) A non-spherical shape increases the energy cost (at constant cell volume) for both the cell wall and the contractile vacuole mechanisms. These conclusions may be used in the interpretation of such phenomena as the lower osmolarity of wall-less than walled cells and the tendency for the resting stages of normally wall-less and flagellate cells to have walls. It appears that the majority of freshwater algae have genetic capabilities to produce both walls and contractile vacuoles for cell volume regulation (e.g, the production of walled cysts by organisms which are normally flagellate, and the production of wall-less motile spores and gametes by normally walled organisms).

summary The typical N source for terrestial vascular plants is the soil in which they are rooted. N exchange between the aerial shoot and its environment, as gaseous, dissolved or particulate N, can involve either net loss or net gain of N by the plant. Net gain of N by the plant shoot at the expense of its aerial environment can be significant, under natural or agricultural conditions, for all three forms of aerial N (NH3,N2,NO3), Gaseous N gain can be substantial, as NH3, for plants growing on soils receiving large inputs of animal excreta and faeces. Soluble N uptake by shoots is the major N source for ‘atmospheric’ epiphytes. Particulate N supply to shoots is a major N source for carnivorous plants. Experimentally, several plants which normally grow with N supply mainly via their roots can be grown with a predominant, or sole, supply via their shoots. Plant growth with CO2, as C source, SO42− as S source and H2PO4, as P source generates excess OH (∼ 0.78 OH−/N assimilated) in the synthesis of core metabolites when the N source is NO3− supplied to roots or shoots. All other N sources (except dicarboxylic amino-acids) yield excess H+during synthesis of core metabolites. NH4+ (in solution) supplied to roots or shoots yields some 1.22 H+/N assimilated, while NH3, N2 or NO2 supplied to shoots as gases, dissolved urea supplied to shoots or roots, and animal protein supplied to shoots of carnivorous plants, yields some 0.22 H+/N assimilated. These biochemically caused acid-base perturbations may be exacerbated (NO2) or partially or, occasionally, completely offset (NH3) by effects related to solution and dissociation of the gas in growing shoots. Assimilation of the shoot-acquired N in the shoots leads to somewhat different constraints on acid-base homoiostasis relative to those found with assimilation of root-acquired N. The shoot cannot generally excrete excess H+ or OH− to its environment (as is so for root-assimilation of N), nor can it dispose of H+ biochemically without co-operation of root metabolism and root–shoot and root-rhizosphere transport processes. Excess OH− from shoot-acquired and shoot-assimilated NO3− can, by contrast, be disposed of biochemically after the fashion of root-acquired, shoot-assimilated NO3−. Tentative conclusions as to the disposal of excess H+ generated in shoot acquisition and assimilation of N-sources other than NO3− and NO2− are different for rhizophytes and for haptophytes. Rhizophytes probably carry out a net synthesis of organic acid in the roots, with subsequent cation-H+ exchange at the root plasmalemma, transport of cation salt of the organic acid to the shoot in the xylem, and metabolism of the organic acid anion in the shoot to yield OH−. Haptophytes probably have an excess of liquid water supply over their needs for growth and transpiration due to the very low N concentration in this water; they thus have the potential for H+ (as strong acid) excretion to the excess surface water which is lost, at least in part, as liquid water.

SUMMARY The transport and distribution of phosphate in Hydrodictyon africanum is reported, and discussed in relation to phosphate transport in other green algae and chloride transport in Hydrodictyon. The inorganic phosphate concentration in both vacuole and cytoplasm is 1–2 mM; the cytoplasm also contains 35 mM of phosphate as organic and polyphosphate, which are absent from the vacuole. Phosphate influx from 0.1 mM phosphate at pH 6.5 was much greater than the efflux, and was more stimulated by light and inhibited by low temperature than was the efflux. The phosphate influx is, on electrochemical criteria, active. The influx at the tonoplast was much higher than that at the plasmalemma. Na stimulated phosphate influx more than did K, in agreement with results for other green algae but in contrast to monovalent cation effects on active Cl influx. Both phosphate and chloride influxes show a simple hyperbolic relationship between anion concentration and influx, in the concentration range tested, but the kinetics for the two ions show different responses to changes in irradiance and Na concentration. pH effects on the influxes are also reported. The passive permeability of the plasmalemma to H2PO-4 is similar to that to Cl-.

SUMMARY CCCP inhibits photosynthetic CO2 fixation more than photosynthetic O2 evolution in Hydrodictyon africanum, while the photosystem two inhibitor, DCMU, inhibits the two processes equally. These results are consistent with CCCP acting as an uncoupler of photophosphorylation, thereby inhibiting CO2 fixation, and the remaining O2 evolution being associated with the ATP-independent reduction of some (unknown) electron acceptor. Analysis of tracer chloride fluxes at the plasmalemma into active influx, exchange diffusion and passive uniport components supports previous conclusions, based on the assumption that all the tracer influx was due to active transport, that active chloride influx is relatively ATP-independent. Some problems inherent in the occurrence of redox-coupled active transport at the plasmalemma of eukaryotes are discussed, and a scheme which accounts for active chloride transport at the plasmalemma of H. africanum is proposed.

SUMMARY Polar transport of IAA in young higher plant shoot tissue requires an asymmetric distribution of membrane-transport sites in the plasmalemma of the cells. The 'chemiosmotic' hypothesis of IAA transport requires that IAA- uniporters be largely on the basal side of the cells. It is suggested that the longitudinal gradient of electrical potential difference (PD, base positive to apex) which may result from activation of plasmalemma H+ efflux in the basal cells by transported IAA, may serve to maintain and reinforce this asynmmetric distribution of carriers by the process of membrane electrophoresis of the carriers. The origin of this asymmetric distribution (and hence of polar IAA transport) may lie in electrical potential differences in maternal tissue around the developing embryo. Maintenance of polarity through seed dormancy may be related to gradients in cell-wall fixed charges (and hence of electrical potential difference) along the shoot axis related to the uronate content of cell walls at different stages of cell development. The possibility of testing this hypothesis, and its relevance to other polar transport phenomena in plants, are briefly discussed.

SUMMARY The molecular weight and ether-water partition coefficient of boric acid are consistent with a PB(OH)3 in plant cell membranes of at least 10-6 cm s-1. This permeability coefficient is high enough to account for the measured magnitude of boric acid fluxes at many plant cell membranes. The use of active transport of boric acid to maintain B distribution across a membrane away from thermodynamic equilibrium is consequently likely to be energetically expensive. The content of m-diols (with which boric acid can form complexes) in cell walls and inside the cells varies widely between different plant species without any obvious correlation with either total B content or with the B content at which deficiency (in B-requirers) or toxicity symptoms are manifested. B distribution at the cell level depends on the relative extents of passive permeation, active transport and cisdiol formation; regulations may be in response to total intracellular B rather than free boric acid. The net uptake of boric acid by intact vascular land plants is influenced by the rate of transpiration; while transport of B within the xylem is probably directly proportional to the rate of transpiration, neither whole plant B uptake nor transfer from root tissue into the xylem exhibit such a simple relationship. Redistribution of B in the phloem from transpirational termini is very limited. This limitation could result from the toxicity of B (which requires a low B concentration in the translocation stream relative to other nutrients and to sink requirements) or to ‘counter-current distribution’ of B from the phloem to the adjacent xylem stream (low in B) through the B-permeable sieve-tube plasmalemma. The transport of B is discussed in relation to the evolution of multi-cellular algae and of vascular land plants.

Summary A quantitative analysis of energy transmission along membranes by diffusion of buffered protons in the aqueous (N and P) phases on either side of the membrane is presented. In thylakoids of plants with chlorophylls a and b, where proton motive force (pmf)-generators and pmf-consumers are laterally separated by up to 1 μm, energy transmission by diffusion of buffered protons in the aqueous phase can occur at the observed in vivo rate with less than a 10% decrement in pmf available to consumers relative to its value at pmf-generators. However, in motile cyanobacteria, it is concluded that motility cannot be maintained in non-illuminated portions of the trichome by transmission of proticity along the plasmalemma when the mean distance of transmission exceeds a few micrometres. The transmitted effect of illumination is probably an informational rather than an energetic effect. Energy transmission by lateral proticity transport is essentially a diffusive process, and is subject to similar constraints to those related to the transport of ‘chemical’ energy (e.g. as ATP): suggestions that energy transmission by lateral proticity movement can occur over distances, and at velocities, similar to those of nervous transmission apply to transient states and do not reflect steady-state power transmission.

Summary An attempt is made to provide a cost-benefit analysis of light harvesting by microalgal cells. The costs relate to the number of mol photons which the cell must absorb to produce a quantity of light-harvesting apparatus containing 1 mol of chromophore; the fraction of the dry weight which is devoted to the light-harvesting machinery containing 1 mol of chromophore; and the number of mols of nitrogen which are used in producing the light-harvesting machinery containing one mol of chromophore. These costs relate to resources (photons; fraction of cell biomass; nitrogen) which may restrict the growth of microalgae in low-light environments (co-limitation by light and nitrogen in the case of nitrogen). The benefits relate to the photon absorption rate in a given light field (photon flux density and spectral distribution) per mol chromophore. Maximum photon absorption rate per mol chromophore requires that the specific absorption coefficient (e) of the pigment shall be high in the wavelength range to which the organism is exposed, and that self-shading (the ‘package’ effect) be minimized by having a large area of cell exposed to the incident photons per mol of chromophore. The costs of the light-harvesting machinery were estimated in terms of photons absorbed by the cell per mol of chromophore (plus associated protein and lipid) synthesized, using known biochemical pathways from carbon dioxide, nitrogen source (ammonium or nitrate) and photons to the light-harvesting apparatus. The fraction of the cell dry weight occupied by light-harvesting apparatus containing one mol of chromophore was deduced from the mass of protein and lipid associated with one mol of the various chromophores. The nitrogen cost was derived from the mols of nitrogen found in the light-harvesting machinery containing 1 mol of each of the various chromophores. These estimates show that, for all three criteria enumerated above, the cheapest tight-harvesting apparati are integral complexes containing chlorophylls a + b + carotenoids in chlorophytes and chlorophylls a+c2 (± C1) + carotenoids in Chromophytes, and the most expensive are the phycocyanins and allophycocyanins of Phycobiliphytes. The benefits of the various kinds of light-harvesting machinery were estimated in terms of the number of photons which were absorbed from a given light field per unit chromophore in solution and, more realistically, in vivo in cells of various sizes. The mean specific coefficient over the blue-green waveband characteristic of ‘aquatic shade’ showed that (in the absence of self-shading) the light-harvesting machinery characteristic of Chlorophytes and Chromophytes was generally superior to that of algae containing phycobilins and, especially, phycocyanin and allophycocyanin. When this disadvantage of the phycocyanins in terms of photon absorption rate per mol chromophore is compounded by considering the high energy (and fraction of biomass, and nitrogen) costs of synthesis of the phycocyanins, these pigments would appear to be contra-indicated as light-harvesting pigments for shade-adapted microalgae. Nevertheless, the phycocyanins occur in Cyanobacterial and Cryptophycean phytoplankters. A partial offset of the high costs of synthesis of the peripheral (phycobilin) light-harvesting complexes may derive from reduced H+ leakage through thylakoid membranes of organisms containing these complexes since the lipid bilayer area per mol chromophore is lower in Phycobiliphytes. A mismatch between prediction and reality analogous to that found for Phycobiliphyte exploitation of extreme shade environments is found when we examine the surface area of organism exposed to incident photons per mol chromophore. While many shade-adapted phytoplankters are small spheres up to a few tens of μm3 in volume or, if of larger volume, are cylinders of small radius or are flattened, there are also phytoplankters of shaded habitats with small projected areas per mol chromophore and hence with an inefficient use of light-harvesting machinery due to self-shading. It would appear that, while cost-benefit analyses of light-harvesting provide a partial answer to the problem of how microalgal cells can grow at very low photon flux densities, there are many exceptions to the generalisations which the cost-benefit analysis generates. These exceptions demand further study.

summary Genotypic variation in the temperature optimum for resource-saturated growth of microalgae has been used to provide envelopes of μm (maximum specific growth rate) as a function of temperature. The Q10 value for μm for batch-cultured algae with optimal growth temperatures in the range 5–40°C is 1.88; rather higher values (Q10= 2.08–2.19) are found, albeit with lower μ values at a given temperature, for continuous cultures. The envelope approach selects μ values for the smallest cells from the taxa (members of the Chlorophyta and Bacillariophyta) with the highest μ values at a given temperature. Larger cell size, or membership of the Dinophyta, gives a decreased μ at a given temperature. Phenotypic change in μ, within a given genotype grown at sub-optimal temperatures, has a Q10 in excess of 1.88. Analysis of constraints on the resource-saturated value of μ in the fastest-growing micro-algae suggest that, at their temperature optima, the cells are close (within a factor of 2) to their maximum potential growth rate, based on the known kinetic properties of their catalysts, the need for kinetic heterogenity in catalyses in metabolic pathways, and the need to allocate some cell resources to structural and storage components. Phenotypic and genotypic responses to lower temperatures for growth, in terms of reallocation of resources to increase the quantity per unit biomass of catalyst? as a means of offsetting lower catalytic capacity at lower temperatures, are limited. An exception is the light-harvesting and reaction centre apparatus which catalyses the temperature-insensitive processes of light absorption, excitation energy transfer and primary photochemistry, and which is present (as assayed by photosynthetic pigment per unit biomass) in smaller relative amounts during resource-saturated growth at lower temperatures. The involvement of other low-temperature ‘adaptations’ (e.g. homeoviscous behaviour of thylakoid membranes) in offsetting low temperature effects on catalytic rates is not clear. The scope for increasing the quantity of temperature-sensitive catalysts in the biomass as a means of offsetting the effects of low temperature on resource-saturated μ is potentially higher in the Dinophyta with their relatively low μ at their temperature optimum; however, this option does not appear to be taken up by the Dinophyta which have unexceptional Q10 values for μ. For resource-limited growth, the phenotypic effect of suboptimal temperatures on growth, when light is the limiting resource, is often less marked than when growth is light saturated. When a chemical nutrient is limiting, the temperature effect on growth of a given genotype is often, but not invariably, decreased. Cases in which the effect of temperature on growth rate is decreased under light-limiting conditions can be interpreted in terms of the intrinsically low Q10 of growth when temperature-insensitive reactions (light absorption, excitation energy transfer, primary photochemistry) are limiting and the acclimatory effects of changed temperature and light regimes for growth on resource allocation between pigment-protein complexes and downstream catalysts of temperature-sensitive reactions. Cases in which light-limited growth rate is quite temperature sensitive may be accounted for by a decrease in absorptance as a result of a lower pigment content per cell at low growth temperatures. For growth limited by chemical nutrients, the variable responses make analysis difficult. It is tempting to assign a low Q10 for μ under these conditions to a limitation by some transport process (diffusion through unstirred layers, or, less plausibly, the cell membrane) with a low Q10, although the evidence favouring this interpretation is not abundant.

SUMMARY Work previously published by the author shows that either cyclic photophosphorylation or oxidative phosphorylation can supply ATP to a variety of ATP-requiring processes in Hydrodictyon africanum. The rate at which the process occurs with ATP supplied from cyclic photophosphorylation is in some cases faster than that when ATP is supplied by oxidative phosphorylation; for other processes the reverse is true. The reasons for this variability in the rate at which the two ATP sources can support different ATP-requiring processes is discussed and it is suggested that regulation of the rate of ATP-requiring processes involves changes in the activity of the ATP-consuming reactions as well as in the ATP concentration (or energy charge). It is concluded that the in vivo capacity for cyclic photophosphorylation is at least as great as that for oxidative phosphorylation. Based on the capacity for respiratory electron flow in vivo, it is suggested that cyclic and oxidative phosphorylation each have capacities of some 20 μMoles (mg chl)−1 h−l. Based on the capacity for non-cyclic electron flow in vivo, it is suggested that the capacity for non-cyclic phosphorylation in vivo is 150 μMoles (mg chl)−1 h−1. Cyclic photophosphorylation saturates at an irradiance of about 1 J m−2 s−1, while non-cyclic photophosphorylation saturates at about 10 J m−2 s−1. These low figures for this shade-Actapted alga contrast with about ten- to twenty-fold higher values for capacity and light requirement for the various phosphorylation processes in sun-Actapted microalgae.