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Stomatal conductance (gs) impacts both photosynthesis and transpiration, and is therefore fundamental to the global carbon and water cycles, food production, and ecosystem services. Mathematical models provide the primary means of analysing this important leaf gas exchange parameter. A nearly universal assumption in such models is that the vapour pressure inside leaves (ei) remains saturated under all conditions. The validity of this assumption has not been well tested, because so far ei cannot be measured directly. Here, we test this assumption using a novel technique, based on coupled measurements of leaf gas exchange and the stable isotope compositions of CO2 and water vapour passing over the leaf. We applied this technique to mature individuals of two semiarid conifer species. In both species, ei routinely dropped below saturation when leaves were exposed to moderate to high air vapour pressure deficits. Typical values of relative humidity in the intercellular air spaces were as low 0.9 in Juniperus monosperma and 0.8 in Pinus edulis. These departures of ei from saturation caused significant biases in calculations of gs and the intercellular CO2 concentration. Our results refute the longstanding assumption of saturated vapour pressure in plant leaves under all conditions.

Concurrent with the trend of rising global average air temperature, there have been worldwide observations of a decline in pan evaporation over the last 30–50 years. This global phenomenon has since received much attention from the scientific community. Better interpretation of the long-term trend of pan evaporation (involving seasonal and inter-annual variations) requires rigorous experimental investigation of the physics of pan evaporation. To do that, we constructed an instrumented US Class A pan that replicated an operational pan at Canberra Airport in Australia. We subsequently monitored pan evaporation and associated meteorological variables at half-hourly intervals over a three-year period (Oct 2007–Jan 2010). Extending our earlier work on the aerodynamics of pan evaporation, we conducted a theoretical and experimental study on the energy balance of the pan under non-steady state conditions. The theory considers heat exchanges at both the pan water surface and the pan wall. We formulate the radiative balance based on the geometry of the system (the pan diameter, the height of the water level, the height of the rim and solar zenith angle) and account for differences in albedo and emissivity between the pan water surface and the pan wall. The theory is used to compute all relevant energy fluxes and thereby close the energy balance. Integration of the half-hourly fluxes to a daily basis showed that we were able to close the energy budget with a RMSE of ∼18 W m −2 or 7.5% of the net short-wave radiation. We find that evaporation from our pan is dominated (∼80%) by the radiative exchanges at the pan water surface; those at the pan wall are smaller but also important. Contrary to expectation, there was little if any sensible heat exchange across the pan wall. Instead we found that the major sensible heat flux occurs across the pan water surface. Importantly, evaporative cooling at the pan water surface can result in a transfer of sensible heat from the (warmer) air to the (cooler) surface of the water body. The sensible heat flux from the overlying air to the pan water surface is minimal at low evaporation rates, but at higher evaporation rates (e.g., >300 W m −2 ) it contributes the equivalent of up to 15% of the total evaporative flux.

In response to worldwide observations reporting a decline in pan evaporation over the last 30–50 years, we developed an instrumented US Class A pan that replicated an operational pan at Canberra Airport in Australia. The aim of the experimental facility was to investigate the physics of pan evaporation under non-steady state conditions. By monitoring the water level at 5-min intervals we were able to calculate the evaporation rate and thereby determine the short-term mass balance of the pan. Over the same time intervals, we also monitored (short- and long-wave) radiation, temperature (air, water surface, bulk water, inner and outer pan wall), atmospheric pressure as well as the air vapour pressure and the wind speed at a standard reference height (2 m above ground level). The experimental pan was operated for three years (2007–2010). In this paper, we develop a framework for quantifying vapour transfer by coupling Fick's First Law of Diffusion with boundary layer theory. This approach adequately represented pan evaporation measurements over short time intervals (half-hourly) under non-steady state conditions provided that surface temperature measurements, that account for the substantial cooling associated with evaporation, are available. It involved estimating the boundary layer thickness and other properties of air above the evaporating surface for a pan. Our results are consistent with the “envelope of theoretical curves” concept for the wind function introduced by Thom et al. (1981) .

In a greenhouse experiment, we grew maize plants at different densities. We added fertilizer to half of the pots and created a temperature gradient. After 10 weeks of plant growth, we measured soil CO2 efflux (SCE) and determined rhizosphere respiration (Rrhizo) and the decomposition rate of soil organic matter (RSOM) using the different δ13C of the C3 soil and C4 plants. Whereas Rrhizo remained stable across the temperature gradient, RSOM significantly increased with growth temperature. Neither plant density, nor the fertilizer treatment affected the relation between Rrhizo or RSOM and growth temperature. Although Rrhizo might still increase with temperature in the short term, long term exposure to higher temperatures revealed full thermal acclimation of Rrhizo, but not of RSOM.

Measurement of soil CO2 efflux using a non-flow-through steady-state (NFT-SS) chamber with alkali absorption of CO2 by soda lime was tested and compared with a flow-through non-steady-state (FT-NSS) IRGA method to assess suitability of using soda lime for field monitoring over large spatial scales and integrated over a day. Potential errors and artifacts associated with the soda lime chamber method were investigated and improvements made. The following issues relating to quantification and reliable measurement of soil CO2 efflux were evaluated: (i) absorption capacity of the soda lime, (ii) additional and thus artifactual absorption of CO2 by soda lime during the experimental procedure, (iii) variation in the CO2 concentration inside the chamber headspace, and (iv) effects of chamber closure on soil CO2 efflux. Soil CO2 efflux, as measured using soda lime (with a range of quantities: 50, 100, and 200 g per 0.082 m2 ground area enclosed in chamber), was compared with transient IRGA measurements as a reference method that is based on well-established physical principles, using several forms of spatial and temporal comparisons. Natural variation in efflux rates ranged from 2 to 5.5 g C m−2 day−1 between different chambers and over different days. A comparison of the IRGA-based assay with measurement based on soda lime yielded an overall correlation coefficient of 0.82. The slope of the regression line was not significantly different from the 1:1 line, and the intercept was not significantly different from the origin. This result indicated that measurement of CO2 efflux by soda lime absorption was quantitatively similar and unbiased in relation to the reference method. The soda lime method can be a highly practical method for field measurements if implemented with due care (in terms of drying and weighing soda lime, and in minimizing leakages), and validated for specific field conditions. A detailed protocol is presented for use of the soda lime method for measurement of CO2 efflux from field soils.

The effectiveness of several leaf water models (‘string-oflakes’, ‘desert river’ and the Farquhar‐Gan model) are evaluated in predicting the enrichment of leaf water along a maize leaf at different humidities. Progressive enrichment of both vein xylem water and leaf water was observed along the blade. At the tip, the maximum observed enrichment for the vein water was 17.6‰ at 50% relative humidity (RH) whereas that for the leaf water was 50‰ at 34% RH and 19‰ at 75% RH. The observed leaf water maximum was a fraction (0.5‐0.6) of the theoretically possible maximum. The ‘string-of-lakes’ and ‘desert river’ models predict well the variation of leaf water enrichment pattern with humidity but overestimate the average enrichment of bulk leaf water. However, the Farquhar‐Gan model gives good prediction for these two aspects of leaf water enrichment. Using the anatomical dimensions of vein xylem overestimates the effective longitudinal Peclet number ( P l ). Possible explanations for this discrepancy between the effective and the xylem-based estimate of P l are discussed. The need to characterize the heterogeneity of transpiration rate over the leaf surface in studies of leaf water enrichment is emphasized. The possibility that past atmospheric humidity can be predicted from the slope of the D 18 O spatial variation of leaf macrofossils found in middens is proposed.

Three leaf water models (two-pool model, Péclet effect, and string-of-lakes) were assessed for their robustness in predicting leaf water enrichment and its spatial heterogeneity. This was achieved by studying the 18O spatial patterns of vein xylem water, leaf water, and dry matter in cotton (Gossypium hirsutum) leaves grown at different humidities using new experimental approaches. Vein xylem water was collected from intact transpiring cotton leaves by pressurizing the roots in a pressure chamber, whereas the isotopic content of leaf water was determined without extracting it from fresh leaves with the aid of a purpose-designed leaf punch. Our results indicate that veins have a significant degree of lateral exchange with highly enriched leaf water. Vein xylem water is thus slightly, but progressively enriched in the direction of water flow. Leaf water enrichment is dependent on the relative distances from major veins, with water from the marginal and intercostal regions more enriched and that next to veins and near the leaf base more depleted than the Craig-Gordon modeled enrichment of water at the sites of evaporation. The spatial pattern of leaf water enrichment varies with humidity, as expected from the string-of-lakes model. This pattern is also reflected in leaf dry matter. All three models are realistic, but none could fully account for all of the facets of leaf water enrichment. Our findings acknowledge the presence of capacitance in the ground tissues of vein ribs and highlight the essential need to incorporate Péclet effects into the string-of-lakes model when applying it to leaves.

Theory suggests that the level of enrichment of18O above source water in plant organic material (Δ) may provide an integrative indicator of control of water loss. However, there are still gaps in our understanding of the processes affecting Δ. One such gap is the observed discrepancy between modeled enrichment of water at the sites of evaporation within the leaf and measured enrichment of the leaf water as a whole (ΔL). Farquhar and Lloyd (1993) suggested that this may be caused by a Péclet effect. It is also unclear whether organic material formed in the leaf reflects enrichment of water at the sites of evaporation within the leaf or ΔL. To investigate this question castor bean (Ricinus communis L.) leaves, still attached to the plant, were sealed into a controlled-environment gas exchange chamber and subjected to a step change in leaf-to-air vapor pressure difference. Sucrose was collected from a cut on the petiole of the leaf in the chamber under equilibrium conditions and every hour for 6 h after the change in leaf-to-air vapor pressure difference. Oxygen isotope composition of sucrose in the phloem sap (Δsuc) reflected modeled ΔL. A model is presented describing Δsuc at isotopic steady state, and accounts for 96% of variation in measured Δsuc. The data strongly support the Péclet effect theory.

Total and forest floor carbon dioxide flux densities (FCO2) and environmental variables were measured for 18 consecutive mid-summer days during July 1996 in a 215-year-old stand of Pinus sylvestris L. trees located 40 km southwest of the village of Zotino in central Siberia, Russia (61°N, 89°E, 160 m asl). Forest floor FCO2 was regulated by surface soil water content, related to the limited storage capacity of the sandy soil equivalent to only 4 mm water per 100 mm depth of soil. Following 12 mm rainfall, forest floor FCO2 increased by 52% to a maximum value of 4.1 μmol m−2 s−1. However, the rate had returned to the general lower level by the next day in response to rapid drying of the surface soil. There was little correspondence between forest floor FCO2 and the distributions of root and soil carbon or soil temperature. However, for soil samples returned to the laboratory, sieved to remove roots and re-watered, microbial respiration rate was positively and exponentially related to temperature. Measurements of forest floor FCO2 by eddy covariance were in good agreement with the chamber data during the daytime when the atmosphere was regularly mixed by turbulence. Micrometeorological flux measurements at the forest floor and above the trees showed how, on average, 77% of the carbon sequestered by tree canopy photosynthesis was lost to the atmosphere by root and soil microbial respiration during the observation period. On a daily basis, the boreal forest was generally a modest net sink (−75 mmol m−2 per day), but also a small carbon source on hot and dry days.

Cuticular properties affect the gas exchange of leaves, but little is known about how much CO2 and water vapor cross the cuticular barrier or whether low water potentials affect the process. Therefore, we measured the cuticular conductances for CO2 and water vapor in grape (Vitis vinifera L.) leaves having various water potentials. The lower leaf surface was sealed to force all gas exchange through the upper surface, which was stoma-free. In this condition both gases passed through the cuticle, and the CO2 conductance could be directly determined from the internal mole fraction of CO2 near the compensation point, the external mole fraction of CO2, and the CO2 flux. The cuticle allowed small amounts of CO2 and water vapor to pass through, indicating that gas exchange occurs in grape leaves no matter how tightly the stomata are closed. However, the CO2 conductance was only 5.7% of that for water vapor. This discrimination against CO2 markedly affected calculations of the mole fraction of CO2 in leaves as stomatal apertures decreased. When the leaf dehydrated, the cuticular conductance to water vapor decreased, and transpiration and assimilation diminished. This dehydration effect was largest when turgor decreased, which suggests that cuticular gas exchange may have been influenced by epidermal stretching.

A simple ‘big leaf’ ecosystem gas exchange model was developed, using eddy covariance data collected at an undisturbed tropical rainforest in south-western Amazonia (Brazil). The model used mechanistic equations of canopy biochemistry combined with an empirical stomatal model describing responses to light, temperature and humidity. After calibration, the model was driven using hourly data from a weather station at the top of the tower at the measurement site, yielding an estimate of gross primary productivity (annual photosynthesis) in 1992/1993 of about 200 mol C m−2 year −. Although incoming photon flux density emerged as the major control on photosynthesis in this forest, at a given PAR CO2 assimilation rates were higher in the mornings than in the afternoons. This was attributable to stomatal closure in the afternoon in response to increasing canopy-to-air vapour pressure differences. Although most morning gas exchange was clearly limited by the rate of electron transport, afternoon gas exchange was generally observed to be very nearly co-limited by both Rubisco activity (Vmax) and electron transport rate. The sensitivity of the model to changes in nitrogen allocation showed that the modelled ratio of Vmax to electron transport (Jmax) served nearly to maximize the annual carbon gain, and indeed, would have resulted in almost maximum annual carbon gain at the pre-industrial revolution atmospheric CO2 concentration of 27 Pa. Modelled gross primary productivity (GPP) was somewhat lower at 27 Pa, being about 160 mol C m−2 year−1. The model suggests that, in the absence of any negative feedbacks on GPP, future higher concentrations of atmospheric CO2 will continue to increase the GPP of this rainforest, up to about 230 mol C m−2 year−1 at 70 Pa.

Shrinkage of palisade cells during transpiration, previously measured for sclerophyllous leaves of Eucalyptus where cells shrank equally, was compared with shrinkage in thin mesophytic leaves of cotton (Gossypium hirsutum L.). Selected vapour pressure differences (Δe) from 0.6 to 2.7 kPa were imposed during steady-state photosynthesis and transpiration. Leaves were then cryo-fixed and cryo-planed paradermally, and images obtained with a cryo-scanning electron microscope (CSEM). Diameters of palisade ‘cavity cells’ within sub-stomatal cavities, and surrounding palisade ‘matrix cells’ were measured on CSEM images. Cavity and spongy mesophyll cells shrank progressively down to Δe = 2.7 kPa, while matrix cells remained at the same diameter at all Δe. Diameters were also measured of cavity and matrix cells quasi-equilibrated with relative humidities (RHs) from 100% to 86%. In leaves quasi-equilibrated with 95% RH, the cavity cells shrank so much as to be almost unmeasurable, while matrix cells shrank by only 6%. These data suggest that there are two distinct pools of water in cotton leaves: cavity plus spongy mesophyll cells (two-thirds of leaf volume) which easily lose water; and matrix cells (one-third of leaf volume), which retain turgor down to relative water loss = 0.4, providing structural rigidity to prevent wilting. This phenomenon is probably widespread among mesophytic leaves.

The oxygen isotope enrichment of bulk leaf water (Δb) was measured in cotton (Gossypium hirsutum) leaves to test the Craig-Gordon and Farquhar-Gan models under different environmental conditions. Δb increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (ea/ei) and to stomatal conductance (gs). The oxygen isotope enrichment of lamina water relative to source water \documentclass[10pt]{article} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{pmc} \usepackage[Euler]{upgreek} \pagestyle{empty} \oddsidemargin -1.0in \begin{document} \begin{equation*}({\bar {{\Delta}}}_{1}),\end{equation*}\end{document} which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. The Craig-Gordon model overestimated Δb (and \documentclass[10pt]{article} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{pmc} \usepackage[Euler]{upgreek} \pagestyle{empty} \oddsidemargin -1.0in \begin{document} \begin{equation*}{\bar {{\Delta}}}_{1}),\end{equation*}\end{document} as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Δb and \documentclass[10pt]{article} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{pmc} \usepackage[Euler]{upgreek} \pagestyle{empty} \oddsidemargin -1.0in \begin{document} \begin{equation*}{\bar {{\Delta}}}_{1}\end{equation*}\end{document} with an L of 7.9 mm, much less than the total radial effective length Lr of 43 mm. The fitted values of L for \documentclass[10pt]{article} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{pmc} \usepackage[Euler]{upgreek} \pagestyle{empty} \oddsidemargin -1.0in \begin{document} \begin{equation*}{\bar {{\Delta}}}_{1}\end{equation*}\end{document} of individual leaves showed little dependence on VPd and temperature, supporting the assumption that the Farquhar-Gan formulation is relevant and useful in describing leaf water isotopic enrichment.

The bacterial cytokinin biosynthesis gene ipt under control of a chalcone synthase promoter (PCHS) was introduced into potato (Solanum tuberosum L.). Two transgenic lines were selected for detailed study, because in these, root development was reduced only moderately, thus, enabling the plants to be grown in pots. Expression of the PCHS-ipt gene elevated the level of zeatin cytokinins markedly in the apical bud, subapical stems and leaves. The transgenic (IPT) plants exhibited a lower and denser leaf canopy relative to wild-type (WT) plants owing to reduction in main stem length, increase in node number per stem and promotion of lateral shoot development. Main stem diameter was increased markedly due to promotion of cell division associated with activation of cyclin-dependent kinase in the subapical stem. Expression of the PCHS-ipt gene induced aerial stolons, promoted growth of underground stolons and increased tuber number but reduced tuber weight and nitrogen content. The gene expression also increased pinnae and pinnule number per leaf, increased thickness of pinnae and promoted transpiration, photosynthesis and stomatal conductance – effects monitored by gas exchange and 18O and 13C analysis. The elevation of [CO2] to 900 μmol mol–1 promoted growth of both WT and IPT plants, ameliorated the negative effect of high cytokinin on tuber weight and interacted additively with ipt gene expression to promote stem growth.

Pretreatment of maize (Zea mays L.) seeds withCaCl2 solution significantly raised calcium andcalmodulin levels and enhanced intrinsic heat tolerance in maize seedlings.This enhancement is specific to Ca2+ and cannot besubstituted by Mg2+. In contrast, treatments with theCa2+ chelator EGTA or plasma membraneCa2+ channel blockers La3+ orverapamil lowered the intrinsic heat tolerance. Concurrent treatment withCa2+ and Ca2+ -channel blockers(La3+, verapamil) also decreased theCa2+ -enhanced heat tolerance. In addition, calmodulinantagonists chlorpromazine or W7 treatment eliminated theCa2+ -enhanced heat tolerance, whilst W5, a less activeanalogue of W7, had little effect on the Ca2+ -enhancedheat tolerance. Measurement of antioxidant enzyme activities and lipidperoxidation showed that heat stress induced an oxidative stress in maizeseedlings. External Ca2+ treatment enabled the seedlingsto keep relatively higher activities of superoxide dismutase (SOD), catalase(CAT) and ascorbate peroxidase (APX) and lower levels of lipid peroxidationthan Ca2+ -deficient treatments; on the contrary, EGTAtreatment led to more rapid loss of SOD, CAT and APX activities and higherlevels of lipid peroxidation in the seedlings under heat stress. In addition,concurrent Ca2+ and W7 treatment weakened the effects ofCa2+ treatment on SOD, CAT and APX activities and causedmore severe lipid peroxidation. These results suggested that externalCa2+ can enhance the intrinsic heat tolerance of maizeseedlings. This enhancement requires the entry of externalCa2+ into cells across plasma membranes and themediation of intracellular calmodulin, and is associated with the increase ofantioxidant system activity.

In order to understand the factors influencing the isotopic composition of air above and within plant canopies, equations quantifying the effects of photosynthesis, respiration and turbulent transport on the isotopic composition of the surrounding CO2 are developed. These equations are then extended to the regional scale, allowing the average isotopic composition of CO2 within the convective boundary layer to be related to the isotopic composition of tropospheric CO2, and to isotopic fractionations during ecosystem carbon exchange. Equations presented have the potential to be inverted, allowing direct estimation of isotopic fractionations by vegetation at the local and regional scales. Equations allowing the estimation of the extent of refixation of respired CO2 ('recycling') at the regional scale are also presented. Using measurements of CO2 carbon isotopic composition in conjunction with ecosystem flux measurements, the theory is applied to a tropical rain forest in Amazonia and a boreal forest in Siberia. When examined on a ground area basis and over the course of a day it is observed that, by virtue of greater fluxes but similar isotopic fractionations, the tropical rainforest exerts much more influence over the isotopic composition of the surrounding air than does the boreal forest. Due to higher rates of ecosystem respiration, recycling of respired CO2 is modelled to be much greater for tropical rainforest, but values presented here are considerably lower than previously published estimates, the latter being based solely on the relationship between the isotopic composition and concentrations of CO2 within forest canopies. The reasons for these differences are examined.

Rates of CO 2 assimilation and leaf conductances to CO 2 transfer were measured in plants of Zea mays during a period of 14 days in which the plants were not rewatered, and leaf water potential decreased from −0.5 to −8.0 bar. At any given ambient partial pressure of CO 2 , water stress reduced rate of assimilation and leaf conductance similarly, so that intercellular partial pressure of CO 2 remained almost constant. At normal ambient partial pressure of CO 2 , the intercellular partial pressure of CO 2 was estimated to be 95 microbars. This is the same as had been estimated in plants of Zea mays grown with various levels of nitrogen supply, phosphate supply and irradiance, and in plants of Zea mays examined at different irradiances. After leaves of Phaseolus vulgaris L. and Eucalyptus pauciflora Sieb. ex Spreng had been exposed to high irradiance in an atmosphere of CO 2 -free N 2 with 10 millibars O 2 , rates of assimilation and leaf conductances measured in standard conditions had decreased in similar proportions, so that intercellular partial pressure of CO 2 remained almost unchanged. As the conductance of each epidermis that had not been directly irradiated had declined as much as that in the opposite, irradiated surface it was hypothesized that conductance may have been influenced by photoinhibition within the mesophyll tissue.

Carbon isotope discrimination (Delta) was analyzed in leaf starch and soluble sugars, which represent most of the recently fixed carbon. Plants of three C(3) species (Populus nigra L. x P. deltoides Marsh., Gossypium hirsutum L. and Phaseolus vulgaris L.) were kept in the dark for 24 hours to decrease contents of starch and sugar in leaves. Then gas exchange measurements were made with constant conditions for 8 hours, and subsequently starch and soluble sugars were extracted for analysis of carbon isotope composition. The ratio of intercellular, p(i), and atmospheric, p(a), partial pressures of CO(2), was calculated from gas exchange measurements, integrated over time and weighted by assimilation rate, for comparison with the carbon isotope ratios in soluble sugars and starch. Carbon isotope discrimination in soluble sugars correlated strongly (r = 0.93) with p(i)/p(a) in all species, as did Delta in leaf starch (r = 0.84). Starch was found to contain significantly more (13)C than soluble sugar, and possible explanations are discussed. The strong correlation found between Delta and p(i)/p(a) suggests that carbon isotope analysis in leaf starch and soluble sugars may be used for monitoring, indirectly, the average of p(i)/p(a) weighted by CO(2) assimilation rate, over a day. Because p(i)/p(a) has a negative correlation with transpiration efficiency (mol CO(2)/mol H(2)O) of isolated plants, Delta in starch and sugars may be used to predict differences in this efficiency. This new method may be useful in ecophysiological studies and in selection for improved transpiration efficiency in breeding programs for C(3) species.

Plants of Zea mays were grown with different concentrations of nitrate (0.6, 4, 12, and 24 millimolar) and phosphate (0.04, 0.13, 0.53, and 1.33 millimolar) supplied to the roots, photon flux densities (0.12, 0.5, and 2 millimoles per square meter per second), and ambient partial pressures of CO(2) (305 and 610 microbars). Differences in mineral nutrition and irradiance led to a large variation in rate of CO(2) assimilation per unit leaf area (A, 11 to 58 micromoles per square meter per second) when measured under standard conditions. The variation was shown, with the plants that had received different amounts of nitrate, to be related to variations in the nitrogen and chlorophyll contents, and phosphoenolpyruvate and ribulose-1,5-bisphosphate carboxylase activities per unit leaf area. Irrespective of growth treatment, A and leaf conductance to CO(2) transfer (g), measured under standard conditions were in almost constant proportion, implying that intercellular partial pressure of CO(2) (p(i)), was almost constant at 95 microbars. The same proportionality was maintained as A and g increased in an initially nitrogen-deficient plant that had been supplied with abundant nitrate. It was shown that p(i) measured at a given ambient partial pressure was not affected by the ambient partial pressure at which the plants had been grown, although it was different when measured at different ambient partial pressures. This suggests that the close coupling between A and g in these experiments is not associated with sensitivity of stomata to change in p(i).Similar, though less comprehensive, experiments were done with Gossypium hirsutum, and yielded similar conclusions, except that the proportionality between A and g at normal ambient partial pressure of CO(2) implied P(i) approximately 200 microbars.

Cotton and maize plants were grown under full sunlight in glass houses containing normal ambient partial pressure of CO2 (330±20 μbar) and enriched partial pressure of CO2 (640 ±15 μbar) with four levels of nitrogen nutrient. In 40 day old cotton plants grown in high CO2, there was a 2-fold increase in day weight and a 1.6-fold increase in leaf area compared with plants grown in ambient CO2. In 30 day old maize plants there was only 20% increase in dry weight in plants grown in 640 μbar CO2 compared with plants grown in 330 μbar and no significant increase in leaf area. In both species, at both CO2 treatments, dry weight and leaf area decreased in similar proportion with decreased nitrogen nutrient. The increase of leaf area in cotton plants at high CO2 caused a reduction of total nitrogen on a dry weight basis. In cotton assimilation rate increased 1.5 fold when plants were grown with high nitrogen and high CO2. The increase was less at lower levels of nitrate nutrient. There was a 1.2 fold increase in assimilation rate in maize grown at high CO2 with high nitrate nutrient. Cotton and maize grown in high CO2 had a lower assimilation rate in ambient CO2 compared to plants grown in normal ambient air. This difference was due to the reduction in RuBP carboxylase activity. Water use efficiency was doubled in both cotton and maize plants grown at high CO2 in all nutrient treatments. However, this increase in water use efficiency was due primarily to reduced transpiration in some treatments and to increased assimilation in others. These data show that plant responses to elevated atmospheric partial pressure of CO2 depend on complex of partially compensatory processes which are not readily predictable.

Measurements of evaporation and net assimilation rate of irrigated wheat at Griffith, New South Wales, were made during 73 days from full cover to physiological maturity. Micrometeorological determinations of flux densities of water vapour and CO 2 were based on the energy balance or Bowen ratio technique. Complementary measurements involving weighing lysimetry, soil water content and plant dry matter indicated that crop water use and growth could be explained in terms of a series of hourly values of these fluxes. Daily rates of evaporation increased ∼3-fold to 10 mm day −1 during the study period. Estimates of potential evaporation consistently underestimated measured evaporation until the final 20 days, when declining soil water and crop senescence reduced evaporation rates. Net daytime assimilation rates of CO 2 were nearly constant with time prior to anthesis, with day-to-day variability being an order of magnitude less than that for evaporation rate. Both daily and peak rates of net assimilation were generally greater than those reported for dryland wheat in southern Australia, but were comparable with those for irrigated sunflower. During the post-anthesis phase, a progressive decline in rate was observed. The initial decline was attributed to crop lodging, but later reductions are thought to be due to the combined effect of soil water depletion and crop sensecence. A loss in water use efficiency of 24% from the pre- to post-anthesis stages was determined from harvests of standing crop material near anthesis and at physiological maturity. The ratio of hourly evaporation to net assimulation was positively correlated with vapour pressure deficit to explain, in part, the observed reduction as a response to decreasing relative humidity with progression into the reproductive stage. Lodging and senescence of the crop were identified as additional factors which depressed water-use efficiency during this stage. An improved understanding of the dynamics of water-use efficiency requires further development in measurement to partition fluxes of gaseous exchange between plant and soil.

When photon flux density incident on attached leaves of Zea mays L. was varied from the equivalent of 0.12 of full sunlight to full sunlight, leaf conductance to CO 2 transfer, g , changed in proportion to the change in rate of CO 2 , assimilation, A , with the result that intercellular partial pressure of CO 2 remained almost constant. The proportionality was the same as that previously found in g and A measured at one photon flux density in plants of Zea mays L. grown at different levels of mineral nutrition, light intensities, and ambient partial pressures of CO 2 . In shade-grown Phaseolus vulgaris L. plants, A as photon flux density was increased from about 0.12 up to about 0.5 full sunlight, the proportionality being almost the same in plants grown at low and at high light intensity. When photon flux density incident on the adaxial and abaxial surfaces of the isolateral leaves of Eucalyptus pauciflora Sieb. ex Spreng was varied, g and A also varied proportionally. The leaf conductance in a particular surface was affected by the photon flux density at the opposite surface to a greater extent than was expected on the basis of transmittance. The results indicated that stomata may, in some way, be sensitive to the photon flux absorbed within the leaf as a whole.

Ultrastructural and physiological characteristics of the C3-C4 intermediate Neurachne minor S. T. Blake (Poaceae) are compared with those of C3 and C4 relatives, and C3-C4Panicum milioides Nees ex Trin. N. minor consistently exhibits very low CO2 compensation points (τ: 1.0, usually 0.3–0.6 Pa) yet has C3-like δ13C values. CO2 assimilation rates (A) respond like those of C3 plants to a decrease in O2 partial pressure (2 × 104–1.9 × 103 Pa) at ambient CO2 levels, but this response is progressively attenuated until negligible at very low CO2. By contrast, other species of the Neurachneae are clearly either C4 (two spp.) or C3 (seven spp.). For plants grown and measured at different photon flux densities (PFDs), τ for N. minor and P. milioides increases from 0.5 to 1.0, and from 1.0 to 2.1 Pa, respectively, as PFD is decreased from 1860 to 460 μmol m−2s−1. In N. minor, the O2 response of τ is either biphasic as in P. milioides, but much diminished and with a higher transition point, or is very C4-like. As in C4 relatives, inner sheath cells contain numerous chloroplasts. Their walls possess a suberized lamella, which may make them more CO2-tight than bundle sheath cells of P. milioides, contributing to the almost C4-like τ characteristics of N. minor. The biochemical basis of C3-C4 intermediacy is considered.

Rates of CO(2) assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO(2) in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25 degrees C. Electron transport activity for CO(2) assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O(2), and between fluorescence and rates of CO(2) assimilation at 19 millibars but not 200 millibars O(2). The values of fluorescence measured at about 220 microbars intercellular CO(2) were not greatly affected by increasing O(2) from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO(2) partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO(2) partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO(2) partial pressures which reversed to a positive correlation at higher values of CO(2) partial pressures. Similar results were observed in the C(3) species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C(4) species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO(2) assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q(E)) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.

Previous studies on the Physiology of stomata in higher plants suggest that stomata influence the rate of CO2 fixation in leaf mesophyll tissue. We believe that an equally important stomatal function has not been fully recognised; that stomatal aperture is determined by the capacity of the mesophyll tissue to fix carbon. We altered the capacity of leaves to fix carbon by various means, and found invariably that the diffusive conductance of the epidermis to CO2 transfer, g, (which mainly depends on the number and dimensions of the stomata) changes in nearly the same proportion as the rate of assimilation of CO2. Thus, the intercellular concentration of CO2 (ci), calculated as ci = ca–A/g (where ca is ambient concentration of CO2, A is assimilation rate of CO2), tends to remain constant providing ca is kept constant. We used routine techniques1 to measure A and estimate g in leaves placed singly in chambers. Conductance takes account of CO2 transfer through both stomata and leaf boundary layer, the conductance of the latter being 0.5 mol m−2 s−1.

Nitrogen metabolism was examined in senescent flag leaves of 90- to 93-day-old wheat (Triticum aestivum L. cv Yecora 70) plants. CO(2) assimilation and the levels of protein, chlorophyll, and nitrogen in the leaves decreased with age. Glutamine synthetase activity decreased to one-eighth of the level in young flag leaves. Detached leaves were incubated (with the cut base) in (15)N-labeled NH(3), glutamate, or glycine in the light (1.8 millieinstein per square meter per second) at 25 degrees C in an open gas exchange system under normal atmospheric conditions for up to 135 minutes. The (15)N-enrichment of various amino acids derived from these (15)N-substrates were examined. The amido-N of glutamine was the first (15)N-labeled product in leaves incubated with (15)NH(4)Cl whereas serine, closely followed by the amido- and amino-N of glutamine, were the most highly (15)N-labeled products during incubation with [(15)N]glycine. In contrast, aspartate and alanine were the first (15)N-labeled products when [(15)N] glutamate was used. These results indicate that NH(3) was assimilated via glutamine synthetase and glutamate synthase activities and the photorespiratory nitrogen cycle remained functional in these senescent wheat flag leaves. In contrast, an involvement of glutamate dehydrogenase in the assimilation of ammonia could not be detected in these tissues.

The effects of abscisic acid (ABA) on photosynthesis in leaves of Helianthusannuus L. were compared with those in leaves of Viciafaba L. After the ABA treatment, the response of photo­ synthetic CO2 assimilation rate, A, to calculated intercellular partial pressure of CO2 , Pi' (A(Pi) relationship) was markedly depressed in H. annuus. A less marked depression was also observed in Vifaba. However, when the abaxial epidermes were removed from these leaves, neither the maximum rate nor the CO 2 response of photosynthetic oxygen evolution was affected by the ap­ plication of ABA. Starch-iodine tests revealed that photosynthesis was not uniform over the leaves of H. an­ nuus treated with ABA. The starch content was diffferent in each bundle sheath extension com­ partment (the smallest subdivision of mesophyll by veins with bundle sheath extensions, hav­ ing an area of ca. 0.25 mrrr' and ca. 50 stomata). In some compartments, no starch was detected. The distribution of open stomata, examined using the silicone' rubber impression tech­ niques, was similar to the pattern of starch accumulation. In Vifaba leaves, which lack bundle sheath extensions, distribution of starch was more homogeneous. These results indicate that the apparent non-stomatal inhibition of photosynthesis by ABA deduced from the depression of A(Pi) relationship is an artifact which can be attributed to the non-uniform distribution of transpiration and photosynthesis over the leaf. Intercellular gaseous environment in the ABA-treated leaves is discussed in relation to mesophyll anatomy.

Cotton plants were grown in late spring under full sunlight in glasshouses containing normal ambient partial pressure of CO2 (32±2Pa) and enriched partial pressure of CO2 (64±1.5Pa) and at four levels of nitrogen nutrition. Thirty-five days after planting, the total dry weights of high CO2-grown plants were 2- to 3.5-fold greater than plants grown in normal ambient CO2 partial pressure. Depending on nitrogen nutrition level, non-structural carbohydrate content (mainly starch) in the leaves of plants grown in normal CO2 was between 4 and 37% of the total leaf dry weight compared to 39 to 52% in the leaves of high CO2-grown plants. Specific leaf weight calculated using total dry weight was 1.6- to 2-fold greater than that based on structural dry weight. In high CO2-grown plants the amount of non-structural carbohydrate translocated from the leaves at night was between 10 and 20% of the level at the end of the photoperiod. This suggests that the plant was unable to utilize all the carbohydrate it assimilated in elevated CO2 atmosphere. While there was a 1.5-fold enhancement in the rate of CO2 assimilation in plants grown in 64 Pa CO2, there was, however, some evidence to suggest that the activities of other metabolic pathways in the plants were not stimulated to the same extent by the enriched CO2 atmosphere. This resulted in massive accumulation of non-structural carbohydrate, particularly at low level of nitrogen nutrition.