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summary Clusia rosea Jacq. is abundant in the moist parts of the Caribbean island of St John (US Virgin Islands, Lesser Antilles) but relatively rare along the dry south coast. Three types of seedlings were encountered, terrestrial seedlings, seedlings growing as humus-epiphytes on other trees, and seedlings growing inside the tanks of the bromeliad Aechmea lingulata (L.) Baker. Free-living trees grow from terrestrial seedlings or from epiphytic seedlings strangling and shading their host trees. Leaf-Na+ levels were always low (1–4 mequiv I−1 tissue water); trees close to the shore were not affected by salinity. In leaves of mature C. rosea trees, levels of Ca2+, Mg2+ and K+ were about 60–90, 40–50, 45–55 mequiv I−1 tissue water, respectively. Epiphytic seedlings tended to contain lower levels of these inorganic cations than seedlings growing terrestrially or in the tanks of Ae. lingulata. Epiphytic seedlings contained significantly less nitrogen than terrestrial seedlings. In the leaves of mature trees N-levels were independent of altitude and location on the island, but shaded leaves had significantly higher N-levels than exposed leaves. Light compensation point of photosynthesis in epiphytic seedlings performing C3-photosynthesis was 17–5 (μmol photons m−2 s−1), photosynthesis was saturated at about 300μmol photons m−2 s−1 showing a maximum rate of CO2-uptake of 2–3 μmol m−2 s−1.

International audience; As a measure of frost hardiness, we determined an index of injury (I-30) in living bark tissue of 20-year-old pedunculate oaks (Quercus robur L.) that had been manually and almost completely defoliated in the first half of May of one or two years, and of non-defoliated control trees. I-30 was calculated as a percentage value on the basis of electrolyte leakage from samples artificially frozen at a temperature of -30 °C, and from unfrozen control samples. In parallel, the bark's concentrations of soluble sugars, of nitrogen and of quercitol, a cyclic polyol, were measured. Repeated defoliation significantly reduced the frost hardiness of the bark as well as its concentrations of raffinose, stachyose, nitrogen and quercitol. The I-30 values were correlated with the total concentration of soluble sugars and with the concentrations of the individual sugar compounds, but not with the quercitol concentration. Less tight, yet significant correlations were obtained between I-30 and nitrogen concentrations. We conclude that repeated defoliation decreases the bark's capability to acclimatize to winter frost due to a reduction in the concentrations of soluble sugars, particularly those of raffinose and stachyose.; Effets de la défoliation sur la résistance au gel et les concentrations de sucres solubles et de cyclitols dans le liber de chênes pédonculés (Quercus robur L.). Comme mesure de la résistance au gel du liber vivant de chênes pédonculés âgés de vingt ans et ayant déjà été défoliés manuellement presque entièrement dans la première moitié du mai d'une ou deux années, on a calculé un index de dommage (I-30) et on a comparé avec la valeur correspondante d'arbres témoins non défoliés. On a déterminé I-30 comme une valeur de pourcentage sur la base de perte d'électrolytes des échantillons congelés artificiellement à -30 °C, et des échantillons pas congelés. Parallèlement on a mesuré les concentrations de sucres solubles, d'azote et de quercétol, un polyalcool cyclique, du liber. La deuxième défoliation a réduit de manière significative tant la résistance au gel du liber que ses concentrations en raffinose, stachyose, azote et quercétol. Les indices de dommage (I-30) étaient en corrélation avec la concentration totale de sucres solubles et avec les concentrations de sucres individuels, mais pas avec la concentration de quercétol. Des corrélations moins étroites mais toutefois significatives ont été mises en évidence entre les valeurs I-30 et les concentrations d'azote. Ces résultats nous permettent de conclure qu'une défoliation répétée réduit la capacité d'acclimatation du liber aux gels d'hiver en raison d'une diminution des concentrations de sucres solubles, surtout de raffinose et de stachyose.

Low temperature driven carbon shortage is often assumed to explain slow growth and treeline formation at high elevations. To test this hypothesis, we analysed mobile carbon pools in Pinus cembra across the treeline ecotone in the Swiss Alps. Concentrations of non-structural carbohydrates (NSC) in needles, branches, stems and roots, as well as lipids (acylglycerols) in all woody tissues were measured throughout the growing season. Starch was the most prominent non-structural carbon compound in needles, whereas lipids represented 50–75% of the mobile carbon compounds in wood. The relative seasonal variation of the lipid fraction was very small, but due to the high absolute amount of lipids, the annual variability of carbon in lipids exceeded that of NSC in woody tissues. Mobile carbon compounds were highly abundant throughout the year and were never significantly depleted. Across a 110 m altitudinal transect from timberline to the uppermost site of tree existence, NSC and lipid concentrations generally increased. This trend became even more pronounced when the increasing structural density of tissues at higher elevations was accounted for. An estimation of the whole tree mobile carbon concentration (fraction of mobile carbon compounds within the whole tree biomass) also revealed an increasing trend of NSC and lipid pools with elevation. We therefore conclude that carbon limitation is unlikely to be responsible for reduced tree growth at the alpine treeline studied. Increased concentrations of NSC and lipids at the upper tree limit rather suggest that sink activity is limited. Hence, treeline formation is most likely the result of a direct thermal restriction of tissue formation (investment in structures) under otherwise sufficient carbon assimilation during the growing season.

The drought-tolerant tree species Ziziphus mauritiana Lamk. and Z. rotundifolia Lamk. were shown to have similar high mucilage concentrations (7-10% dry weight) in their leaves, with large numbers of mucilage-containing cells in the upper epidermis and extracellular mucilage-containing cavities in the leaf veins and stem cortex. The main sugar constituents of the water-soluble mucilage extract were rhamnose, glucose and galactose. During drought-stress in two independent studies, foliar mucilage content was unaffected in both species, but glucose and starch contents declined significantly in crude mucilage extracts from droughted leaves. Enzymatic hydrolysis of the mucilage extract using alpha-amylase and amyloglucosidase released glucose, indicating that a mucilage-associated water-soluble glucan, with alpha-1,4- and alpha-1,6-linkages, may exist which was extracted together with the mucilage. From the current data, it is not possible to localize the glucan to determine whether or not it is associated with mucilage-containing cells. Data from pressure-volume analyses of drought-stressed and control leaves showed that, in line with their similar mucilage contents, the relative leaf capacitance isotherm (change in relative water content per unit change in water potential) was similar in both species. During drought-stress, reduced relative capacitance resulted from osmotic adjustment and decreased wall elasticity. Data suggest that in Ziziphus leaves, intracellular mucilages play no part in buffering leaf water status during progressive drought. In Ziziphus species, growing in environments with erratic rainfall, the primary role of foliar mucilage and glucans, rather than as hydraulic capacitors, may be as sources for the remobilization of solutes for osmotic adjustment, thus enabling more effective water uptake and assimilate redistribution into roots and stems prior to defoliation as the drought-stress intensified.

Seasonal changes in the activity of phosphoenolpyruvate carboxylase (PEPCase, EC 4.1.1.31), a key enzyme in the interaction of carbohydrate and nitrogen metabolism, were studied in leaves of the C3 semiparasitic mistletoe, Viscum album, growing on different host trees. Maximum extractable PEPCase activities were higher in leaves of mistletoes growing on Betula pendula and Alnus glutinosa hosts compared with those on the conifers, Abies alba and Larix decidua. Independent of host, maximum extractable PEPCase activities were high in spring and autumn while low in summer. Samples with higher PEPCase activities showed higher amounts of PEPCase protein and higher PEPCase mRNA levels. A curvilinear correlation between leaf total nitrogen content and the maximum extractable PEPCase activity as well as PEPCase mRNA level suggested that nitrogen might affect the activity of PEPCase of mistletoe by up-regulating gene expression. In addition to extractable activity, seasonal changes of the PEPCase activation state, the ratio of activities resulting from limited:non-limited assays, were found, which was correlated to the variation of malate content in leaves of mistletoe. ATP-dependent activation of PEPCase was characterized by an increase in I0.5(L-malate), indicating that PEPCase of leaves of mistletoes is probably regulated via phosphorylation.

Summary As part of an ongoing project to understand the effects of elevated atmospheric CO 2 on plants in complex, tropical communities, we studied biomass accumulation in a simplified model seedling community consisting of two species of tropical trees ( Ficus insipida , a fast growing pioneer species, and Virola surinamensis , a slow-growing, shade-tolerant late successional species). The plants were grown at ambient and elevated (about two times ambient) CO 2 concentrations using open-top chambers at a field site in Panama. Communities grew in heavily fertilized soil. Compared to a previous experiment with model communities of F. insipia and V. surinamensis grown on unfertilized soil (Winter et al., Flora [2000] 195, 289) application of soil fertilizer markedly accelerated community growth rates at ambient CO 2 , and biomass accumulation was enhanced by an additional 52% at elevated CO 2 . In contrast, elevated CO 2 had no significant effect on biomass accumulation in unfertilized communities. Communities growing on fertilized soil showed greater biomass allocation into leaves, i.e. higher leaf weight ratios (LWRs) than did communities on unfertilized soil. Higher LWRs were related to lower root: shoot ratios and together with greater specific leaf areas (area per unit leaf mass), largely a consequence of lower leaf starch contents, resulted in higher leaf area ratios (LARs). While elevated CO 2 caused the relatively low LARs in unfertilized communities to decrease further, by strongly increasing leaf starch levels and decreasing specific leaf areas, these leaf characteristics changed only slightly in fertilized communities exposed to elevated CO 2 . Thus, by maintaining relatively high LARs at elevated CO 2 , fertilized plants were able to effectively use enhanced CO 2 concentrations for increased carbon gain and growth. Leaves of plants on fertilized soil were characterized by relatively low C: N ratios which were largely unaffected by CO 2 concentration. In contrast, C: N ratios in leaves of unfertilized plants were higher than those of fertilized plants and increased in response to elevated CO 2 .

Summary Biomass accumulation of model seedling communities of two tropical tree species was studied at ambient and elevated CO 2 levels (four replicates per CO 2 concentration), using open-top chambers situated in a cleared area at the edge of a tropical forest near Panama City, Republic of Panama. Each chamber (diameter about 2 m) contained a mixture of 18 plants of Ficus insipida , a fast-growing pioneer species, and 18 plants of Virola surinamensis , a slow-growing late-successional species. Plants grew in well-drained, well-watered, loose, unfertilized soil. During the 30 week experimental period, biomass accumulation by F. insipida , which increased in height from about 20 to 180 cm, predominated, whereas V. surinamensis grew only slowly in the strongly shaded understorey. Neither community biomass accumulation (above- plus belowground) nor the biomass ratio ( F. insipida: V. surinamensis ) of the two species was significantly affected by elevated CO 2 , although community biomass accumulation was on average slightly higher at elevated than at ambient CO 2 , and the biomass of F. insipida increased on average relative to V. surinamensis at elevated CO 2 . Plants grown at elevated CO 2 contained greater levels of starch and lower levels of nitrogen per unit leaf dry mass, and, therefore, showed higher ratios of C: N than plants at ambient CO 2 . Specific leaf area, the area per unit leaf dry mass; and the leaf area ratio (LAR), the total leaf area per total plant dry mass, decreased in response to elevated CO 2 . This decrease in LAR counteracted the potential effect on biomass gain of increases in both net assimilation rate (the rate of dry mass accumulation per unit leaf area) and rates of photosynthetic leaf net CO 2 uptake at elevated as compared to ambient CO 2 , particularly in Ficus insipida .

Summary Intact plants and cells (photoautotrophic, photomixotrophic, heterotrophic) in suspension culture of Cicer arietinum L. were compared for their ability to synthesize and accumulate O -methyl-inositols. In intact plants (leaves, shoot axis, roots), pinitol (1D-3- O -methyl- chiro -inositol) was the major cyclitol of the polyol fraction and it was accumulated in response to drought stress. Withholding water from the plants for 14 days caused an increase in pinitol from 5 to 195 mmol kg -1 dry matter in the leaves, from 9 to 67 mmol kg -1 dry matter in the shoot axis and from 5 to 22 mmol kg -1 dry matter in the roots, which was far beyond the levels in controls of the same age. In contrast to the intact plants pinitol was not detectable in most of the suspension cultured cells. It was only present in small amounts ( -1 dry matter) in photoautotrophic cells when cells were stressed with either 100 mol m -3 NaCl or 200 mol m -3 sorbitol. The precursor of pinitol, ononitol (1D-4- O -methyl- myo -inositol), appeared in traces under control conditions in photomixotrophic cells, but was accumulated up to 27 mmol kg -1 dry matter under salinity conditions (100 mol m -3 NaCl). No methylated cyclitol was detectable in heterotrophic cells. However, when photoautotrophic cells were reverted to heterotrophic conditions they maintained the ability to synthesize ononitol for more than 6 months. The presented data provide evidence that O-methyl-inositol synthesis can be induced on the cellular level. Photosynthetic activity of the cells is not a necessary prerequisite for accumulation of methylated polyols.

The foliar content of nitrogen and the relative abundances of 13C and 15N were analysed in vascular epiphytes collected from six sites along an altitudinal gradient from tropical dry forests to humid montane forests in eastern Mexico. The proportion of epiphyte species showing crassulacean acid metabolism (CAM) (atmospheric bromeliads, thick-leaved orchids, Cactaceae, and Crassulaceae) decreased with increasing elevation and precipitation from 58 to 6%. Atmospheric bromeliads, almost all of which had δ13C values indicating CAM, were more depleted in 15N (x=−10·9‰± 2·11) than the C3 bromeliads which form water-storing tanks (−6·05‰± 2·26). As there was no difference in δ15N values between C3 and CAM orchids, the difference in bromeliads was not related to photosynthetic pathways but to different nitrogen sources. While epiphytes with strong 15N depletion appear to obtain their nitrogen mainly from direct atmospheric deposition, others have access to nitrogen in intercepted water and from organic matter decomposing on branches and in their phytotelmata. Bromeliads and succulent orchids had a lower foliar nitrogen content than thin-leaved orchids, ferns and Piperaceae. Ground-rooted hemi-epiphytes exhibited the highest nitrogen contents and δ15N values.

Mature trees have already experienced substantial increases in CO2 concentrations during their lifetimes, and will experience continuing increases in the future. Small open-top chambers were used to enclose branchlets that were at a height of between 20 and 25 m in the canopy of the tree species Luehea seemannii Tr. & Planch. in a tropical forest in Panama. Elevated concentrations of CO2 increased the rate of photosynthetic carbon fixation and decreased stomatal conductance of leaves, but did not influence the growth of leaf area per chamber, the production of flower buds and fruit nor the concentration of nonstructural carbohydrates within leaves. The production of flower buds was highly correlated with the leaf area produced in the second flush of leaves, indicating that the branchlets of mature trees of Luehea seemannii are autonomous to a considerable extent. Elevated levels of CO2 did increase the concentration of nonstructural carbohydrates in woody stem tissue. Elevated CO2 concentration also they increased the ratio of leaf area to total biomass of branchlets, and tended to reduce individual fruit weight. These data suggest that the biomass allocation patterns of mature trees may change under future elevated levels of CO2. Although there were no effects on growth during the experiment, the possibility of increased growth in the season following CO2 enrichment due to increased carbohydrate concentrations in woody tissue cannot be excluded.

Communities of ten species of tropical forest tree seedlings from three successional classes were grown at ambient and elevated CO2 in large open-top chambers on the edge of a forest in Panama. Communities grew from 20 cm to approximately 2 m in height in 6 months. No enhancements in plant biomass accumulation occurred under elevated CO2 either in the whole communities or in growth of individual species. Reductions in leaf area index under elevated CO2 were observed, as were decreases in leaf nitrogen concentrations and increases in the C:N ratio of leaf tissue. Species tended to respond individualistically to elevated CO2, but some generalizations of how successional groupings responded could be made. Early and mid-successional species generally showed greater responses to elevated CO2 than late-successional species, particularly with respect to increases in photosynthetic rates and leaf starch concentrations, and reductions in leaf area ratio. Late-successional species showed greater increases in C:N ratios in response to elevated CO2 than did other species. Our results indicate that there may not be an increase in the growth of regenerating tropical forest under elevated CO2, but that there could be changes in soil nutrient availability because of reductions in leaf tissue quality, particularly in late-successional species.

bulk tissue elastic modulus (wall rigidity) which resulted in turgor loss at the same h in both stressed Ber (Ziziphus mauritiana Lamk.) is a major fruit tree and unstressed leaves. The possible ecological signicrop of the north-west Indian arid zone. In a study of ficance of maintenance of turgor potential and cell the physiological basis of drought tolerance in this volume at low water potentials for drought tolerance species, two glasshouse experiments were conducted in Ziziphus is discussed. in which trees were droughted during single stresscycles. In the first experiment, during a 13 d drying Key words: Ziziphus mauritiana, drought, solute accumulacycle, pre-dawn leaf water (Y leaf ) and osmotic (Y p ) tion, osmotic adjustment, proline. potentials in droughted trees declined from ’0.5 and ’1.4 MPa to ’1.7 and ’2.2 MPa, respectively, for a decrease in relative water content (h) of 14%. During Introduction drought stress, changes in sugar metabolism were associated with significant increases in concentra- Drought is probably the most important factor limiting tions of hexose sugars (3.8-fold), cyclitol (scyllo- crop productivity world-wide (Jones and Corlett, 1992). inositol; 1.5-fold), and proline (35-fold; expressed per In semi-arid regions, where the amounts and patterns of unit dry weight), suggesting that altered solute parti- seasonal rainfall are often erratic and unpredictable, deeptioning may be an important factor in drought toler- rooting perennial species may exploit deep soil water ance of Ziziphus. On rewatering, pre-dawn Y leaf and h reserves. This facilitates production of yields in situations recovered fully, but Y p remained depressed by 0.4 MPa of high light coupled with drought, where shallow-rooted relative to control values, indicating that solute con- annual crops would normally fail. The use of perennials centration per unit water content had changed during may help buVer farmers’ production against year-to-year the drought cycle. fluctuations in yields from annual crop species. Under Evidence for osmotic adjustment was provided from such conditions, an ability to maintain cell water status a second study during which a gradual drought (e.g. through osmotic adjustment and/or leaf area adjustwas imposed. Pressure‐volume analysis revealed a ment) and cell integrity (by protecting cells against photo0.7 MPa reduction in osmotic potential at full turgor, and chemical oxidation) may be an advantage. There are with Y leaf at turgor loss depressed by ~1 MPa in many cases where plants growing in hostile environments drought-stressed leaves. Coupled with osmotic adjust- exhibit increased oxy-stress enzyme activities to combat lipid peroxidation by free radicals, hydrogen peroxide

Kalanchoe pinnata (Lam.) Pers. (Crassulaceae), a succulent-leaved crassulacean-acid-metabolism plant, was grown in open-top chambers at ambient and elevated (two times ambient) CO2 concentrations under natural conditions at the Smithsonian Tropical Research Institute, Republic of Panama. Nocturnal increase in titratable acidity and nocturnal carbon gain were linearly related, increased with leaf age, and were unaffected by CO2 treatments. However, under elevated CO2, dry matter accumulation increased by 42–51%. Thus, the increased growth at elevated CO2 was attributable entirely to increased net CO2 uptake during daytime in the light. Malic acid was the major organic acid accumulated overnight. Nocturnal malate accumulation exceeded nocturnal citrate accumulation by six-to eightfold at both CO2 concentrations. Basal (predawn) starch levels were higher in leaves of plants grown at elevated CO2 but diurnal fluctuations of starch were of similar magnitude under both ambient and elevated CO2. In both treatments, nocturnal starch degradation accounted for between 78 and 89% of the nocturnal accumulation of malate and citrate. Glucose, fructose, and sucrose were not found to exhibit marked day-night fluctuations.

Changes in chlorophyll a fluorescence during the day and diurnal-changes of net CO2-exchange and organic acid contents were determined in two species of the genus Clusia during the dry season in Venezuela. The investigations included plants of the C3/CAM intermediate species Clusia minor and the C3 species C. multiflora growing at exposed and shaded sites. Both species showed a C3 pattern of net CO2-exchange at the exposed site. In the shade under extreme drought stress C. minor showed a weak expression of CAM without CO2-uptake during the afternoon (phase IV of CAM). C. multiflora growing in the shade exhibited a C3-pattern of net CO2-exchange and a small but significant nocturnal accumulation of citrate. Shaded plants of C. minor were able to double their light utilisation for electron transport and to reduce non-photochemical quenching during phase III compared to phase II of CAM. Furthermore, increase of electron transport rate through photosystem II in phase III of CAM is correlated to decarboxylation of malate. At the exposed site C. multiflora was less negatively affected by high PPFD than C. minor. This was shown by a lower reduction of potential electron quantum yield (Fv/Fm) and higher light utilisation of electron transport of C. multiflora compared to C. minor. At the exposed site C. minor did not make use of the CAM option to increase light utilisation of electron transport and to reduce non-photochemical quenching as did the plants growing in the shade.

Four different Clusia species showing C 3 photosynthesis (C. multiflora) or crassulacean acid metabolism (CAM) (C. rosea, an unidentified Clusia sp. and C. alata) co-occur in the same habitat in the northwest of Venezuela. The aim of this field study was to correlate the adaptive changes in chlorophyll a fluorescence with gas exchange patterns and deacidification of nocturnally accumulated organic acids. The results indicate that in C. rosea and Clusia sp. a lowering of the potential quantum yield, measured as F v /F m after 10 min of dark adaptation, is restricted to the early morning hours. With the onset of deacidification there was no further decline in F v /F m , although the irradiance upon the leaves remained high. For the C 3 -plant, C. multiflora, changes in F v /F m were correlated with changes in irradiance incident on the leaves before predarkening for 10 min for F v /F m measurements. The fast relaxation of F v /F m under low light indicates that C. multiflora was not suffering photoinhibitory damage. Effective quantum yield as determined by ΔF/F' m , was low under high irradiances for C. rosea and Clusia sp. and did not change significantly for C. rosea during phase III of CAM. During the time of low incident irradiance C. alata and C. multiflora reached an effective quantum yield that was close to the optimal quantum yield of electron transport. It is noteworthy that apparent electron transport rates through photosystem II (PS II) ((ΔF/F' m ) x PPFD) were highest for C. rosea, Clusia sp. and C. alata during phase III of CAM. Electron transport rates in C. multiflora were lower under high irradiance and did not differ from those found in phases II and IV of CAM. The possible role of CO 2 evolution from acid decarboxylation with respect to photosynthetic efficiency is discussed.

In search of a reliable drying method, which might be used even under field conditions, microwave drying was compared to freeze-drying of plant material. Leaves of Ananas comosus and Avicennia germinans as well as buds and phloem of Acer pseudoplatanus were used and checked for one or more of the following substances: sugars, sugar alcohols, organic and amino acids, total nitrogen, and glycinebetaine. With most samples good agreement was achieved between the two drying methods. Only in the case of the Ananas comosus leaves, which exhibited low pH and high water content, did appreciable differences occur in organic and amino acids. Besides that, sucrose was the compound most susceptible to alterations, which was especially evident when leaves of Sambucus nigra were dried in the two different compartments (condenser compartment, drying bell jar) of the freeze-dryer in use. For Ananas comosus leaf samples it was shown that microwaving can also be used prior to extraction of tissue sap.

Gas exchange patterns and water relations of two mistletoes, Tapinanthus oleifolius and Viscum rotundifolium on the same deciduous host tree, Acacia nebrownii, have been studied intensively in the dry and wet seasons in semi-arid south-eastern Namibia, where rain falls only in the summer. Comparative measurements of Tapinanthus oleifolius on the stem-succulent CAM plant, Euphorbia virosa, and on Ziziphus mucronata in the same area were also made. All measurements of CO2 exchange and transpiration were performed in fully acclimatized cuvettes of a gas exchange system which allowed continuous measurements of the host and the parasite simultaneously. The water potential results confirm previous observations that mistletoes have a lower water potential throughout the day than their hosts. In the dry season, pre-dawn and minimum water potential was about 1 MPa lower than in the wet season, for both the leafless and fully foliated host and the evergreen mistletoes. There was an effective stomatal control in all mistletoe plants investigated. The results obtained for transpiration, leaf conductance, internal CO2 concentration and water-use efficiency (WUE) contradicted commonly accepted generalizations. Transpiration and leaf conductance were found to be higher for the host than for any of its common mistletoes as long as the leaf-to-air water vapour pressure difference (Δw) did not exceed 30 mPa Pa− 1, At higher values of Δw, Tapinanthus oleifolius, but not Viscum rotundifolium, transpired at higher rates than the host. CO2 balance over 24 hours and photosynthetic CO2 uptake were significantly higher for the host than for any of its parasites. Although the WUE over 24 h was slightly better for Acacia nebrownii compared with Tapinanthus oleifolius, apparent WUE during photosynthetic CO2 uptake was always better for both mistletoes. Measurements of CO2 and water balance over 24 h, photosynthetic CO2 uptake, night respiration, internal CO2 concentration, leaf conductance and WUE indicate that Tapinanthus oleifolius and Viscum rotundifolium behaved in opposite ways in the dry and wet seasons, indicating that different phenological cycles of both species might be involved in determining mistletoe performance on the same host. The results presented contribute to a better understanding of the host/parasite relationship and indicate that commonly accepted generalizations may not always be applicable. Gaswisselingspatrone en waterverhoudings van twee voelente Tapinanthus oleifolius en Viscum rotundifolium op dieselfde bladwisselende gasheerplant Acacia nebrownii is bestudeer in die droe en nat seisoene in Suid-Oos-Namibie. Vergelykende studies van Tapinanthus oleifolius op Ziziphus mucronata en die stamsukkulente CAM plant Euphorbia virosa is ook uitgevoer. Alle bepalings is in ten voile ge-akklimatiseerde kuvette uitgevoer wat deurlopende bepalings op die gasheer en parasiet moontlik gemaak het. Die resultaat van die waterpotensiaalbepalings bevestig vorige waarnemings dat voelente’n laer waterpotensiaal gedurende die dag het as hulle gashere. Die minimum waterpotensiaal voor dagbreek in die droe seisoen was ongeveer 1 MPa laer as in die nat seisoen vir beide die blaarlose en blaarbevattende gasheerplant en die immergroen voelente wat ondersoek is. In al die voelente was daar effektiewe stomatere beheer teenwoordig. Die resultate wat behaal is vir transpirasie, blaargeleiding, interne koolsuurgaskonsentrasle en waterbenuttingsdoeltreffendheid het nie ooreengestem met aanvaarde veralgemenings nie. Transpirasie en blaargeleiding was hoer vir die gasheer as vir enige van die voelente solank as die blaar-tot-lug waterdampdruk verskil nie 30 mPa Pa− 1 oorskry het nie. By hoer waardes het Tapinanthus oleifolius, maar nie Viscum rotundifolium nie, teen’n hoer tempo as die gasheerplant getranspireer. Die koolsuurgasbalans oor 24 uur en die fotosintetiese koolsuurgasopname was betekenisvol hoer vir die gasheer as vir enige van die parasiete. Alhoewel die waterbenuttingsdoeltreffendheid oor 24 uur effens beter vir Acacia nebrownii as vir Tapinanthus oleifolius was, was die skynbare waterbenuttingsdoeltreffendheid gedurende fotosintetiese koolsuurgasopname altyd beter. Bepaling van die koolsuurgas en waterbalans oor 24 uur, fotosintetiese koolsuurgasopname, nagrespirasie, interne koolsuurgaskonsentrasie, blaargeleiding en waterbenuttingsdoeltreffendheid het aangedui dat die twee parasiete teenoorgesteld gereageer het in die nat en droe seisoene. Dit mag beteken dat verskillende fenologiese siklusse van albei spesies betrokke mag wees in die bepaling van voelentprestasie op dieselfde gasheer. Die resultate lei tot’n beter begrip van gasheer/parasietverwantskappe en dui daarop dat die aanvaarde veralgemenings nie altyd korrek is nie.

The inorganic ion and organic solute composition of Tapinanthus oleifolius (Loranthaceae) and Viscum rotundifolium (Viscaceae) growing on various host trees in Namibia were investigated. Organic osmolytes accounted for 22.8–45.1% of the total solutes determined in leaves of the mistletoes parasitizing Acacia nebrownii, Catophractes alexandri, Grewia flava and Ziziphus mucronata. On other hosts (Acacia karroo, Euphorbia virosa, Salvadora persica and Tamarix usneoides) T. oleifolius showed distinct succulence with increasing leaf age, with leaves more than 3 mm thick on E. virosa. In the more succulent leaves (> 1.1 kg H2O m−2 leaf area) organic solutes were only of minor importance. Water content per leaf area was significantly correlated with chloride content per leaf area, suggesting that succulence served as a means to keep ion concentration at a physiological tolerable level. At whole plant levels 93.7% of the total sodium of the mistletoe bush was located in leaves thicker than 1 mm for T. oleifolius parasitizing Tamarix usneoides. This pronounced sequestration of sodium in older leaves as well as the high variability of the K/Na ratio in various parts of the parasite-host system point to highly selective ion distribution processes in this association.

SUMMARY Investigations of Viscum album on 16 different host species revealed an endogenous cyclitol pattern with pinitol, 1-O-methyl-muco-inositol, quebrachitol, chiro-inositol, an unidentified O-methyl-inositol and traces of ononitol. Host-specific cyclitols including bornesitol, quercitol, viburnitol, scyllo-inositol and the hexitol, sorbitol, were also stored in the mistletoe. The endogenous cyclitols were accumulated to such high concentrations that they made a large contribution to the osmotic potential. It was estimated that 22.6–43 % of mistletoe carbon is derived from the host.

Clusia rosea Jacq. is abundant in the moist parts of the Caribbean island of St John (US Virgin Islands, Lesser Antilles) but relatively rare along the dry south coast. Three types of seedlings were encountered, terrestrial seedlings, seedlings growing as humus-epiphytes on other trees, and seedlings growing inside the tanks of the bromeliad Aechmea lingulata (L.) Baker. Free-living trees grow from terrestrial seedlings or from epiphytic seedlings strangling and shading their host trees. Leaf-Na+ levels were always low (1-4 mequiv I-1 tissue water); trees close to the shore were not affected by salinity. In leaves of mature C. rosea trees, levels of Ca2+ , Mg2+ and K+ were about 60-90, 40-50, 45-55 mequiv I-1 tissue water, respectively. Epiphytic seedlings tended to contain lower levels of these inorganic cations than seedlings growing terrestrially or in the tanks of Ae. lingulata. Epiphytic seedlings contained significantly less nitrogen than terrestrial seedlings. In the leaves of mature trees N-levels were independent of altitude and location on the island, but shaded leaves had significantly higher N-levels than exposed leaves. Light compensation point of photosynthesis in epiphytic seedlings performing C3 -photosynthesis was 17-5 (μmol photons m-2 s-1 ), photosynthesis was saturated at about 300μmol photons m-2 s-1 showing a maximum rate of CO2 -uptake of 2-3 μmol m-2 s-1 .

Populations of Acrostichum aureum growing under contrasting conditions of soil salinity and sunlight exposure were studied in two sites in the north coast of Puerto Rico. Plant size and density of clumps under full sun exposure are larger in sites with lower soil salinity. In shady habitats with relatively high soil salinity (understory of Laguncularia forests), plants grow larger but occur at lower densities as compared with nonshaded sites. Apparently salt stress in shady habitats is reduced due to lower evaporative demands. Osmotic concentration of leaf cell sap increases markedly with soil salinity (except in shade plants), a process accompanied by corresponding increases in leaf water content per unit leaf area, and reductions in leaflet size and specific leaf area. The major solutes responsible for increases in cell sap osmotic pressure are Na+, Mg2+, and sucrose. However, the largest relative response to salinity is shown by the cyclitol D-1-0-methyl-muco-inositol, a cytoplasmic compatible solute. ...

Antarctic bryophyte communities presently tolerate physiological extremes in water availability, surviving both desiccation and submergence events. We investigated the relative ability of three Antarctic moss species to tolerate physiological extremes in water availability and identified physiological, morphological, and biochemical characteristics that assist species performance under such conditions. Tolerance of desiccation and submergence was investigated using chlorophyll fluorescence during a series of field- and laboratory-based water stress events. Turf water retention and degree of natural habitat submergence were determined from gametophyte shoot size and density, and δ13C signatures, respectively. Finally, compounds likely to assist membrane structure and function during desiccation events (fatty acids and soluble carbohydrates) were determined. The results of this study show significant differences in the performance of the three study species under contrasting water stress events. The results indicate that the three study species occupy distinctly different ecological niches with respect to water relations, and provide a physiological explanation for present species distributions. The poor tolerance of submergence seen in Ceratodon purpureus helps explain its restriction to drier sites and conversely, the low tolerance of desiccation and high tolerance of submergence displayed by the endemic Grimmia antarctici is consistent with its restriction to wet habitats. Finally the flexible response observed for Bryum pseudotriquetrum is consistent with its co-occurrence with the other two species across the bryophyte habitat spectrum. The likely effects of future climate change induced shifts in water availability are discussed with respect to future community dynamics.

The term “mistletoe” has a double meaning: sensu stricto it applies to Viscum album L. (European mistletoe), but it is more generally used to describe perennial (usually woody) flowering plants which are attached to the shoot of trees or shrubs (Barlow 1987). The majority of the approximately 1300 mistletoe species fall into the two closely related families Loranthaceae and Viscaceae within the order Santalales (Calder 1983).

The unusual gas-exchange stoichiometries of succulent plants which engage in crassulacean acid metabolism (CAM) have been recognized since de Saussure (1804), whose observations are best accessible today via Thomas (1949). In beautifully crafted sentences, sometimes exceeding 100 words, Thomas reviewed the history of gas-exchange experimentation in CAM plants. He left his indelible mark on the dark reactions of CAM with the words “The positive contribution made by the Leipzig workers was to draw attention to the possible cardinal importance (cf. Gustafson’s qualified statement) of carbon dioxide rather than oxygen in controlling diurnal fluctuations in the acidity of the green cells of plants showing crassulacean acid metabolism. The present writer felt that this possibility became a probability when he had considered the experimental results obtained at Leipzig and elsewhere in the light of Wood and Werkman’s discovery”. There is no clearer, and more generous, expression of the origins of the hypothesis that led us to an understanding of the mechanisms of dark CO2 fixation and malic-acid synthesis in CAM plants. Appreciation of photosynthetic CO2 fixation in CAM plants is less clear, historically. Bennet-Clark (1933) attributed the recognition of malate decarboxylation as an internal CO2 source for photosynthesis to A. Mayer (1878, 1899) and to H.A. Spoehr (1913) (cited in Bennet-Clark 1933).

One of the now widely used definitions of halophytes is that of Jennings (1976), describing them as “the native flora of saline habitats”. This is very close to the description by Warming (1909 English Edition, 1895 Plantesamfund Danish Edition), who coined this and a larger number of still commonly used terms like hydro-, meso-, and xerophytes. According to Warming (1909), “a certain amount of soluble salts must be present before halophytic vegetation is called into existence; but the nature of the salts seems to be a matter of indifference”. Following Munns et al. (1983), such a habitat is one “containing saline water with an osmotic pressure (π) of more than 3.3 bar” (equivalent to 70 mM monovalent salts).

The various components of salt resistance were tested for their importance for salt adaptation in mangroves. Evidently avoidance of salt uptake by special mechanisms in the mangrove roots is of paramount importance. Retention of NaCl in roots and lower parts of the stem was negligible in young, glasshouse-grown mangroves. Salt excretion by specialized glands contributed essentially to the maintenance of a certain NaCl level in mangrove leaves. However, only a few mangrove species are salt-excreting. For non-excreting species retranslocation of NaCl via the phloem and increasing succulence seemed to contribute decisively to the maintenance of a stable NaCl level in the leaves. Although there is not a lot of direct evidence, it might be assumed that NaCl is mainly stored in the vacuole. In 29 out of 30 mangrove species organic solutes, which are thought to serve for the intracellular osmotic adjustment, were present in appreciable amounts. In a number of cases it was shown that concentrations of these organic solutes corresponded to the salinity stress applied.

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. The C3-crassulacean acid metabolism tropical hemiepiphytic tree, Clusia minor L., is highly flexible in terms of ecological sites occupied, life forms, and photosynthetic and metabolic pathways. We studied nitrogen uptake patterns in two glasshouse 15N-labelling experiments using hydroponically-grown plants and excised roots of pot-grown C. minor, and investigated leaf and root nitrate reductase activity (NRA) in a field study in Venezuela. The results of both 15N-uptake experiments indicate that C. minor utilized all offered nitrogen sources, but clearly preferred to take up NH4+ over glycine (GLY) and NO3-. The uptake pattern of NH4+ and NO3- was identical in intact plants and excised roots, and NH4+ was taken up to a much larger extent in both experiments. Gas chromatography-mass spectrometry (GC-MS) analysis of amino acids in excised roots revealed that glutamine (45 atom-%), glutamate (23 atom-%) and alanine (34 atom-%) accounted for most of the labelled soluble amino acids after 2-h labelling with 15NH4+. High amounts of 15N in GLY and serine confirmed that GLY was taken up as an intact molecule and metabolized in the excised roots. With 15NO3- labelling, only a small amount of 15N was found in the amino acid fraction, indicating a low NO3- assimilation rate by nitrate reductase. This was confirmed by low NRA of leaves and roots in C. minor plants in Venezuela. It appears, therefore, that a high degree of plasticity is not reflected in nitrogen uptake and metabolism. Although C. minor exhibits clear preferences for NH4+ uptake, it can utilize other sources of nitrogen, and the nitrogen uptake pattern represents an adaptation to the natural environments in which C. minor can grow.

Tolerance of desiccation was examined in three species of moss, Grimmia antarctici Card., Ceratodon purpureus (Hedw.) Brid. and Bryum pseudotriquetrum (Hedw.) Gaertn., Meyer et Scherb. collected from two sites of contrasting water availability in the Windmill Islands, continental Antarctica. Physiological tolerance to desiccation was measured using chlorophyll fluorescence in plugs of moss during natural drying in the laboratory. Differences in relative water content, rate of drying and the response of photosynthesis to desiccation were observed among the three species and between sites. Of the three species studied, G. antarctici showed the lowest capacity to sustain photosynthetic processes during desiccation, B. pseudotriquetrum had an intermediate response and showed the greatest plasticity and C. purpureus showed the greatest capacity to sustain photosynthesis during desiccation. These results fit well with the known distribution of the three species with G. antarctici being limited to relatively wet sites, C. purpureus being common in the driest sites and B. pseudotriquetrum showing a wide distribution between these two extremes. Levels of soluble carbohydrates were also measured in these samples following desiccation and these indicate the presence of stachyose, an oligosaccharide known to be important in desiccation tolerance in seeds, in B. pseudotriquetrum. Both gross morphology and carbohydrate content are likely to contribute to differences in desiccation tolerance of the moss species. These results indicate that if the Casey region continues to dry out, as a result of local geological uplifting or global climate change, we would expect to see not only reductions in the moss community but also changes in community composition. G. antarctici is likely to become more limited in distribution as C. purpureus and B. pseudotriquetrum expand into drying areas.

1d-1-O-Methyl-muco-inositol was isolated from Viscum album as well as from Rhizophora mucronata and identified by GC retention times, GC-MS, 13C NMR and by specific rotation. In V. album and in various members of the Rhizophoraceae the compound was stored in large amounts up to 8.4% of dry wt. Increase of concentrations under salinity stress (Ceriops tagal) and during the cold season (V. album) suggested a role of 1d-1-O-methyl-muco-inositol as a stress metabolite and cryoprotectant respectively.

To investigate the importance of phosphorus and carbohydrate concentrations in influencing photosynthetic capacity of tropical forest tree seedlings under elevated CO2, we grew seedlings of Beilschmiedia pendula (Sw.) Hemsl. (Lauraceae) under elevated CO2 concentrations either with or without vesicular-arbuscular (VA) mycorrhizae. VA-mycorrhizae increased phosphorus concentrations in all plant organs (leaves, stems and roots). Maximum rates of photosynthesis (Amax) measured under saturating levels of CO2 and light were correlated with leaf phosphorus concentrations. VA-mycorrhizae also increased leaf carbohydrate concentrations, particularly under elevated CO2, but levels were low and within the range observed in naturally occurring forest species. Root carbohydrate concentrations were reduced in VA-mycorrhizal plants relative to non-mycorrhizal plants. These results indicate an important role for VA-mycorrhizae in controlling photosynthetic rates and sink strength in tropical trees, and thus in determining their response to future increases in atmospheric CO2 concentrations.

The perennial halophytes, Batis maritima L., Sesuvium portulacastrum L., and Portulaca rubricaulis H.B.K. which inhabit the extreme environment on parts of the salt plain and at the edges of the vegetation islands of Cienega el Ostional were examined to determine their strategies for growth and survival in the rainy and dry seasons. All of the three species are leaf-succulents. High leaf-sap osmotic pressures, xylem tensions and Na+ and Cl- levels indicated that S. portulacastrum and B. maritima are salt-accumulating halophytes. Succulence, leaf-sap osmolalities and Na+ and Cl- levels increased in the dry season in B. maritima by a factor of 1.5-2.0 and in S. Portulacastrum by a factor of 1.9-2.7. B. maritima also accumulated sulphate with a two-fold increase of concentrations in the dry season. In S. portulacastrum Na+ accumulation much exceeded Cl- accumulation and oxalate synthesis was found to serve charge balance. In this species the compatible solutes, proline and pinitol, were clearly detectable in both seasons; their levels increased by a factor of about 6 in the dry season. Exchange of water vapour and CO2 was measured with a portable steady-state porometer. Photosynthesis in B. maritima showed little response to the transition from the rainy to the dry season while S. portulacastrum was severely impaired in the dry season, showing pronounced midday depressions of gas exchange and about 40% inhibition of light-saturated rates of CO2 uptake. P. rubricaulis shed its leaves in the dry season. According to carbon isotope ratios (δ13 C), B. maritima (δ13 C =-26.4‰) and S. portulacastrum (δ13 C =-25.8 ‰) are C3 plants while P. rubricaulis (δ13 C =-12.3‰) performed C4 photosynthesis.

Summary The capacity for nitrate reduction was shown to be present in the roots and shoots of all fern species examined. In many species the root was the major site of nitrate reduction, irrespective of the availability of external nitrate. The capacity for root nitrate reduction was positively correlated with root glutamine synthetase activity. Some species exhibited multiple forms of glutamine synthetase in their shoots. The activity of chloroplastic glutamine synthetase appeared to be correlated with photorespiratory potential. Root nitrate assimilation was most pronounced in shade ferns.

Summary Low molecular weight carbohydrates (LMWC) occurring in both young and old leaves of 22 mangrove species from Northern Queensland (Australia) were identified using gas liquid and gas capillary chromatography. The cyclitol, pinitol, was the most prevalent compound in all members of the Rhizophoraceae and in the mangrove fern Acrostichum speciosum. In addition, Aegialitis annulata stored chiro-inositol. Another cyclitol, quebrachitol, occurred in Excoecaria agallocha. Mannitol, a polyol, was dominating in Aegiceras corniculatum, Lumnitzera littorea, Lumnitzera racemosa, Sonneratia alba and Scyphiphora hydrophylacea. The sugars sucrose, glucose, and fructose were not generally stored in high concentrations (except Xylocarpus granatum, 300 mol · m−3 plant water). Those species which contained no cyclitols or mannitol were low in their LMWC content. LMWC concentrations were higher in young leaves than in old ones in nearly all species under investigation. Polyols and cyclitols may have taxonomically associated distributions and may play a role in osmotic adjustment.

Summary 19 species of halophytes, mainly Chenopodiaceae and Poaceae were investigated for their free amino acids and the total and water-soluble nitrogen content. All plants under investigation except Phragmites contained proline, but only in Lepidium crassifolium, Artemisia monogyna and Triglochin maritimum in such high amounts, that it can be thought to serve as cytoplasmic osmoticum. The concentrations of the other free amino acids are rather low, especially in the members of the Chenopodiaceae . Only in Crypsis aculeata and Phragmites australis other amino acids than proline are accumulated to a considerable extent. Marked differences also occur in the nitrogen levels: Chenopodiaceae usually show high values of total and water-soluble nitrogen, while the opposite is true for most of the Poaceae . The free amino acids contribute very little to the nitrogen content, except in the case of Lepidium , where the proline-N makes up for 20 % of the total nitrogen.

Zusammenfassung An einigen ausgewahlten Halophytenarten (Lepidium crassifolium, Triglochin maritimum, Puccinellia distans, Aster tripolium subsp. pannonicus, Suaeda maritima und Plantago maritima) der Salzgebiete am Neusiedlersee wurden im Verlauf von zwei Vegetationsperioden jeweils im Juni, August und Oktober (bei uberdauernden Pflanzen auch im Februar) die Gehalte an freien Aminosauren, Gesamtstickstoff und anorganischen Kationen bestimmt. Nur bei Lepidium bilden die freien Aminosauren einen wesentlichen Anteil am Gesamtstickstoff. Das von dieser Pflanze in besonders hohem Ausmas gespeicherte Prolin macht im Verlauf des Jahres immer etwa 20 % des Stickstoffgehaltes aus. In allen anderen Pflanzen finden sich wesentlich weniger freie Aminosauren bzw. Prolin, wobei der Prolingehalt starken jahreszeitlichen Schwankungen unterliegt. Diese zeigen keinen Zusammenhang mit Anderungen im Salzgehalt, sondern durften — da die maximalen Prolinkonzentrationen im Oktober bzw. Februar auftreten — mit der Kalteresistenz in Beziehung stehen. Eine Differenzierung des Pflanzenmaterials in junge und alte Blatter bei Lepidium, Plantago und Phragmites austrails zeigt, das die Prolinakkumulation vor allem in den jungen Pflanzenteilen stattfindet. Wahrend bei Lepidium in den jungen Blattern etwa doppelt soviel wie in den alten enthalten ist, kann es bei Plantago und Phragmites uberhaupt nur in den jungen Blattern festgestellt warden.

The performance of crassulacean acid metabolism (CAM) by dicotyledonous trees of the genusClusia sampled at three sites in the state of Falcon in northern Venezuela is characterized.Clusia leaves have a somewhat succulent appearance. Unlike leaves of many other CAM plants, which are uniformly built up of very large isodiametric cells, there are distinct layers of palisade and spongy mesophyll, with individual cells being smaller. There is no specialized water storage tissue. δ13C values indicate thatC. multiflora in the elfin-cloud forest on top of Cerro Santa Ana, at ∼800 m altitude, performs C3 photosynthesis (δ13 −27.1‰). However,C. rosea in the tall cloud forest on Cerro Santa Ana (∼600m altitude), andC. rosea andC. alata in the dry forest on Serrania San Luis (∼900 m altitude) perform CAM (δ13C −14.1 to −19.2‰). InC. alta andC. rosea there were large day-night changes in the levels of malic and citric acids ranging from 63 to 240 mmol 1−1 for malid acid and from 35 to 112 mmol 1−1 for citric acid. The sum of the changes in malate and citrate levels accounts for the changes of titratable protons measured. With a day-night change of titratable protons of 768 mmol 1−1 in one of the analyses,C. rosea showed the highest value yet encountered in a CAM plant. Oscillations of free sugars (fructose, glucose, sucrose) and of starch were also analysed in the CAM performingClusia species. Carbon skeletons of the precursors involved in nocturnal malate and citrate synthesis largely derive from free sugars and not from polyglucan. Unlike some other CAM plants, there is no clear and quantitative correlation between day-night changes of organic acid levels and cell sap osmolality.

Samples from the Crassulacean species (Sedum maximum, S. album, S. acre, S. sexangulare, Jovibarba hirta, Sempervivum pittonii) were collected from sites differing in their edaphic conditions throughout the year. The samples were investigated for their cation content to proof their calciotrophic character (molar ratios K/Ca equal or below unity) and for their organic acid pattern. In most cases, low K/Ca ratios were maintained in the shoots even on habitats where Ca 2+ was not as abundant as in calcareous soils. Roots, in contrast, tended toward high K + concentrations although in some plants from serpentine habitats, Mg 2+ became the dominant cation either in shoots (J. hirta, Se. pittonii, S. album) or in roots (Se. pittonii, S. acre) . Isocitrate concentrations were higher than those of malate and citrate in all species except J. hirta . The concentration of isocitrate was positively correlated with both Ca and cation excess concentrations.

summary Seasonal effects on leaf gas exchange and water relations were compared for Avicennia germinans, a true mangrove, and Conocarpus erectus, a mangrove associate, at coastal sites in northern Venezuela. On the Cienega el Ostional at Chichiriviche, A. germinans was most abundant around lagoons on the seaward side of the vegetation-free alluvial sand plain. C. erectus was the dominant shrub in inland communities, but the two species co-occurred on vegetation islands at the landward edge of the alluvial plain. On the vegetation islands of the Cienega el Ostional, gas exchange in A. germinans (a species with foliar salt glands) was less severely curtailed in the dry season compared with the rainy season than was gas exchange in C. erectus (a species lacking salt glands). Average rates of photosynthesis at near-saturating light intensities and total diurnal CO2 uptake were reduced in the dry season to 69 and 61%, respectively, of their values in the rainy for A. germinans, but to 48 and 30%, respectively, of their rainy-season values for C. erectus. Similarly, stomatal conductance and transpirational water loss were more reduced in the dry season for C. erectus than for A. germinans, with the result that C. erectus showed a 3.4-fold increase in water-use efficiency in the dry season compared with the rainy season. The importance of the soil environment in determining plant gas-exchange Patterns was evidenced by large seasonal shifts in dawn xylem tension for the two species (which increased from 1.34 MPa in the rainy season to 5.50 MPa in the dry season for A. germinans, and from 0.40 to 5.78 MPa for C. erectus). These values reflected changes in the soil environment caused by inundation of the upper soil layers by fresh water in the rainy season and a progressive increase in salt concentrations (to almost twice those in sea water) by evaporation from the soil in the dry season. Large changes in xylem tension were observed for both species during individual day–night cycles, reaching a maximum of 2.36 MPa for A. germinans. For C. erectus, the magnitude of these day-night changes was greatly reduced in the dry season, consistent with its very low transpiration rates at this time of year. Leaf-cell osmotic pressures also tended to be higher in A. germinans than C. erectus (attaining a maximum of 8.3 MPa for A. germinans in the dry season), and were related to the more seaward distribution of the true mangrove on the alluvial plain. Whereas leaves of A. germinans did not show any changes in succulence, leaf succulence in C. erectus increased with leafage and was slightly higher in the dry season than the rainy season. The more succulent leaves also had higher cell-sap osmotic pressures and NaCl concentrations. The most succulent leaves of C. erectus were observed for exposed shrubs growing on the shoreline. During the dry season, these shoreline plants showed high rates of gas exchange and low values for dawn xylem tension (0.89 MPa), indicating that they had access to relatively non-saline water from the shallow water table. On individual plants, exposed shoots had more succulent leaves and higher osmotic pressure and NaCl concentrations than sheltered leaves, demonstrating the importance of foliar absorption of salt borne in sea spray for the ionic relations of C. erectus. Thus, although the distribution of C. erectus is centred on brackish-water zones, this species can apparently extend from habitats with permanent access to a shallow water table through to areas where it is seasonally exposed to low soil water potentials and high salt concentrations in the substratum.

Summary Free amino acids, total methylated onium compounds (TMOC) and total nitrogen were investigated in young and old leaves of 22 mangrove species from Northern Queensland (Australia). Total nitrogen ranged between 0.35 to 2.5% of dry matter depending on leaf age and species. The soluble nitrogen compounds accounted only in a few cases for more than 10% of the total nitrogen. High proline accumulation was observed in only three species: Aegialitis annulata (28.5 mol · m-3 plant water), Xylocarpus granatum (76.0 mol · m-3 pw) and Xylocarpus mekongensis (56.7 mol · m-3 pw). The latter two species contained also hydroxyproline as a major component of the amino acid fraction. GABA, GLU, ASP and ALA were the most frequently occurring amino acids in the other species. Avicennia eucalyptifolia, Avicennia marina, Hibiscus tiliaceus and Heritiera littoralis accumulated high concentrations of TMOC (up to 107 mol · m-3 plant water), while Acanthus ilicifolius stored both TMOC and PRO. The occurrence of organic solutes in mangroves is discussed in comparison to herbaceous halophytes.

Summary Young and old leaves from 22 mangrove species of Northern Queensland (Australia) were investigated for inorganic ions and organic acids. Na + and Cl − concentrations expressed on plant water basis were close to sea water with the exception of Heritiera littoralis and Hibiscus tiliaceus , which are both regarded as brackish water species. Leaf age did not appear to effect Na + and Cl − storage much. SO 4 2− and Mg 2+ increased markedly in old leaves of salt-secreting species such as Avicennia marina , Avicennia eucalyptifolia , Aegialitis annulata , Aegiceras corniculatum and Acanthus ilicifolius as well as in the members of the Rhizophoraceae. Free oxalate was found in all salt-secreting species and young leaves of Lumnitzera racemosa . Malate and citrate were present in all species, while quinate and shikimate occurred frequently. The contribution of organic acids to the anion content was important in only a few cases. Mechanisms of salt-regulation and problems in the classification of mangroves are discussed.

Young and old leaves of twenty-three mangrove species from northern Queensland (Australia) were investigated for their mineral ion and organic solute content. With a few exceptions, the Na+ and the Cl− concentrations calculated on the basis of plant water (p.w.) were close to that of seawater and showed little age-induced changes. In some species, especially in Ceriops tagal, SO 4 2− accumulated with increasing leaf age. The most widely distributed organic solutes were pinitol and mannitol, which were stored up to 280 mM plant water. A negative correlation between pinitol and SO 4 2− was found in the case of Ceriops tagal. Other compatible solutes known for halophytes, such as proline and methylated quaternary ammonium compounds (MQAC), were present only in a few mangrove species. Proline occurred in two Xylocarpus species, while MQAC were accumulated by Avicennia eucalyptifolia, A. marina, Acanthus ilicifolius, Heritiera littoralis and Hibiscus tiliaceus. In all other species, low molecular weight carbohydrates (LMWC) were the main organic solutes.

Halophytes, metallophytes, serpentine plants, calcicoles, and calcifuges are adapted to soil conditions which are deleterious for other plants. Some mechanisms responsible for these adaptations are described. The respective changes in the genome for these alterations of the mineral metabolism are probably not too far-reaching, because already intraspecific differentiation may lead to resistant ecotypes. On the other hand it seems likely that some families or species are favoured in occupying extreme soil types in consequence of their basic physiological features (i.e. Chenopodiaceae, Poaceae in saline habitats, Brassicaceae, Caryophyllaceae on heavy metal soils).

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. We investigated the nitrogen (N) nutrition of Clusia osaensis, C. peninsulae and C. valerii during the seedling, epiphytic and hemiepiphytic phase in a lowland tropical forest in Costa Rica in order to elucidate nutritional adaptations of different plant growth stages to their habitat. Although all Clusia individuals were non-mycorrhizal, excised roots of seedlings, but also of epiphytic and hemiepiphytic stages, showed a distinct preference for glycine uptake. The shift in main rooting site from canopy soil to terrestrial soil was not reflected by changes in uptake rate or preference, although N availability and the composition of the available N pool changed significantly. High foliar N concentrations indicated that epiphytic seedlings seemed to be sufficiently supplied with N by maternal seed resources and canopy soils. With development, the epiphytic plants of Clusia may face N limitation due to higher N demands with increasing growth and restricted resources in the tree crowns. 15N natural abundance data indicate that epiphytes mainly accessed atmospheric and, to a lesser extent, canopy soil N sources and, after becoming terrestrially anchored, in the hemiepiphytic life stage exploited the larger nutrient reservoir of the ground soil. In consequence, Clusia species did not show an adaptation to the different N availability situations that they experienced whether canopy or ground-rooted.

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. Carbon isotope fractionation and nocturnal acid accumulation in Clusia osaensis Hammel-ined., C. �peninsulae Hammel-ined. and C. valerii Standl. were investigated during the seedling, epiphytic and hemiepiphytic phases in a lowland tropical forest in Costa Ricato study photosynthetic adaptations of different plant growth stages to their habitat. Foliar δ 13C values around -24 to -32‰ indicate predominant C3 fixation of CO2 and low crassulacean acid metabolism (CAM) activity in all three Clusia species. Only terrestrially rooted plants of C. osaensis showed increased CAM expression. However, all developmental stages exhibited significant CAM cycling as shown by significant day-night fluctuations of titratable protons and of malic and citric acid. In C. valerii and C. peninsulae, an increase in CAM expression with plant development was not observed, and CAM cycling in hemiepiphytic-stage plants was completely repressed during the high rainfall season. The expression of CAM in the three Clusia species is therefore not developmentally controlled but triggered by environmental factors such as water availability and light intensity. These factors remain relatively stable in this ecosystem and CAM is therefore not fully expressed. However, CAM cycling may be of ecophysiological significance in all life stages as it serves as a mechanism to improve carbon economy by reducing respiratory CO2 losses.