Decline In Photosynthetic Capacity Research Articles

Seasonal dynamics and punctuated carbon sink reduction suggest photosynthetic capacity of boreal silver birch is reduced by the accumulation of hexose.

The 'assimilates inhibition hypothesis' posits that accumulation of nonstructural carbohydrates (NSCs) in leaves reduces leaf net photosynthetic rate, thus internally regulating photosynthesis. Experimental work provides equivocal support mostly under controlled conditions without identifying a particular NSC as involved in the regulation. We combined 3-yr in situ leaf gas exchange observations (natural dynamics) in the upper crown of mature Betula pendula simultaneously with measurements of concentrations of sucrose, hexoses (glucose and fructose), and starch, and similar measurements during several one-day shoot girdling (perturbation dynamics). Leaf water potential and water and nitrogen content were measured to account for their possible contribution to photosynthesis regulation. Leaf photosynthetic capacity (A/Ci) was temporally negatively correlated with NSC accumulation under both natural and perturbation states. For developed leaves, leaf hexose concentration explained A/Ci variation better than environmental variables (temperature history and daylength); the opposite was observed for developing leaves. The weaker correlations between NSCs and A/Ci in developing leaves may reflect their strong internal sink strength for carbohydrates. By contrast, the strong decline in photosynthetic capacity with NSCs accumulation in mature leaves, observed most clearly with hexose, and even more tightly with its constituents, provides support for the role of assimilates in regulating photosynthesis under natural conditions.

Grapevine (Vitis vinifera) responses to salt stress and alkali stress: transcriptional and metabolic profiling

BackgroundSoil salinization and alkalization are widespread environmental problems that limit grapevine (Vitis vinifera L.) growth and yield. However, little is known about the response of grapevine to alkali stress. This study investigated the differences in physiological characteristics, chloroplast structure, transcriptome, and metabolome in grapevine plants under salt stress and alkali stress.ResultsWe found that grapevine plants under salt stress and alkali stress showed leaf chlorosis, a decline in photosynthetic capacity, a decrease in chlorophyll content and Rubisco activity, an imbalance of Na+ and K+, and damaged chloroplast ultrastructure. Fv/Fm decreased under salt stress and alkali stress. NPQ increased under salt stress whereas decreased under alkali stress. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed the differentially expressed genes (DEGs) induced by salt stress and alkali stress were involved in different biological processes and have varied molecular functions. The expression of stress genes involved in the ABA and MAPK signaling pathways was markedly altered by salt stress and alkali stress. The genes encoding ion transporter (AKT1, HKT1, NHX1, NHX2, TPC1A, TPC1B) were up-regulated under salt stress and alkali stress. Down-regulation in the expression of numerous genes in the ‘Porphyrin and chlorophyll metabolism’, ‘Photosynthesis-antenna proteins’, and ‘Photosynthesis’ pathways were observed under alkali stress. Many genes in the ‘Carbon fixation in photosynthetic organisms’ pathway in salt stress and alkali stress were down-regulated. Metabolome showed that 431 and 378 differentially accumulated metabolites (DAMs) were identified in salt stress and alkali stress, respectively. L-Glutamic acid and 5-Aminolevulinate involved in chlorophyll synthesis decreased under salt stress and alkali stress. The abundance of 19 DAMs under salt stress related to photosynthesis decreased. The abundance of 16 organic acids in salt stress and 22 in alkali stress increased respectively.ConclusionsOur findings suggested that alkali stress had more adverse effects on grapevine leaves, chloroplast structure, ion balance, and photosynthesis than salt stress. Transcriptional and metabolic profiling showed that there were significant differences in the effects of salt stress and alkali stress on the expression of key genes and the abundance of pivotal metabolites in grapevine plants.

The severe toxicity of CuO nanoparticles to the photosynthesis of the prokaryotic algae Arthrospira sp.

This research first verified that prokaryotic algae are more sensitive to toxicity of CuO nanoparticles (CuO NPs) than eukaryotic algae and that CuO NPs damaged photosynthesis of prokaryotic algae (Arthrospira sp.) but had no effect on respiration. The Cu2+ released by CuO NPs caused a bending deformation of the thylakoid, which was an important cause of the decline in photosynthetic capacity. In addition, the D1 protein was the most susceptible site to CuO NPs. The degradation of D1 protein reduced photosynthetic electron transport, which enhanced the excess excitation energy to cause the accumulation of reactive oxygen species (ROS) to further result in oxidative stress on algae. Dissolved organic matter (DOM) increased the toxicity of CuO NPs to photosynthesis of Arthrospira sp. The damage of photosynthesis caused by CuO NPs is an important reason why CuO NPs have a serious toxicity to algae.

Transcription-associated metabolomic adjustments in maize occur during combined drought and cold stress.

Although simultaneous drought and cold stress occurs, especially in northwestern and eastern regions of China, and is an important factor limiting agricultural productivity, there are few studies focusing on plant responses to a combination of drought and cold stress. Here, by partially overlapping drought and cold stresses, we characterized the acclimation of maize (Zea mays B73) to these two stresses using physiological measurements, as well as comparative transcriptomics combined with metabolomics and hormonal analyses during the stress treatments and recovery stages. The combined drought and cold stress and drought stress alone were accompanied by a decline in photosynthetic capacity and enhanced transcriptional response, and subsequent recovery of these following removal from stress, whereas cold stress alone was accompanied by irreversible damage to photosynthetic capacity and chloroplast structure. The stress combination induced transcription-associated metabolomic alterations, in which raffinose, trehalose-6-phosphate, and proline accumulated, and monosaccharide abundance increased. Concomitantly, the increased abscisic acid (ABA) content and upregulated ABA signaling pathway may have provided the transcriptional regulation for the metabolic changes. In a parallel experiment, ABA treatments prior to exposure of the plants to cold stress primed the plants to survive the cold stress, thus confirming a key role for the endogenous ABA activated by the drought pretreatment in acclimation of the plants to cold. We present a model showing that the plant response to the combined stress is multi-faceted and reveal an ABA-dependent maize acclimation mechanism to the stress combination.

Mechanisms Regulating the Dynamics of Photosynthesis Under Abiotic Stresses.

Photosynthesis sustains plant life on earth and is indispensable for plant growth and development. Factors such as unfavorable environmental conditions, stress regulatory networks, and plant biochemical processes limits the photosynthetic efficiency of plants and thereby threaten food security worldwide. Although numerous physiological approaches have been used to assess the performance of key photosynthetic components and their stress responses, though, these approaches are not extensive enough and do not favor strategic improvement of photosynthesis under abiotic stresses. The decline in photosynthetic capacity of plants due to these stresses is directly associated with reduction in yield. Therefore, a detailed information of the plant responses and better understanding of the photosynthetic machinery could help in developing new crop plants with higher yield even under stressed environments. Interestingly, cracking of signaling and metabolic pathways, identification of some key regulatory elements, characterization of potential genes, and phytohormone responses to abiotic factors have advanced our knowledge related to photosynthesis. However, our understanding of dynamic modulation of photosynthesis under dramatically fluctuating natural environments remains limited. Here, we provide a detailed overview of the research conducted on photosynthesis to date, and highlight the abiotic stress factors (heat, salinity, drought, high light, and heavy metal) that limit the performance of the photosynthetic machinery. Further, we reviewed the role of transcription factor genes and various enzymes involved in the process of photosynthesis under abiotic stresses. Finally, we discussed the recent progress in the field of biodegradable compounds, such as chitosan and humic acid, and the effect of melatonin (bio-stimulant) on photosynthetic activity. Based on our gathered researched data set, the logical concept of photosynthetic regulation under abiotic stresses along with improvement strategies will expand and surely accelerate the development of stress tolerance mechanisms, wider adaptability, higher survival rate, and yield potential of plant species.

Optimal leaf life strategies determine Vc,max dynamic during ontogeny.

Leaf photosynthetic properties, for example the maximum carboxylation velocity or Vc,max , change with leaf age due to ontogenetic processes. This study introduces an optimal dynamic allocation scheme to model changes in leaf-level photosynthetic capacity as a function of leaf biochemical constraints (costs of synthesis and defence), nitrogen availability and other environmental factors (e.g. light). The model consists of a system of equations describing RuBisCO synthesis and degradation within chloroplasts, defence and ageing at leaf levels, nitrogen transfer and carbon budget at plant levels. Model results show that optimal allocation principles explained RuBisCO dynamics with leaf age. An approximated analytical solution can reproduce the basic pattern of RuBisCO and Vc,max in rice and in two tropical tree species. The model also reveals leaf life complementarities that remained unexplained in previous approaches, as the interplay between Vc,max at maturation, life span and the decline in photosynthetic capacity with age. Furthermore, it explores the role of defence, which is not implemented in current models. This framework covers some of the existing gaps in integrating multiple processes across plant organs (chloroplast, leaf and whole plant) and is a first-step towards representing mechanistically leaf ontogenetic processes into physiological and ecosystem models.

Interactive effects of global warming and eutrophication on a fast-growing Mediterranean seagrass

Coastal ecosystems, such as seagrasses, are subjected to local (e.g. eutrophication) and global (e.g. warming) stressors. While the separate effects of warming and eutrophication on seagrasses are relatively well known, their joint effects remain largely unstudied. In order to fill this gap, and using Cymodocea nodosa as a model species, we assessed the joint effects of warming (three temperatures, 20 °C, 30 °C and 35 °C) with two potential outcomes of eutrophication: (i) increase in nutrients concentration in the water column (30 and 300 μM), and (ii) organic enrichment in the sediment). Our results confirm that temperature in isolation clearly affects plant performance; while plants exposed to 30 °C performed better than control plants, plants exposed to 35 °C showed clear symptoms of deterioration (e.g. decline of photosynthetic capacity, increase of incidence of necrotic tissue). Plants were unaffected by high ammonium concentrations; however, organic enrichment of sediment had deleterious effects on plant function (photosynthesis, growth, demographic balance). Interestingly, these negative effects were exacerbated by increased temperature.Our findings indicate that in addition to the possibility of the persistence of C. nodosa being directly jeopardized by temperature increase, the joint effects of warming and eutrophication may further curtail its survival. This should be taken into consideration in both predictions of climate change consequences and in local planning.

Warming delays autumn declines in photosynthetic capacity in a boreal conifer, Norway spruce (Picea abies).

Climate change, via warmer springs and autumns, may lengthen the carbon uptake period of boreal tree species, increasing the potential for carbon sequestration in boreal forests, which could help slow climate change. However, if other seasonal cues such as photoperiod dictate when photosynthetic capacity declines, warmer autumn temperatures may have little effect on when carbon uptake capacity decreases in these species. We investigated whether autumn warming would delay photosynthetic decline in Norway spruce (Picea abies (L.) H. Karst.) by growing seedlings under declining weekly photoperiods and weekly temperatures either at ambient temperature or a warming treatment 4 °C above ambient. Photosynthetic capacity was relatively constant in both treatments when weekly temperatures were >8 °C, but declined rapidly at lower temperatures, leading to a delay in the autumn decline in photosynthetic capacity in the warming treatment. The decline in photosynthetic capacity was not related to changes in leaf nitrogen or chlorophyll concentrations, but was correlated with a decrease in the apparent fraction of leaf nitrogen invested in Rubisco, implicating a shift in nitrogen allocation away from the Calvin cycle at low autumn growing temperatures. Our data suggest that as the climate warms, the period of net carbon uptake will be extended in the autumn for boreal forests dominated by Norway spruce, which could increase total carbon uptake in these forests.

Long-term water stress leads to acclimation of drought sensitivity of photosynthetic capacity in xeric but not riparian Eucalyptus species.

Experimental drought is well documented to induce a decline in photosynthetic capacity. However, if given time to acclimate to low water availability, the photosynthetic responses of plants to low soil moisture content may differ from those found in short-term experiments. This study aims to test whether plants acclimate to long-term water stress by modifying the functional relationships between photosynthetic traits and water stress, and whether species of contrasting habitat differ in their degree of acclimation. Three Eucalyptus taxa from xeric and riparian habitats were compared with regard to their gas exchange responses under short- and long-term drought. Photosynthetic parameters were measured after 2 and 4 months of watering treatments, namely field capacity or partial drought. At 4 months, all plants were watered to field capacity, then watering was stopped. Further measurements were made during the subsequent 'drying-down', continuing until stomata were closed. Two months of partial drought consistently reduced assimilation rate, stomatal sensitivity parameters (g1), apparent maximum Rubisco activity (V'(cmax)) and maximum electron transport rate (J'(max)). Eucalyptus occidentalis from the xeric habitat showed the smallest decline in V'(cmax) and J'(max); however, after 4 months, V'(cmax) and J'(max) had recovered. Species differed in their degree of V'(cmax) acclimation. Eucalyptus occidentalis showed significant acclimation of the pre-dawn leaf water potential at which the V'(cmax) and 'true' V(cmax) (accounting for mesophyll conductance) declined most steeply during drying-down. The findings indicate carbon loss under prolonged drought could be over-estimated without accounting for acclimation. In particular, (1) species from contrasting habitats differed in the magnitude of V'(cmax) reduction in short-term drought; (2) long-term drought allowed the possibility of acclimation, such that V'(cmax) reduction was mitigated; (3) xeric species showed a greater degree of V'(cmax) acclimation; and (4) photosynthetic acclimation involves hydraulic adjustments to reduce water loss while maintaining photosynthesis.

Growth, physiological, and biochemical responses of Camptotheca acuminata seedlings to different light environments

Light intensity critically affects plant growth. Camptotheca acuminata is a light-demanding species, but its optimum light intensity is not known. To investigate the response of C. acuminata seedlings to different light intensities, specifically 100% irradiance (PAR, 1500 ± 30 μmol m−2 s−1), 75% irradiance, 50% irradiance, and 25% irradiance, a pot experiment was conducted to analyze growth parameters, photosynthetic pigments, gas exchange, chlorophyll fluorescence, stomatal structure and density, chloroplast ultrastructure, ROS concentrations, and antioxidant activities. Plants grown under 75% irradiance had significantly higher total biomass, seedling height, ground diameter, photosynthetic capacity, photochemical efficiency, and photochemical quenching than those grown under 100%, 25%, and 50% irradiance. Malondialdehyde (MDA) content, relative electrolyte conductivity (REC), superoxide anion (O.−2) production, and peroxide (H2O2) content were lower under 75% irradiance. The less pronounced plant growth under 100% and 25% irradiance was associated with a decline in photosynthetic capacity and photochemical efficiency, with increases in the activity of specific antioxidants (i.e., superoxidase dismutase, peroxidase, and catalase), and with increases in MDA content and REC. Lower levels of irradiance were associated with significantly higher concentrations of chlorophyll (Chl) a and b and lower Chla/b ratios. Stomatal development was most pronounced under 75% irradiance. Modification of chloroplast development was found to be an important mechanism of responding to different light intensities in C. acuminata. The results indicated that 75% irradiance is optimal for the growth of C. acuminata seedlings. The improvement in C. acuminata growth under 75% irradiance was attributable to increased photosynthesis, less accumulation of ROS, and the maintenance of the stomatal and chloroplast structure.

ASPECTOS ECOFISIOLÓGICOS E CRESCIMENTO EM PLANTAS DE MILHO SUBMETIDAS AO ALAGAMENTO

Todos os aspectos de crescimento e desenvolvimento das plantas podem ser afetados pela deficiência hídrica e saturação de água nos tecidos, causada pela excessiva demanda evaporativa e/ou limitado suprimento de água. As conseqüências na planta pelo alagamento vão ser clorose, murchamento prematuro, queda da capacidade fotossintética, do potencial hídrico e da concentração de nutrientes nas folhas e diminuição no crescimento. O objetivo desse trabalho foi avaliar os efeitos do alagamento sob conteúdo relativo de água, transpiração, condutância estomática, relação raiz/ parte aérea e massa seca total em plantas de milho submetidas ao alagamento. O experimento foi conduzido em casa de vegetação da Universidade Federal Rural da Amazônia (UFRA - Capitão - Poço), utilizando-se plantas de milho variedade BR 5102, o delineamento experimental foi inteiramente casualizado (DIC) com fatorial 2 x 4, com duas condições hídricas: controle, déficit hídrico e quatro ciclos de estresse com cinco repetições. Obtiveram-se resultados significativos em todos os parâmetros avaliados, concluindo que o alagamento influenciou negativamente na ecofisiologia da planta e na massa seca total. PALAVRAS-CHAVE: anoxia, conteúdo relativo de água, transpiração.

Fuel treatment effects on soil chemistry and foliar physiology of three coniferous species at the Teakettle Experimental Forest, California, USA

A full factorial design crossing overstory (O) and understory (U) thinning and prescribed burning (B) was started at Teakettle Experimental Forest, California, in 2001 with the aim of achieving shifts in species composition to favor fire-resistant pines over fir. The goal of the present study was to evaluate the use of metabolic changes as early indicators for predicting the effects of these treatments on species-specific growth and long-term productivity of Pinus jeffreyi, Pinus lambertiana, and Abies concolor. Foliar polyamines, amino acids, soluble proteins and chlorophyll were quantified and related with foliar and soil chemistry. Most changes in soil chemistry occurred only with combined treatments (BU or BO). Changes in foliar metabolism were not driven by soil chemistry. In general, the pines were more sensitive to water stress as compared to fir as indicated by proline content. Understory thinning had no effect on any species. Both O and BO had species-specific effects on foliar amino acids, chlorophyll, soluble proteins, and inorganic ions. In addition, both O and BO were the most extreme thinning treatments for pines as they greatly increase the time needed to replenish carbon stocks. With the exception of some amino acids, B by itself had no major effects on foliar inorganic ions, chlorophyll content, and polyamines for any of the three species while BU caused a decrease in chlorophyll in white fir, suggesting a decline in photosynthetic capacity. These data point to BU as the best management option to specifically decrease the growth of fire-sensitive white fir without a major impact on pines.

Foliage type specific susceptibility to ozone in Picea abies, Pinus cembra and Larix decidua at treeline: A synthesis

Cumulative ozone uptake (COU, mmolm−2) and O3 flux (FO3, nmolm−2s−1) were related to physiological, morphological and biochemical characteristics of field-grown mature evergreen Norway spruce [Picea abies (L.) Karst.], Cembran pine [Pinus cembra L.], and deciduous European larch [Larix decidua Mill.] trees at treeline. The threshold COU causing a statistically significant decline in photosynthetic capacity (Amax) ranged between 19.6mmolm−2 in current-year needles of evergreen conifers and 22.0 6mmolm−2 in short-shoot needles of deciduous L. decidua subjected to exposure periods of ≥84 and ≥43 days, respectively. The higher O3 sensitivity of deciduous L. decidua than of evergreen P abies and P. cembra was associated with differences in FO3 and specific leaf area (SLA), both being significantly higher in L. decidua. FO3 was 5.9nmolm−2s−1 in L. decidua and 2.7nmolm−2s−1 in evergreen conifers. Species-dependent differences were also related to detoxification capacity expressed through total surface area based concentrations of reduced ascorbate and α-tocopherol that both increased with SLA. Findings suggest that differences in O3 sensitivity between evergreen and deciduous conifers can be attributed to foliage type specific differences in SLA, the latter determining physiological and biochemical characteristics of the treeline conifers.

Growth and Physiological Responses ofPhaseolusSpecies to Salinity Stress

This paper reports the changes on growth, photosynthesis, water relations, soluble carbohydrate, and ion accumulation, for two salt-tolerant and two salt-sensitivePhaseolusspecies grown under increasing salinity (0, 60 and 90 mM NaCl). After 20 days exposure to salt, biomass was reduced in all species to a similar extent (about 56%), with the effect of salinity on relative growth rate (RGR) confined largely to the first week. RGR of salt-tolerant species was reduced by salinity due to leaf area ratio (LAR) reduction rather than a decline in photosynthetic capacity, whereas unit leaf rate and LAR were the key factors in determining RGR on salt-sensitive species. Photosynthetic rate and stomatal conductance decreased gradually with salinity, showing significant reductions only in salt-sensitive species at the highest salt level. There was little difference between species in the effect of salinity on water relations, as indicated by their positive turgor. Osmotic adjustment occurred in all species and depended on higher K+, Na+, and Cl−accumulation. Despite some changes in soluble carbohydrate accumulation induced by salt stress, no consistent contributions in osmotic adjustment could be found in this study. Therefore, we suggest that tolerance to salt stress is largely unrelated to carbohydrate accumulation inPhaseolusspecies.

Effects of leaf aging and light duration on photosynthetic characteristics in a cucumber canopy

Photosynthetic characteristics, chlorophyll index and leaf area were examined in selected leaves of cucumber ( Cucumis sativus L. cv. Euphorbia). In the first experiment, plants of cucumber were grown horizontally at a lighting period of 20 h day −1. Photosynthetic measurements in horizontally growing cucumbers showed that there was no decline in photosynthetic capacity when cucumber leaves are developing under good light conditions. In a second experiment, plants were grown in a traditional high-wire cultivation system under 20 h day −1 lighting period until they reached final height and then exposed to different lighting periods (20 and 24 h day −1) for 3 weeks. In stands of cucumber plants photosynthetic measurements showed that the lower leaves have a significant reduction in photosynthetic capacity due to reduced light conditions. Three weeks exposure to 24 h day −1 lighting period reduced leaf area by 20%. Plant grown under continuous light had also lower chlorophyll index compared to plants grown under 20 h day −1 lighting period.

Long-term drought results in a reversible decline in photosynthetic capacity in mango leaves, not just a decrease in stomatal conductance

The negative effects of drought on plant growth, development of natural plant communities and crop productivity are well established, but some of the responses remain poorly characterized, particularly the effect of long-term drought on photosynthetic capacity. We hypothesized that long-term drought results in a decline in leaf photosynthetic capacity, and not just a decrease in diffusive conductance. To test this hypothesis, we studied the effect of drought, slowly developed over 3.5 months, in leaves of eight potted mango (Mangifera indica L.) trees. We found that photosynthesis was not only limited by stomatal closure, but was also downregulated as a consequence of a strong decrease in photosynthetic capacity assessed by the measurements of maximal net photosynthesis (A(max)) and the light-saturated rate of photosynthetic electron transport (J(max)). The rapid recovery of A(max) and J(max), after only 1 week of rewatering, the maintenance of a stable pool of leaf nitrogen throughout the trial, and the decrease in quantum efficiency of open centers of photosystem II, indicate that the photosynthetic machinery escaped photodamage in the drought-treated trees and was simply downregulated during drought. The hexose-to-sucrose ratio was higher in leaves from drought-treated trees than in control leaves, suggesting that photosynthetic capacity decreased as a consequence of sink limitation.

The different fates of mitochondria and chloroplasts during dark‐induced senescence in Arabidopsis leaves

Senescence is an active process allowing the reallocation of valuable nutrients from the senescing organ towards storage and/or growing tissues. Using Arabidopsis thaliana leaves from both whole darkened plants (DPs) and individually darkened leaves (IDLs), we investigated the fate of mitochondria and chloroplasts during dark-induced leaf senescence. Combining in vivo visualization of fates of the two organelles by three-dimensional reconstructions of abaxial parts of leaves with functional measurements of photosynthesis and respiration, we showed that the two experimental systems displayed major differences during 6 d of dark treatment. In whole DPs, organelles were largely retained in both epidermal and mesophyll cells. However, while the photosynthetic capacity was maintained, the capacity of mitochondrial respiration decreased. In contrast, IDLs showed a rapid decline in photosynthetic capacity while maintaining a high capacity for mitochondrial respiration throughout the treatment. In addition, we noticed an unequal degradation of organelles in the different cell types of the senescing leaf. From these data, we suggest that metabolism in leaves of the whole DPs enters a 'stand-by mode' to preserve the photosynthetic machinery for as long as possible. However, in IDLs, mitochondria actively provide energy and carbon skeletons for the degradation of cell constituents, facilitating the retrieval of nutrients. Finally, the heterogeneity of the degradation processes involved during senescence is discussed with regard to the fate of mitochondria and chloroplasts in the different cell types.

Growth, ion content, gas exchange, and water relations of wheat genotypes differing in salt tolerances

Although the mechanisms of salt tolerance in plants have received much attention for many years, genotypic differences influencing salt tolerance still remain uncertain. To investigate the key physiological factors associated with genotypic differences in salt tolerance of wheat and their relationship to salt stress, 13 wheat genotypes from Egypt, Australia, India, and Germany, that differ in their salt tolerances, were grown in a greenhouse in soils of 4 different salinity levels (control, 50, 100, and 150 mm NaCl). Relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR), photosynthesis, chlorophyll content (SPAD value), and leaf water relations were measured at Days 45 and 60 after sowing. Mineral nutrient content in leaves and stems was determined at Day 45 and final harvest. Salinity reduced RGR, NAR, photosynthetic rate, stomatal conductance, water and osmotic potentials, and K+ and Ca2+ content in stems and leaves at all times, whereas it increased leaf respiration, and Na+ and Cl– content in leaves and stems. LAR was not affected by salinity and the effect of salinity on SPAD value was genotype-dependent. Growth of salt-tolerant genotypes (Sakha 8, Sakha 93, and Kharchia) was affected by salinity primarily due to a decline in photosynthetic capacity rather than a reduction in leaf area, whereas NAR was the more important factor in determining RGR of moderately tolerant and salt-sensitive genotypes. We conclude that Na+ and Cl– exclusion did not always reflect the salt tolerance, whereas K+ in the leaves and Ca2+ in the leaves and stems were closely associated with genotypic differences in salt tolerance among the 13 genotypes even at Day 45. Calcium content showed a greater difference in salt tolerance among the genotypes than did K+ content. The genotypic variation in salt tolerance was also observed for the parameters involved in photosynthesis, and water and osmotic potentials, but not for turgor pressure.

PHOTOSYNTHETIC FUNCTION IN DUNALIELLA TERTIOLECTA (CHLOROPHYTA) DURING A NITROGEN STARVATION AND RECOVERY CYCLE

Phytoplankton can be exposed to periods of N starvation with episodic N resupply. N starvation in Dunaliella tertiolecta (Butcher) measured over 4 days was characterized by slow reduction in cell chl and protein content and chl/carotenoid ratio and a decline in photosynthetic capacity and maximum quantum yield of photosynthesis (Fv/Fm). In the early stages of N starvation, cell division was maintained despite reduction in cellular chl. Chl content was more sensitive than carotenoids to N deprivation, and cellular chl a was maintained preferentially over chl b under N starvation. NO3− resupply stimulated rapid and complete recovery of Fv/Fm (from 0.4 to 0.7) within 24 h and commencement of cell division after 10 h, although N‐replete levels of cell chl and protein were not reestablished within 24 h. Recovery of Fv/Fm was correlated with increases in cell chl and protein and was more related to increases in Fm than to changes in F0. Recovery of Fv/Fm was biphasic with a second phase of recovery commencing 4–6 h after resupply of NO3−. Uptake of NO3− from the external medium and the recovery of Fv/Fm, cell chl, and protein were inhibited when either cytosolic or chloroplastic protein synthesis was inhibited by cycloheximide or lincomycin, respectively; a time lag observed before maximum NO3− uptake was consistent with synthesis of NO3− transporters and assimilation enzymes. When both chloroplastic and cytosolic translation was inhibited, Fv/Fm declined dramatically. Dunaliella tertiolecta demonstrated a capacity to rapidly reestablish photosynthetic function and initiate cell division after N resupply, an important strategy in competing for limiting inorganic N resources.

Decline of photosynthetic capacity with leaf age and position in two tropical pioneer tree species

The effect of leaf age on photosynthetic capacity, a critical parameter in the theory of optimal leaf longevity, was studied for two tropical pioneer tree species, Cecropia longipes and Urera caracasana, in a seasonally dry forest in Panama. These species continuously produce short-lived leaves (74 and 93 d, respectively) during the rainy season (May-December) on orthotropic branches. However, they differ in leaf production rate, maximum number of leaves per branch, light environment experienced by the leaves, leaf mass per unit area, and nitrogen content. Light-saturated photosynthetic rates for marked leaves of known ages (±1 wk) were measured with two contrasting schemes (repeated measurements vs. chronosequence within branch), which overall produced similar results. In both species, photosynthetic rates and nitrogen use efficiency were negatively correlated with leaf age and positively correlated with light availability. Photosynthetic rates declined faster with leaf age in Cecropia than in Urera as predicted by the theory. The rate of decline was faster for leaves on branches with faster leaf turnover rates. Nitrogen per unit leaf area decreased with leaf age only for Urera. Leaf mass per unit area increased with leaf age, either partly (in Cecropia) or entirely (in Urera) due to ash accumulation.