Influence of Photoperiod and Leaf Age on Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq

The possibility that Crassulacean acid metabolism (CAM) is subject to long day photoperiodic control in Portulacaria afra (L.) Jacq., a facultative CAM plant, was studied. Periodic measurements of ¹⁴CO₂ uptake, stomatal resistance, and titratable acidity were made on plants exposed to long and short day photoperiods. Results indicates that waterstressed P. afra had primarily nocturnal CO₂ uptake, daytime stomatal closure, and a large diurnal acid fluctuation in either photoperiod. Mature leaf tissue from nonstressed plants under long days exhibited a moderate diurnal acid fluctuation and midday stomatal closure. Under short days, there was a reduced diurnal acid fluctuation in mature leaf tissue. Young leaf tissue taken from nonstressed plants did not utilize the CAM pathway under either photoperiod as indicated by daytime CO₂ uptake, lack of diurnal acid fluctuation, and incomplete daytime stomatal closure.

The induction of CAM in P. afra appears to be related to the water status of the plant and the age of the leaf tissue. The photosynthetic metabolism of mature leaves may be partly under the control of water stress and of photoperiod, where CAM is favored under long days.

     

Effect of plant growth promoting Bacillus subtilis and Pseudomonas fluorescens on growth and pigment composition of radish plants (Raphanus sativus) under NaCl stress

Soil salinity is one of the most severe factors limiting growth and physiological response in Raphanus sativus. In this study, the possible role of plant growth promoting bacteria (PGPB) in alleviating soil salinity stress during plant growth under greenhouse conditions was investigated. Increasing salinity in the soil decreased plant growth, photosynthetic pigments content, phytohormones contents (indole-3-acetic acid, IAA and gibberellic acid, GA₃) and mineral uptake compared to soil without salinity. Seeds inoculated with Bacillus subtilis and Pseudomonas fluorescens caused significantly increase in fresh and dry masses of roots and leaves, photosynthetic pigments, proline, total free amino acids and crude protein contents compared to noninoculated ones under salinity. The bacteria also increased phytohormones contents (IAA and GA₃) and the contents of N, P, K⁺, Ca²⁺, and Mg²⁺ but decreased ABA contents and Na⁺ and Cl^? content which may contribute in part to activation of processes involved in the alleviation of the effect of salt.     

Photosynthesis in the Higher Plant Vicia faba: V. Role of Malate as a Precursor of the Tricarboxylic Acid Cycle

In the higher plant Vicia faba, anomalous labeling patterns in the organic acids and related amino acids of the tricarboxylic acid cycle which result from photosynthetic ¹⁴CO₂ fixation (in conjunction with an enzyme localization pattern unique to plant mitochondria) suggest that the tricarboxylic acid cycle functions primarily as a pathway leading to glutamic acid biosynthesis during autotrophic growth. The distribution of isotope in citrate indicates little recycling of oxaloacetate for the resynthesis of citrate. Rather, malate appears to provide both the C₂ and C₄ fragments for the synthesis of citrate, and [³H]formate and ¹⁴CO₂-labeling patterns implicate serine as the ultimate C₃ precursor of malate.     

Role of the two PsaE isoforms on O2 reduction at photosystem I in Arabidopsis thaliana

Leaves of Arabidopsis thaliana plants grown in short days (8 h light) generate more reactive oxygen species in the light than leaves of plants grown in long days (16 h light). The importance of the two PsaE isoforms of photosystem I, PsaE1 and PsaE2, for O₂ reduction was studied in plants grown under these different growth regimes. In short day conditions a mutant affected in the amount of PsaE1 (psae1-1) reduced more efficiently O₂ than a mutant lacking PsaE2 (psae2-1) as shown by spin trapping EPR spectroscopy on leaves and by following the kinetics of P700⁺ reduction in isolated photosystem I. In short day conditions higher O₂ reduction protected photosystem II against photoinhibition in psae1-1. In contrast in long day conditions the presence of PsaE1 was clearly beneficial for photosynthetic electron transport and for the stability of the photosynthetic apparatus under photoinhibitory conditions. We conclude that the two PsaE isoforms have distinct functions and we propose that O₂ reduction at photosystem I is beneficial for the plant under certain environmental conditions.     

Inorganic Carbon Source for Photosynthesis in the Seagrass Thalassia hemprichii (Ehrenb.) Aschers

Photosynthetic carbon uptake of the tropical seagrass Thalassia hemprichii (Ehrenb.) Aschers was studied by several methods. Photosynthesis in buffered seawater in media in the range of pH 6 to pH 9 showed an exponentially increasing rate with decreasing pH, thus indicating that free CO₂ was a photosynthetic substrate. However, these experiments were unable to determine whether photosynthesis at alkaline pH also contained some component due to HCO₃⁻ uptake. This aspect was further investigated by studying photosynthetic rates in a number of media of varying pH (7.8-8.61) and total inorganic carbon (0.75-13.17 millimolar). In these media, photosynthetic rate was correlated with free CO₂ concentration and was independent of the HCO₃⁻ concentration in the medium. Short time-course experiments were conducted during equilibration of free CO₂ and HCO₃⁻ after injection of ¹⁴C labeled solution at acid or alkaline pH. High initial photosynthetic rates were observed when acidic solutions (largely free CO₂) were used but not with alkaline solutions. The concentration of free CO₂ was found to be a limiting factor for photosynthesis in this plant.     

Physiological and Biochemical Mechanisms Preventing Cd Toxicity in the New Hyperaccumulator <Emphasis Type="Italic">Abelmoschus manihot</Emphasis>

Abelmoschus manihot, an ornamental plant, was examined for phytoremediation purposes in accordance with the ability to accumulate cadmium and physiological mechanisms of cadmium tolerance. A net photosynthetic rate (A _N) glasshouse experiment for 60 days was conducted to investigate the influence of different cadmium amounts (0–100 mg kg^?1) on the growth, biomass, photosynthetic performance, reactive oxygen species (ROS) production, antioxidative enzyme activities, Cd uptake and accumulation of A. manihot. Exposure to cadmium enhanced plant growth even at 100 mg kg^?1, without showing symptoms of visible damage. The cadmium concentration of shoots (stems or leaves) and roots was more than the critical value of 100 mg kg^?1 and reached 126.17, 185.26 and 210.24 mg kg^?1, respectively. BCF values of A. manihot plants exceeded the reference value 1.0 for all the Cd treatments, and TF values were greater than 1 at 15–60 mg kg^?1 Cd treatment. The results also showed that cadmium concentrations of 60 mg kg^?1 or less induced a significant enhancement in plant net photosynthetic rate (A _N), stomatal conductance (G _s), transpiration rate (T _r), photosynthetic pigments and F _v/F _m. These parameters were slightly decreased at the higher concentration (100 mg kg^?1). The ROS production (O₂ ^?, H₂O₂) and antioxidative response including SOD, CAT and POD were significantly enhanced by increasing cadmium. These results suggest that A. manihot can be considered as a Cd-hyperaccumulator and the hormetic effects may be taken into consideration in remediation of Cd contamination soil.     

Glycolic Acid Labeling During Photosynthesis with CO(2) and Tritiated Water

Chlorella pyrenoidosa were allowed to photosynthesize for short periods of time in the presence of ¹⁴CO₂ and HTO. Analysis of tritium and ¹⁴C labeling of photosynthetic intermediate compounds showed that the T/¹⁴C ratio of glycolic acid was comparable to that of intermediate compounds of the photosynthetic carbon reduction cycle when photosynthesis was performed in nearly 100% oxygen and only slightly higher under steady-state conditions. It is concluded that formation of labeled glycolic acid as a consequence of its proposed hydrogen transport role in photosynthesis is quantitatively of limited importance compared to the net synthesis of glycolic acid from CO₂.     

Effect of Ca/Mg ions ratio on copper accumulation, photosynthetic activity and growth of Cu2+-treated Salix viminalis L. ‘Cannabina’

The aim of the study was to the assess the influence of Ca/Mg ions ratio on the photosynthetic activity of Salix viminalis L. ??Cannabina?? plants cultivated in medium enriched with Cu(NO₃)₂. The experiment was conducted in controlled conditions in a phytotron for 21 days; hence the early plant response was tested. Plants were cultivated with different Ca/Mg ions ratios, i.e. (4:1)_l, (4:1)_h, and 1:10. Plants were additionally treated with Cu(NO₃)₂ at 1, 2, and 3 mM concentration in cultivation medium. Net photosynthetic rate, stomatal conductance and transpiration were measured after the first, second and third week of cultivation. Additionally, chlorophyll content, leaf morphology, root biomass and copper accumulation in leaves and roots were investigated. The investigations revealed differences in plant response to particular treatments ?? differences in Cu accumulation for particular Ca/Mg ions ratios were detected. It seems that plants are adapted to high Cu²⁺ concentrations, when 1:10 Ca/Mg ions ratio is applied. The highest Cu accumulation in roots was noted for plants fertilized with 1:10 Ca/Mg ions ratio, together with high Cu translocation to above-ground plant organs, which suggests its higher potential in phytoremediation.     

Effects of gradual versus step increases in carbon dioxide on Plantago photosynthesis and growth in a microcosm study

This study investigated the effects of a gradual versus step increases in carbon dioxide (CO₂) on plant photosynthesis and growth at two nitrogen (N) levels. Plantago lanceolata were grown for 80 days and then treated with the ambient CO₂ (as the control), gradual CO₂ increase and step CO₂ increase as well as low and high N additions for 70 days. While [CO₂] were kept at constant 350 and 700 μmol mol⁻¹ for the ambient and step CO₂ treatments, respectively, [CO₂] in the gradual CO₂ treatment was raised by 5 μmol mol⁻¹ day⁻¹, beginning at 350 μmol mol⁻¹ and reaching 700 μmol mol⁻¹ by the end of experiment. The step CO₂ treatment immediately resulted in an approximate 50% increase in leaf photosynthetic carbon fixation at both the low and high N additions, leading to a 20–24% decrease in leaf N concentration. The CO₂-induced nitrogen stress, in return, resulted in partial photosynthetic downregulation since the third week at the low N level and the fourth week at the high N level after treatments. In comparison, the gradual CO₂ treatment induced a gradual increase in photosynthetic carbon fixation, leading to less reduction in leaf N concentration. In comparison to the ambient CO₂, both the gradual and step CO₂ increases resulted in decreases in specific leaf area, leaf N concentration but an increase in plant biomass. Responses of plant shoot:root ratio to CO₂ treatments varied with N supply. It decreased with low N supply and increased with high N supply under the gradual and step CO₂ treatments relative to that under the ambient CO₂. Degrees of those changes in physiological and growth parameters were usually larger under the step than the gradual CO₂ treatments, largely due to different photosynthetic C influxes under the two CO₂ treatments.     

Photosynthetic acclimation in shade-developed leaves of Euterpe edulis Mart (Arecaceae) after long-term exposure to high light

To analyze acclimation of Euterpe edulis seedlings to changes in light availability, we transferred three-year-old seedlings cultivated for six months under natural shade understory [≈ 1.3 mol(photon) m^?2 d^?1] to a forest gap [≈ 25.0 mol(photon) m^?2 d^?1]. After the transfer, changes in chlorophyll fluorescence and leaf gas-exchange parameters, as well as in the light-response curves of photosynthesis and photosynthetic induction parameters, were analyzed during the following 110 days. Simultaneously measured photosynthetic characteristics in the shaded seedlings grown in understory served as the control. Despite the fact that the understory seedlings were under suboptimal conditions to achieve their light-saturated net photosynthetic rate (P _Nmax), light-response curves and photosynthetic induction parameters indicated that the species had the low respiration rate and a fast opening of stomata in response to the intermittent occurrence of sunflecks, which exerted a feed-forward stimulation on P _Nmax. Sudden exposure to high light induced photoinhibition during the first week after the transfer of seedlings to gap, as it was shown by the abrupt decline of the maximal quantum yield of PSII photochemistry (F_v/F_m). The photoinhibition showed the time-dependent dynamics, as the F_v/F_m of the seedlings transferred to the forest gap recovered completely after 110 days. Furthermore, the net photosynthetic rate increased 3.5-fold in relation to priorexposure values. In summary, these data indicated that more than 21 days was required for the shade-acclimated seedlings to recover from photoinhibition and to relax induction photosynthetic limitations following the sudden exposure to high light. Moreover, the species responded very quickly to light availability; it highlights the importance of sunflecks to understory seedlings.     

The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves: III. A Comparison between Some C(3) and C(4) Plants

The chlorophyll-based specific activity of cytochrome oxidase and three exclusively mitochondrial enzymes of the tricarboxylic acid cycle showed little variation between leaves of C₃ and C₄ plants or between mesophyll and bundle sheath cells of Atriplex spongiosa and Sorghum bicolor. However, a large, light-dependent transfer of label from intermediates of the tricarboxylic acid cycle to photosynthetic products was a feature of leaves of C₄ plants. This light-dependent transfer of label was barely detectable in leaves of C₃ plants and in leaves of F₁ and F₃ hybrids of Atriplex rosea (C₄) and Atriplex patula spp hastata (C₃). The light-dependent transfer of label to photosynthetic products in leaves of C₄ plants was inhibited by the tricarboxylic acid cycle inhibitors malonate and fluoroacetate. The requirement for continued tricarboxylic acid cycle activity was also indicated in experiments with specifically labeled succinate-¹⁴C. These experiments, together with the distribution of ¹⁴C in glucose prepared from sucrose-¹⁴C formed during the metabolism of succinate-2,3-¹⁴C, confirmed that the photosynthetic metabolism of malate and aspartate derived from the tricarboxylic acid cycle, and not the refixation of respiratory CO₂, was the main path of carbon from the cycle to photosynthesis.     

Effect of Triacontanol on Chlamydomonas: I. Stimulation of Growth and Photosynthetic CO(2) Assimilation

Treatment of Chlamydomonas reinhardtii cells, cultured at 5% CO₂, with 1 to 1000 micrograms triacontanol (TRIA) per liter resulted in 21 to 35% increases in cell density, 7 to 31% increases in total chlorophyll, and 20 to 100% increases in photosynthetic CO₂ assimilation. The increase in CO₂ fixation with TRIA treatment occurred before, and was independent of, increases in total chlorophyll or cell number. Chlamydomonas cells responded to a broad range of TRIA concentrations that were at least one order of magnitude above the optimum concentration established for higher plants. The necessity for larger concentrations of TRIA may be due to destabilizing effects of Ca²⁺ and K⁺ present in the Chlamydomonas growth medium. These ions caused flocculation of the colloidally dispersed TRIA in apparent competition with binding of [¹⁴C]TRIA to Chlamydomonas cells. Octacosanol inhibited the effect of TRIA on photosynthetic CO₂ assimilation. TRIA treatment did not alter the distribution of ¹⁴C-label among photosynthetic products. The effect of TRIA on photosynthetic CO₂ assimilation increased with time after treatment up to 3 days. Chlamydomonas cells that had been grown at low-CO₂ (air) did not respond to TRIA, and transfer of high-CO₂ (5%) grown cells that had responded to TRIA to a low-CO₂ atmosphere resulted in a loss of the effect of TRIA. The effect of pH on photosynthetic CO₂ assimilation indicated that CO₂ is probably the species of inorganic carbon utilized by control and TRIA-treated Chlamydomonas cells.     

Salicylic acid may be involved in the regulation of drought-induced leaf senescence in perennials: A case study in field-grown Salvia officinalis L. plants

Leaf senescence is a highly regulated physiological process that contributes to nutrient remobilization during stress, thus allowing the rest of the plant to benefit from the nutrients accumulated during the life span of the leaf. Here we studied drought-induced leaf senescence in a perennial plant, common sage (Salvia officinalis L.) grown under Mediterranean field conditions, with an emphasis on the possible involvement of the phytohormones, salicylic acid and jasmonic acid in the process. The initial stages of leaf senescence (0–27 days of water deficit) were characterized by salicylic acid accumulation (by 80%) and decrease of jasmonic acid levels (by 40%), which occurred in parallel with a severe loss of photosynthetic pigments (up to 65%) and increases in the de-epoxidation state (DPS) of the xanthophyll cycle (by 55%), while the maximum efficiency of photosystem II (F_v/F_m ratio) was maintained above 0.80, thus indicating the absence of damage to the photosynthetic apparatus. The latest stages of leaf senescence (until 42 days of water deficit) were instead characterized by maintenance of the levels of jasmonic acid and salicylic acid, while β-carotene and the F_v/F_m ratio decreased significantly, which was followed by cell death. Exogenous applications of methyl salicylic acid in leaves of water-stressed plants led to reductions in chlorophyll levels, thus confirming the promoting effects of salicylic acid on leaf senescence. It is therefore concluded that salicylic acid may be involved, together with other phytohormones, in the regulation of drought-induced leaf senescence in perennials.     

Effects of cytokinin on photosynthetic gas exchange, chlorophyll fluorescence parameters and antioxidative system in seedlings of eggplant (Solanum melongena L.) under salinity stress

The effects of 6-benzyladenine (6-BA) on plant growth, photosynthetic gas exchange, chlorophyll fluorescence and antioxidant systems of eggplant (Solanum melongena L.) under salt stress were investigated. Eggplant seedlings were exposed to 90?mM NaCl with four levels of 6-BA (5, 10, 20 and 50???M) for 10?days. 6-BA at lower concentrations increased chlorophyll concentration, the net photosynthetic rate (P _N), stomatal conductance (g _s), and transpiration rate (E), intercellular CO₂ concentration (C _i) and water use efficiency (WUE), as well as the quantum efficiency of PSII photochemistry (??PSII), photochemical quenching (q _p), and decreased non-photochemical quenching (NPQ), while higher concentrations reduced the effects or even exacerbated the occurrence of photosynthetic capacity. The activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) increased significantly during salt treatments, and induced the increase of the activities of these enzymes at certain concentrations of 6-BA. 6-BA also reduced significantly malonaldehyde (MDA) contents and O ₂ ^·? production. It was concluded that 6-BA could alleviate the detrimental effects of salt stress on plant growth by increasing photosynthetic efficiency and enhancing antioxidant enzyme systems in leaves at a proper concentration and of the varying 6-BA concentrations used, the most effective concentration for promoting growth was 10???M under saline conditions.     

Ecological studies on the timberline of Mt. Fuji

An ecological study of dry matter production was made in a dwarf forest dominated byAlnus maximowiczii at the timberline of Mt. Fuji. Annual gross production was estimated by two methods, namely the summation method using stem analysis and total photosynthesis calculated from leaf area and photosynthetic rate per leaf area. Seasonal changes in relative light intensity and in leaf area were measured in a quadrat. Photosynthesis and respiration rates of samples were measured in temperature-regulated assimilation chambers. The phytomass was 2,989 g d.w.m^?2, and those of stems and branches, leaves, and roots were 1,672 g, 293 g, and 1,024 g respectively. The growing period of this plant was about four months and this plant expanded leaves quickly. The maximum gross photosynthetic rate was 21 mg CO₂dm^?2 h^?1 on September 1. Annual net production estimated by examining the annual rings was 922 g d.w.m^?2 year^?1 and annual respiration was 735 g. Annual gross production estimated from photosynthetic rates was 1,747 g d.w.m^?2 year^?1. The sum of annual net production by stem analysis and respiration agree closely with gross production estimated from photosynthetic rate. Gross production of this dwarf forest is comparable to the beech forest of the upper cool temperate zone owing to the high photosynthetic rate ofAlnus maximowiczii.     

Effect of bicarbonate treatment on photosynthetic assimilation of inorganic carbon in two plant species of Moraceae

Excessive levels of bicarbonate adversely affect the growth and metabolism of plants. Broussonetia papyrifera (L.) Vent. and Morus alba L., belonging to family Moraceae, possess the favorable characteristics of rapid growth and adaptability to adverse environments. We examined the response of these two plant species to bicarbonate stress in terms of photosynthetic assimilation of inorganic carbon. They were exposed to 10 mM sodium bicarbonate in the culture solution for 20 days. The photosynthetic response was determined by measuring the net photosynthetic rate of the leaf, water-use efficiency, and chlorophyll fluorescence on days 10 and 20. The bicarbonate-use capacity of the plants was studied by measuring the carbonic anhydrase activity and the compositions of the stable carbon and hydrogen isotopes. The photosynthetic response to high concentration of bicarbonate varied with plant species and treatment durations. High concentrations of bicarbonate decreased the photosynthetic assimilation of inorganic carbon in the two plant species to half that in the control plants on day 10. Bicarbonate treatment did not cause any damage to the reaction centers of photosystem II in Morus alba; it, however, caused a decline in the quantum efficiency of photosystem II in B. papyrifera on day 20. Moreover, B. papyrifera had a greater bicarbonate-use capacity than M. alba because carbonic anhydrase converted bicarbonate to CO₂ and H₂O to a greater extent in B. papyrifera. This study showed that the effect of bicarbonate on photosynthetic carbon metabolism in plants was dual. Therefore, the concentration of bicarbonate in the soil should first be considered during afforestation and ecological restoration in karst areas.     

Photosynthetic physiological performance and proteomic profiling of the oleaginous algae <Emphasis Type="Italic">Scenedesmus acuminatus</Emphasis> reveal the mechanism of lipid accumulation under low and high nitrogen supplies

In this study, we presented cellular morphological changes, time-resolved biochemical composition, photosynthetic performance and proteomic profiling to capture the photosynthetic physiological response of Scenedesmus acuminatus under low nitrogen (3.6 mM NaNO₃, N^?) and high nitrogen supplies (18.0 mM NaNO₃, N⁺). S. acuminatus cells showed extensive lipid accumulation (53.7% of dry weight) and were enriched in long-chain fatty acids (C16 & C18) under low nitrogen supply. The activity of PSII and photosynthetic rate decreases, whereas non-photochemical quenching and dark respiration rates were increased in the N^? group. In addition, the results indicated a redistribution of light excitation energy between PSII and PSI in S. acuminatus exists before lipid accumulation. The iTRAQ results showed that, under high nitrogen supply, protein abundance of the chlorophyll biosynthesis, the Calvin cycle and ribosomal proteins decreased in S. acuminatus. In contrast, proteins associated with the photosynthetic machinery, except for F-type ATPase, were increased in the N⁺ group (N⁺, 3 vs. 9 days and 3 days, N⁺ vs. N^?). Under low nitrogen supply, proteins involved in central carbon metabolism, fatty acid synthesis and branched-chain amino acid metabolism were increased, whereas the abundance of proteins of the photosynthetic machinery had decreased, with exception of PSI (N^?, 3 vs. 9 days and 9 days, N⁺ vs. N^?). Collectively, the current study has provided a basis for the metabolic engineering of S. acuminatus for biofuel production.     

Effect of elevated CO2 concentration and nitrate: ammonium ratios on gas exchange and growth of cassava (Manihot esculenta Crantz)

Aims

This study evaluated how different nitrogen forms affect growth and photosynthetic responses of cassava to CO₂ concentration.

Methods

Cassava was grown in 14-L pots in a greenhouse at 390 or 750 ppm of CO₂. Three nitrogen treatments were applied: (a) 12?mM NO₃ ^?, (b) 6?mM NO₃ ^??+?6?mM NH₄ ⁺, and (c) 12?mM NH₄ ⁺.

Results

Thirty-six days after treatments began, plants grown under elevated CO₂ and fertilized only with NO₃ ^? (750_NO₃ ^?) had photosynthetic rates similar to plants grown under 390_NO₃ ^?, indicating significant photosynthetic acclimation to CO₂. In contrast, photosynthetic rates at elevated CO₂ increased as NH₄ ⁺ increased in the nutrient solution, such that photosynthetic acclimation was reduced for plants fertilized with only NH₄ ⁺. However, this positive effect of NH₄ ⁺ on photosynthesis was not observed in more advanced growth stages, and the toxic effects of NH₄ ⁺ severely reduced total dry mass for these plants measured at the end of the experiment.

Conclusions

Our results indicate that cassava will respond with increased biomass accumulation in response to raising atmospheric CO₂ levels, and that N form can have an important impact on the photosynthetic response. However, the positive effect of NH₄ ⁺ fertilization on cassava photosynthetic CO₂ response eventually led to a toxicity problem that reduced biomass production. The challenge is to determine how to manage NH₄ ⁺ fertilization so that the photosynthetic benefit observed in the initial phase may persist throughout the crop cycle.     

Changes in photosynthesis caused by adaptation of maize seedlings to short-term drought

Detached leaf is in the state of increasing water deficit; it is a good experimental model for looking into the hardening effect of adaptation of eight-day-old maize (Zea mays L.) seedlings to short-term drought (five days without watering). The light stage of photosynthesis and photosynthetic CO₂/H₂O exchange in detached leaves were studied. Specific surface density of leaf tissue (SSDL), the content of chlorophylls a and b, proline, MDA as well as photosynthetic parameters: quantum yield of photosystem II fluorescence, assimilation of CO₂, and transpiration at room temperature and light saturation (density of PAR quantum flux of 2000 μmol/(m² s)) at normal and half atmospheric CO₂ concentration were determined. The leaves of seedlings exposed to short-term drought differed from control material by a greater SSDL and higher content of proline. The hardening effect of the stress agent on the dark stage of photosynthesis was detected; it was expressed in the maintenance of the higher photosynthetic CO₂ assimilation against control material due to the elevation of stomatal conductance for CO₂ diffusing into the leaf. Judging from the lack of differences in the MDA content, short-term drought did not injure photosynthetic membranes. In detached leaves of experimental maize seedlings, photosynthesis was maintained on a higher level than in control material.     

Developmental Control of CAM in Peperomia scandens

Experiments were conducted to examine the development of photosynthetic carbon metabolism in Peperomia scandens, a tropical epiphyte. Leaves were sampled during a 10-day period when they were between 30 to 165 days old. P. scandens exhibits a C₃ to CAM-cycling to CAM shift during maturation with the magnitude of CAM increasing with age. Initially, during both day and night, no significant CO₂ uptake or diurnal acid flux was evident. C₃ gas exchange was detected at 41 days of age with a gradual shift towards CAM gas exchange maximized thereafter. An acidity flux of 130 to 150 microequivalents per gram fresh weight was evident by 41 days. Between 40 and 90 days, the leaves shifted their CO₂ uptake pattern from a daytime to a nighttime peak. After 90 days, the leaves remained in CAM. The δ¹³C values became progressively less negative as the leaves matured. In the 30-day-old leaves, the δ¹³C value was −21.1% while in the 165-day-old leaves the δ¹³C value was −18.3%. The time-dependent shift from C₃ to CAM-cycling to CAM in P. scandens does not appear to result from changes in water, light, or temperature regimes since these variables were constant for all leaves sampled.