Effects of water stress and high temperature on gas exchange and chlorophyll fluorescence in Triticum aestivum L.

Wheat plants grown in controlled growth chambers were exposed to drought stress (DS) and high temperature (HT) singly and in combination (DS+HT). The effects of these two stresses on net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), quantum efficiency of photosystem 2 (Φ_PS2), variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m), photochemical (q_p) and non-photochemical (NPQ) Chl fluorescence, and yield were investigated. Grain yield was decreased by 21 % due to DS, while it was increased by 26 % due to HT. P _N, g _s, C _i, and Chl fluorescence were dramatically reduced to DS, HT, and their interaction, except NPQ which showed an increase due to HT.     

Photosynthetic activity of homoiochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> during dehydration and rehydration

The functional state of the photosynthetic apparatus of flowering homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and subsequent rehydration was investigated in order to characterize some of the mechanisms by which resurrection plants survive drought stress. The changes in the CO₂ assimilation rate, chlorophyll fluorescence parameters, thermoluminescence, fluorescence imaging and electrophoretic characteristics of the chloroplast proteins were measured in control, moderately dehydrated (50% water content), desiccated (5% water content) and rehydrated plants. During the first phase of desiccation the net CO₂ assimilation decline was influenced by stomatal closure. Further lowering of net CO₂ assimilation was caused by both the decrease in stomatal conductance and in the photochemical activity of photosystem II. Severe dehydration caused inhibition of quantum yield of PSII electron transport, disappearance of thermoluminescence B band and mainly charge recombination related to S₂Q_A⁻ takes place. The blue and green fluorescence emission in desiccated leaves strongly increased. It could be suggested that unchanged chlorophyll content and amounts of chlorophyll–proteins, reversible modifications in PSII electron transport and enhanced probability for non-radiative energy dissipation as well as increased polyphenolic synthesis during desiccation of Haberlea contribute to drought resistance and fast recovery after rehydration.     

Brassinosteroids accelerate recovery of photosynthetic apparatus from cold stress by balancing the electron partitioning,carboxylation and redox homeostasis in cucumber

The aim of this study was to examine the role of brassinosteroids (BRs) in protecting the photosynthetic apparatus from cold‐induced damage in cucumber (Cucumis sativus) plants. Recovery at both high light (HL) and low light (LL) after a cooling at 10/7°C induced irreversible inhibition of CO₂ assimilation, photoinhibition at photosystem I (PSI) and inhibition of enzyme activities of Calvin cycle and ascorbate (AsA)‐reduced glutathione (GSH) cycle, followed by accumulation of H₂O₂ and malondialdehyde. However, cold‐induced photoinhibition at PSII was fully recovered at LL but not at HL. Meanwhile, recovery at HL increased electron flux to O₂‐dependent alternative pathway [Ja(O₂‐dependent)]. Foliar application of 24‐epibrassinolide (EBR) accelerated recovery from photoinhibition of PSII but not of PSI. EBR also significantly increased CO₂ assimilation, activity of Calvin cycle enzymes and electron flux to carbon reduction [Je(PCR)], with a concomitant decrease in Ja(O₂‐dependent); meanwhile EBR increased the activity of enzymes in AsA‐GSH cycle and cellular redox states. However, the positive effect of EBR on plant recovery was observed only at HL, but not LL. These results indicate that BR accelerates the recovery of photosynthetic apparatus at HL by activation of enzymes in Calvin cycle and increasing the antioxidant capacity, which in turn mitigate the photooxidative stress and the inhibition of plant growth during the recovery.     

Dynamic modelling of limitations on improving leaf CO2 assimilation under fluctuating irradiance

A dynamic model of leaf CO₂ assimilation was developed as an extension of the canonical steady‐state model, by adding the effects of energy‐dependent non‐photochemical quenching (qE), chloroplast movement, photoinhibition, regulation of enzyme activity in the Calvin cycle, metabolite concentrations, and dynamic CO₂ diffusion. The model was calibrated and tested successfully using published measurements of gas exchange and chlorophyll fluorescence on Arabidopsis thaliana ecotype Col‐0 and several photosynthetic mutants and transformants affecting the regulation of Rubisco activity (rca‐2 and rwt43), non‐photochemical quenching (npq4‐1 and npq1‐2), and sucrose synthesis (spsa1). The potential improvements on CO₂ assimilation under fluctuating irradiance that can be achieved by removing the kinetic limitations on the regulation of enzyme activities, electron transport, and stomatal conductance were calculated in silico for different scenarios. The model predicted that the rates of activation of enzymes in the Calvin cycle and stomatal opening were the most limiting (up to 17% improvement) and that effects varied with the frequency of fluctuations. On the other hand, relaxation of qE and chloroplast movement had a strong effect on average low‐irradiance CO₂ assimilation (up to 10% improvement). Strong synergies among processes were found, such that removing all kinetic limitations simultaneously resulted in improvements of up to 32%.     

The acceptor side of photosystem II is damaged more severely than the donor side of photosystem II in ‘Honeycrisp’ apple leaves with zonal chlorosis

To investigate the photoinhibition of photosynthesis in ‘Honeycrisp’ apple (Malus domestica Borkh. cv. Gala) leaves with zonal chlorosis, we compared pigments, CO₂ assimilation and chlorophyll (Chl) a fluorescence (OJIP) transient between chlorotic leaves and normal ones. Chl and carotenoids (Car) contents, Chl a/b ratio, and absorptance were lower in chlorotic leaves than in normal ones, whereas Car/Chl ratio was higher in the former. Although CO₂ assimilation and stomatal conductance were lower in chlorotic leaves, intercellular CO₂ concentration did not differ significantly between the two leaf types. Compared with normal leaves, chlorotic ones had increased deactivation of oxygen-evolving complexes (OEC), minimum fluorescence (F _o), dissipated energy, relative variable fluorescence at L-, W-, J- and I-steps, and decreased maximum fluorescence (F _m), maximum quantum yield for primary photochemistry (F _v /F _m or TR_o/ABS), quantum yield for electron transport (ET_o/ABS), quantum yield for the reduction of end acceptors of photosystem I (PSI) (φ_Ro and RE_o/ABS), maximum amplitude of IP phase, amount of active photosystem II (PSII) reaction centers (RCs) per cross section (CS) and total performance index (PI_tot,abs). In conclusion, photoinhibition occurs at both the donor (i.e., the OEC) and the acceptor sides of PSII in chlorotic leaves. The acceptor side is damaged more severely than the donor side, which possibly is the consequence of over-reduction of PSII due to the slowdown of Calvin cycle. In addition to decreasing light absorptance by lowering Chl level, energy dissipation is enhanced to protect chlorotic leaves from photo-oxidative damage.     

Drought tolerance of photosynthetic electron transport under CO2‐enriched and normal air in cereal species

The quantum yield of photosynthetic electron transport (ΦPSII), evaluated by means of chlorophyll (Chl) fluorescence analysis, has proven to be a useful screening test for drought tolerance in durum wheat (Triticum durum Desf.). To explore the potential of this parameter further in detecting drought-tolerant genotypes, three cereal species were studied; ΦPSII measurements were carried out under two different gas mixtures, at three points of the induction curve (to obtain the maximal ΦPSII and both the transient and steady-state actual ΦPSII), and at three different water stress levels (moderate, severe and drastic). The species investigated were durum and bread wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.); two cultivars per species, characterized by different levels of drought tolerance, were tested. The two gas mixtures used were normal air (21% O₂, 0.035% CO₂ in N₂) to monitor the whole photosynthetic process under physiological conditions, and CO₂ enriched-low O₂ air (1% O₂, 5% CO₂ in N₂) to monitor ΦPSII reduction under stress mainly related to Calvin cycle activity. When ΦPSII related to both assimilatory and non-assimilatory metabolism was evaluated, the cultivar differences observed under normal Air were more representative of the agronomic performance upon drought stress than under high CO₂-low O₂ air. Maximal ΦPSII showed no difference among either cultivars, gas mixtures or stress levels, the efficiency of excitation capture being highly resistant to drought. The ΦPSII evaluated during the transient yielded predictable values in respect of drought tolerance for durum wheat and barley cultivars, highlighting the key role of regulatory processes such as the Mehler peroxidase reaction and possibly also cyclic electron transport, in preventing overreduction under stress. The results clearly show that when Chl fluorescence analysis is used as a parameter in plant breeding, different experimental conditions should be used depending on the physiological mechanism that is bred or selected for.     

Photoprotective function of photorespiration in Reaumuria soongorica during different levels of drought stress in natural high irradiance

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as H₂O₂ content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub. The rate of photorespiration was estimated by combined measurement of gas exchange and Chl fluorescence. The rate of photorespiration increased with the increasing drought stress (DS). The ratio of carboxylation electron flow to oxygenation electron flow (J_c/J_o) and the maximal photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m) decreased with the increasing DS. F_v/F_m in isonicotinic acid hydrazide (INH)-sprayed plants was lower than that in normal plants under moderate DS, but no significant difference was observed under severe DS. H₂O₂ content in INH-sprayed plants was significantly lower than that in normal plants under severe DS. Taken together, photorespiration in R. soongorica consumed excess electrons and protected photosynthetic apparatus under moderate DS, whereas it accelerated H₂O₂ accumulation markedly and induced the leaf abscission under severe DS.     

Role of xanthophyll cycle-mediated photoprotection in <Emphasis Type="Italic">Arbutus unedo</Emphasis> plants exposed to water stress during the Mediterranean summer

We analyzed the response of potted strawberry tree (Arbutus unedo L.) seedlings exposed to water stress by withholding water for 10 d (WS). Leaf water potential, net CO₂ assimilation, and stomatal conductance decreased with increasing water deficit. A 30 % reduction of chlorophyll (Chl) content in the antenna complexes was observed in WS-plants. Simultaneously, a decline of photochemical efficiency (F_v/F_m) occurred as a result of an excess of solar radiation energy when carbon assimilation was limited by stomata closure due to soil water deficit. The non-photochemical quenching of Chl fluorescence (Φ_NPQ) significantly increased, as well as the leaf contents of zeaxanthin (Z) and antheraxanthin (A) at the expense of violaxanthin during the WS-period. Elevated predawn contents of de-epoxidized xanthophyll cycle components were associated with a sustained lowering of predawn photosystem 2 efficiency; this suggested an engagement of Z+A in a state primed for energy dissipation. Thus, the ability of strawberry trees to maintain the functionality of the xanthophyll cycle during the Mediterranean summer is an efficient mechanism to prevent irreversible damages to the photosynthetic machinery through thermal energy dissipation in the antenna and the reduction in photochemical efficiency.     

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Reaumuria soongorica</Emphasis> during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (P _N), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.     

Drought effects on photosynthesis of the mangrove, Avicennia germinans, under contrasting salinities

 Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, μmol CO₂ m⁻² s⁻¹) declined during the dry season from 11.9±1.8 to 7.0±1.5 and from 9.6±2.0 to 4.7±2.5 in plants from low and high salinity sites, respectively. Changes in carbon assimilation per unit of chlorophyll (A/Chl, mmol CO₂ mol⁻¹ Chl) were from 31.6±4.7 to 20.5±4.3 and from 21.9±4.7 to 15.2±8.2 in the low and high salinity plants, respectively. Therefore, neither changes in Chl nor seasonal differences in photoprotective down regulation could account fully for the decrease in leaf photosynthesis during drought. A reduction in CO₂ diffusion due to lowered stomatal conductance was not large enough to explain such a dramatic effect of drought on leaf photosynthesis. Stomatal response could be mitigated by the capability of A. germinans for osmotic adjustment under high salinity and/or drought. However, this intracellular salt accumulation may reduce carbon assimilation capacity further by decreasing the metabolism of leaf cells, increasing dark respiration and/or photorespiration. Received: 10 June 1998 / Accepted: 5 October 1998     

Effects of Nitrogen Deficiency on Gas Exchange,Chlorophyll Fluorescence,and Antioxidant Enzymes in Leaves of Rice Plants

Gas exchange, chlorophyll (Chl) fluorescence, and contents of photosynthetic pigments, soluble proteins (ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBPCO), and antioxidant enzymes were characterized in the fully expanded 6^th leaves in rice seedlings grown on either complete (CK) or on nitrogen-deficient nutrient (N-deficiency) solutions during a 20-chase period. Compared with the control plants, the lower photosynthetic capacity at saturation irradiance (P _max) was accompanied by an increase in intercellular CO₂ concentration (C_i), indicating that in N-deficient plants the decline in P _max was not due to stomatal limitation but due to the reduced carboxylation efficiency. The fluorescence parameters _PS2, F_v/F_m, electron transport rate (ETR), and q_P showed the same tendency as P _max in N-deficient plants. Correspondingly, a higher q_N paralleled the rise of the ratio of carotenoid (Car) to Chl contents. However, F_v/F_m was still diminished, suggesting that photoinhibition did occur in the photosystem 2 (PS2) reaction centres. In addition, the activities of antioxidant enzymes on a fresh mass basis were gradually lowered, leading to the aggravation of membrane lipid peroxidation with the proceeding N-deficiency. The accumulation of malonyldialdehyde resulted in the lessening of Chl and soluble protein content. Analyses of regression showed PS2 excitation pressure (1 - q_P) was linearly correlated with the content of Chl and inversely with soluble protein (particularly RuBPCO) content. There was a lag phase in the increase of PS2 excitation pressure compared to the decrease of RuBPCO content. Therefore, the increased excitation pressure under N-deficiency is probably the result of saturation of the electron transport chain due to the limitation of the use of reductants by the Calvin cycle. Rice plants responded to N-deficiency and high irradiance by decreasing light-harvesting capacity and by increasing thermal dissipation of absorbed energy.     

The synergistic effect of drought and light stresses in sorghum and pearl millet

The effects of drought stress and high irradiance and their combination were studied under laboratory conditions using young plants of a very drought-resistant variety, ICMH 451, of pearl millet (Pennisetum glaucum) and three varieties of sorghum (Sorghum bicolor)—one drought-resistant from India, one drought-tolerant from Texas, and one drought-sensitive variety from France. CO₂ assimilation rates and photosystem II fluorescence in leaves were analyzed in parallel with photosynthetic electron transport, photosystem II fluorescence, and chlorophyll-protein composition in chloroplasts isolated from these leaves. High irradiance slightly increased CO₂ assimilation rates and electron transport activities of irrigated plants but not fluorescence. Drought stress (less than −1 megapascal) decreased CO₂ assimilation rates, fluorescence, and electron transport. Under the combined effects of drought stress and high irradiance, CO₂ assimilation rates and fluorescence were severely inhibited in leaves, as were the photosynthetic electron transport activities and fluorescence in chloroplasts (but not photosystem I activity). The synergistic or distinctive effect of drought and high irradiance is discussed. The experiments with pearl millet and three varieties of sorghum showed that different responses of plants to drought and light stresses can be monitored by plant physiological and biochemical techniques. Some of these techniques may have a potential for selection of stress-resistant varieties using seedlings.     

Overexpression of SBPase enhances photosynthesis against high temperature stress in transgenic rice plants

Activity of the Calvin cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase) was increased by overexpression of a rice plants 9,311 (Oryza sativa L.) cDNA in rice plants zhonghua11 (Oryza sativa L.). The genetic engineering enabled the plants to accumulate SBPase in chloroplasts and resulted in enhanced tolerance to high temperature stress during growth of young seedlings. Moreover, CO₂ assimilation of transgenic plants was significantly more tolerant to high temperature than that of wild-type plants. The analyses of chlorophyll fluorescence and the content and activation of SBPase indicated that the enhancement of photosynthesis to high temperature was not related to the function of photosystem II but to the content and activation of SBPase. Western blotting analyses showed that high temperature stress led to the association of SBPase with the thylakoid membranes from the stroma fractions. However, such an association was much more pronounced in wild-type plants than that in transgenic plants. The results in this study suggested that under high temperature stress, SBPase maintained the activation of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) by preventing the sequestration of Rubisco activase to the thylakoid membranes from the soluble stroma fraction and thus enhanced the tolerance of CO₂ assimilation to high temperature stress. The results suggested that overexpression of SBPase might be an effective method for enhancing high temperature tolerance of plants.     

Salicylic acid induces physiological and biochemical changes in Torreya grandis cv. Merrillii seedlings under drought stress

Key message

SA treatment effectively ameliorated the negative effect of moderate drought stress on T. grandis Seedlings through increasing the water content, Pn, proline content, antioxidant enzymes activity and reducing MDA.

Abstract

Water availability is one of the most critical factors that limits the growth and development of plants. Salicylic acid (SA) is an important signal molecule that modulates plant responses to abiotic stress. To elucidate the regulating mechanism of exogenous SA on Torreya grandis cv. Merrillii under different water stresses, a pot experiment was conducted in a greenhouse. Exposure of T. grandis seedlings to drought conditions resulted in reduced growth rate that was associated with a decline in water content and CO₂ assimilation. Foliar application of SA effectively increased the water content, net CO₂ assimilation rate, proline content and antioxidant enzymes activity in the plants, which helped T. grandis to acclimate to moderate drought stress and increase the shoot dry matter. However, when the plants were under severe drought stress, the relative water content and CO₂ assimilation in the SA-treated plants were significantly lower than those in the control plants. Therefore, our results indicated that SA can effectively ameliorate the negative effect of moderate drought stress on T. grandis seedling growth.     

Steady-State Chlorophyll a Fluorescence Transients during Ammonium Assimilation by the N-Limited Green Alga Selenastrum minutum

The assimilation of ammonium by the N-limited green alga Selenastrum minutum results in the suppression of photosynthetic electron flow from H₂O to CO₂ (6, 7, 18). In this study, results are presented which describe the correponding change in steady-state chlorophyll a fluorescence. The addition of ammonium resulted in a transient decline in fluorescence followed by a marked increase. Fluorescence did not return to control levels until the added ammonium had been assimilated. Analysis of the fluorescence transients showed that ammonium assimilation resulted in a rapid increase in nonphotochemical quenching (Q_e) peaking 10 to 15 seconds after ammonium addition. Q_e then decreased dramatically reaching a minimum value approximately 45 seconds following ammonium addition and returned to the control level only after the added ammonium had been assimilated. There were no effects of ammonium addition on photochemical quenching (Q_q) for approximately 10 to 15 seconds at which time both gross O₂ evolution (as measured by mass spectrometry) and Q_q declined. In the presence of d,l-glyceraldehyde or when cells were held at the CO₂ compensation point, the addition of ammonium resulted in a decline in Qe 10 to 15 seconds after addition. The Q_e peak and the Q_q decline were absent. These results imply that the transient increase in Q_e and the subsequent decline in Q_q may be attributed to the decline in Calvin cycle activity during ammonium assimilation. The decline in Q_e is apparently a direct result of ammonium assimilation. The observation that the Q_e peak precedes the Q_q decline would be consistent with the decreases in Calvin cycle carbon flow occurring at the kinase reactions prior to glyceraldehyde-3-phosphate dehydrogenase.     

Responses of leaf photosynthesis,pigments and chlorophyll fluorescence within canopy position in a boreal grass (<Emphasis Type="Italic">Phalaris arundinacea</Emphasis> L.) to elevated temperature and CO<Subscript>2</Subscript> under varying water regimes

The effects of elevated growth temperature (ambient + 3.5°C) and CO₂ (700 μmol mol⁻¹) on leaf photosynthesis, pigments and chlorophyll fluorescence of a boreal perennial grass (Phalaris arundinacea L.) under different water regimes (well watered to water shortage) were investigated. Layer-specific measurements were conducted on the top (younger leaf) and low (older leaf) canopy positions of the plants after anthesis. During the early development stages, elevated temperature enhanced the maximum rate of photosynthesis (P _max) of the top layer leaves and the aboveground biomass, which resulted in earlier senescence and lower photosynthesis and biomass at the later periods. At the stage of plant maturity, the content of chlorophyll (Chl), leaf nitrogen (N_L), and light response of effective photochemical efficiency (Φ_PSII) and electron transport rate (ETR) was significantly lower under elevated temperature than ambient temperature in leaves at both layers. CO₂ enrichment enhanced the photosynthesis but led to a decline of N_L and Chl content, as well as lower fluorescence parameters of Φ_PSII and ETR in leaves at both layers. In addition, the down-regulation by CO₂ elevation was significant at the low canopy position. Regardless of climate treatment, the water shortage had a strongly negative effect on the photosynthesis, biomass growth, and fluorescence parameters, particularly in the leaves from the low canopy position. Elevated temperature exacerbated the impact of water shortage, while CO₂ enrichment slightly alleviated the drought-induced adverse effects on P _max. We suggest that the light response of Φ_PSII and ETR, being more sensitive to leaf-age classes, reflect the photosynthetic responses to climatic treatments and drought stress better than the fluorescence parameters under dark adaptation.     

Effect of glutamate on Pyricularia oryzae infection of rice monitored by changes in photosynthetic parameters and antioxidant metabolism

Considering the importance of blast caused by Pyricularia oryzae in the decrease of rice yield worldwide, this study aimed to assess the photosynthetic performance [leaf gas exchange and chlorophyll (Chl) a fluorescence parameters as well as the photosynthetic pigments concentration], the activities of antioxidant enzymes [ascorbate peroxidase, catalase (CAT), peroxidase (POX), superoxide dismutase (SOD), glutathione peroxidase (GPX), glutathione reductase (GR) and glutathione-S-transferase] and concentrations of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) in the leaves of rice plants non-supplied (−Glu) or supplied (+Glu) with glutamate (Glu) and non-infected or infected by P. oryzae. Blast severity was reduced in the leaves of +Glu plants. On the infected leaves of +Glu plants, the values for internal CO₂ concentration were lower while the values for net carbon assimilation rate, stomatal conductance as well as for the concentrations of Chl a, Chl b and carotenoids were higher in comparison to infected leaves of −Glu plants. The functionality of the photosynthetic apparatus was preserved in the infected leaves of +Glu plants. The activities of CAT, GPX, GR, POX and SOD increased in the infected leaves of both −Glu and +Glu plants compared to their non-inoculated counterparts, but their activities were lower for +Glu plants. The lower activity of these antioxidative enzymes was triggered by the reduced hydrogen peroxide concentration in the infected leaves of +Glu plants resulting in lower MDA concentration. It can be concluded that photosynthesis was less impaired in infected plants supplied with glutamate due to the lower biochemical constraints for CO₂ fixation. Moreover, there was a need for lower activity of reactive oxygen species scavenging enzymes in infected leaves of plants supplied with glutamate due to the lower oxidative stress imposed by P. oryzae infection.     

CO(2) Assimilation by Etiolated Hordeum vulgare Seedlings during the Onset of Photosynthesis

The assimilation of CO₂ by etiolated Hordeum vulgare seedlings during an illumination period indicates a conversion of the organisms to autotrophy.

After 1 hour illumination, increases in the photo-assimilation of CO₂ are observed and the distribution of C¹⁴ in the soluble fraction of the plants is predominantly in intermediates of the Calvin cycle.

     

Oxygen effects on photosynthesis and C metabolism in desert plants

The effect of 1% and 21% O₂ upon ¹⁴CO₂ assimilation by desert plants exposed for 10 to 90 seconds has been studied. The plants studied can be divided into three groups with respect to O₂. The C₃ plants display the usual Warburg effect. No changes could be observed in the intensity of photosynthesis as a function of O₂ content in another group of plants (showing signs of Crassulacean acid metabolism). In still another group of plants (C₄ plants) the stimulating effect of O₂ on photosynthesis could be detected. In C₃ plants, O₂ inhibits the processing of carbon through the Calvin cycle intermediates. The involvement of carbon in the glycolate pathway fails to explain completely the inhibiting effect of O₂ on photosynthesis. It is assumed that O₂ inhibits the enzymes of the Calvin cycle. In C₄ plants O₂ stimulates the incorporation of ¹⁴C into malate and aspartate. The incorporation of ¹⁴C into the intermediates of the Calvin cycle in C₄ plants is inhibited much like that in typical C₃ plants.     

Responses to flooding and drought stress by two citrus rootstock seedlings with different water-use efficiency

Leaf water relations, net gas exchange and leaf and root constituent responses to 9 days of drought stress (DS) or soil flooding were studied in 6‐month‐old seedlings of Carrizo citrange [Citrus sinensis (L.) Osb. ×Poncirus trifoliata L.; Carr] and Cleopatra mandarin (Citrus resnhi Hort. ex Tanaka; Cleo) growing in containers of native sand in the greenhouse. At the end of the drought period, both species had similar minimum stem water potentials but Cleo had higher leaf relative water content (RWC) and higher leaf osmotic potential at full turgor () than Carr. Flooding had no effect on RWC but osmotic adjustment (OA) and were higher in Cleo than in Carr. Net CO₂ assimilation rate (A_CO2) in leaves was decreased more by drought than by flooding in both species but especially in Carr. Leaf water‐use efficiency (A_CO2/transpiration) was lower in Carr and was decreased more by DS and flooding stress than in Cleo. Higher values of intercellular CO₂ concentration (C_i) in stressed plants than in control plants indicated that non‐stomatal factors including chlorophyll degradation and chlorophyll fluorescence [maximum quantum efficiency of PSII (Fv/Fm, where Fm is the maximum fluorescence and F₀, minimum fluorescence in dark‐adapted leaves)] were more important limitations on A_CO2 than stomatal conductance. In both genotypes, leaf proline was increased by drought but not by flooding, whereas both stresses increased proline in roots. Soluble sugars in leaves were increased by DS, and flooding decreased leaf sugars in Cleo. In general, DS tended to increase the concentrations of Ca, K, Mg, Na and Cl in both leaves and roots, whereas flooding tended to decrease these ions with the exception of leaf Ca in Cleo. Based on water relations and net gas exchange, Cleo was more tolerant to short‐term DS and flooding stress than Carr.