Light intensity affects chlorophyll synthesis during greening process by metabolite signal from mitochondrial alternative oxidase in Arabidopsis

Although mitochondrial alternative oxidase (AOX) has been proposed to play essential roles in high light stress tolerance, the effects of AOX on chlorophyll synthesis are unclear. Previous studies indicated that during greening, chlorophyll accumulation was largely delayed in plants whose mitochondrial cyanide‐resistant respiration was inhibited by knocking out nuclear encoded AOX gene. Here, we showed that this delay of chlorophyll accumulation was more significant under high light condition. Inhibition of cyanide‐resistant respiration was also accompanied by the increase of plastid NADPH/NADP⁺ ratio, especially under high light treatment which subsequently blocked the import of multiple plastidial proteins, such as some components of the photosynthetic electron transport chain, the Calvin–Benson cycle enzymes and malate/oxaloacetate shuttle components. Overexpression of AOX1a rescued the aox1a mutant phenotype, including the chlorophyll accumulation during greening and plastidial protein import. It thus suggests that light intensity affects chlorophyll synthesis during greening process by a metabolic signal, the AOX‐derived plastidial NADPH/NADP⁺ ratio change. Further, our results thus revealed a molecular mechanism of chloroplast–mitochondria interactions.     

A hemiparasite in the forest understorey: photosynthetic performance and carbon balance of Melampyrum pratense

• Melampyrum pratense is an annual root‐hemiparasitic plant growing mostly in forest understorey, an environment with unstable light conditions. While photosynthetic responses of autotrophic plants to variable light conditions are in general well understood, light responses of root hemiparasites have not been investigated.
• We carried out gas exchange measurements (light response and photosynthetic induction curves) to assess the photosynthetic performance of M. pratense in spring and summer. These data and recorded light dynamics data were subsequently used to model carbon balance of the hemiparasite throughout the entire growth season.
• Summer leaves had significantly lower rates of saturated photosynthesis and dark respiration than spring leaves, a pattern expected to reflect the difference between sun‐ and shade‐adapted leaves. However, even the summer leaves of the hemiparasite exhibited a higher rate of light‐saturated photosynthesis than reported in non‐parasitic understorey herbs. This is likely related to its annual life history, rare among other understorey herbs. The carbon balance model considering photosynthetic induction still indicated insufficient autotrophic carbon gain for seed production in the summer months due to limited light availability and substantial carbon loss through dark respiration.
• The results point to potentially high importance of heterotrophic carbon acquisition in M. pratense, which could be of at least comparable importance as in other mixotrophic plants growing in forests – mistletoes and partial mycoheterotrophs. It is remarkable that despite apparent evolutionary pressure towards improved carbon acquisition from the host, M. pratense retains efficient photosynthesis and high transpiration rate, the ecophysiological traits typical of related root hemiparasites in the Orobanchaceae.

     

Oxaloacetate and malate transport by plant mitochondria

The permeability of mitochondria from pea (Pisum sativum L. var Kleine Rheinländerin) leaves, etiolated pea shoots, and potato (Solanum tuberosum) tuber for malate, oxaloacetate, and other dicarboxylates was investigated by measurement of mitochondrial swelling in isoosmolar solutions of the above mentioned metabolites. For the sake of comparison, parallel experiments were also performed with rat liver mitochondria. Unlike the mammalian mitochondria, the plant mitochondria showed only little swelling in ammonium malate plus phosphate media but a dramatic increase of swelling on the addition of valinomycin. Similar results were obtained with oxaloacetate, maleate, fumarate, succinate, and malonate. n-Butylmalonate and phenylsuccinate, impermeant inhibitors of malate transport in mammalian mitochondria, had no marked inhibitory effect on valinomycin-dependent malate and oxaloacetate uptake of the plant mitochondria. The swelling of plant mitochondria in malate plus valinomycin was strongly inhibited by oxaloacetate, at a concentration ratio of oxaloacetate/malate of 10⁻³. From these findings it is concluded: (a) In a malate-oxaloacetate shuttle transferring redox equivalents from the mitochondrial matrix to the cytosol, malate and oxaloacetate are each transported by electrogenic uniport, probably linked to each other for the sake of charge compensation. (b) The transport of malate between the mitochondrial matrix and the cytosol is controlled by the oxaloacetate level in such a way that a redox gradient can be maintained between the NADH/NAD systems in the matrix and the cytosol. (c) The malate-oxaloacetate shuttle functions mainly in the export of malate from the mitochondria, whereas the import of malate as a respiratory substrate may proceed by the classical malate-phosphate antiport.     

Transgenic potato plants with altered expression levels of chloroplast NADP-malate dehydrogenase: interactions between photosynthetic electron transport and malate metabolism in leaves and in isolated intact chloroplasts

The contribution of the malate valve in the regulation of steady-state photosynthesis was studied in transgenic potato (Solanum tuberosum L. cv Désirée) plants with altered expression of plastidic NADP-dependent malate dehydrogenase (NADP-MDH; EC 1.1.1.82). Mutant plants were obtained after transformation with the homologous Nmdh gene in antisense orientation, or with the Nmdh gene from pea (Pisum sativum L.) in sense orientation. A total number of nine stable sense and antisense lines with 10% or 30%, and 400% of wild-type NADP-MDH capacity were selected. Intact chloroplasts were isolated from leaves of wild-type and mutant plants. In chloroplasts from sense transformants the increased enzyme amount was activated as in wild-type chloroplasts, but increased rates of oxaloacetate-dependent malate formation were only measured upon partial uncoupling. In contrast, chloroplasts from antisense transformants produced only little malate upon oxaloacetate addition. Measurements with intact leaves during steady-state photosynthesis yielded no differences in gas-exchange parameters and chlorophyll fluorescence. The leaf malate content was unchanged in NADP-MDH underexpressors, but twice as high in overexpressing plants. The altered NADP-MDH expression clearly influences the redox state of ferredoxin, especially in low light. Furthermore, the malate valve can successfully compete for electrons with cyclic electron flow, but the conditions under which this occurs are quite artificial. Received: 14 February 1998 / Accepted: 12 May 1998     

Light‐induced systemic signalling down‐regulates photosynthetic performance of soybean leaves with different directional effects

• When plants are exposed to a heterogeneous environment, photosynthesis of leaves is not only determined by their local condition, but also by certain signals from other parts of the same plant, termed systemic regulation. Our present study was conducted to investigate the effects of light‐dependent systemic regulation on the photosynthetic performance of soybean (Glycine max L. Merr.) under heterogeneous light conditions.
• Soybean plants were treated with heterogeneous light. Then gas exchange characteristics were measured to evaluate the photosynthetic performance of leaves. Parameters related to photosynthetic pigments, chlorophyll fluorescence, Rubisco and photosynthates were examined to study the mechanisms of light‐dependent systemic regulation on photosynthesis.
• Light‐induced systemic signalling by illuminated leaves reduced the P_n of both upper and lower non‐illuminated leaves on the same soybean plant. The decrease in g_s and increase in C_i in these non‐illuminated leaves indicated restriction of carbon assimilation, which was further verified by the decline in content and activity of Rubisco. However, the activation state of Rubisco decreased only in upper non‐illuminated leaves. Quantum efficiency of PSII (ΦPSII) and ETR also decreased only in upper non‐illuminated leaves. Moreover, the effects of light‐induced systemic signalling on carbohydrate content were also detectable only in upper non‐illuminated leaves.
• Light‐induced systemic signalling by illuminated leaves restricts carbon assimilation and down‐regulates photosynthetic performance of non‐illuminated leaves within a soybean plant. However, effects of such systemic regulation differed when regulated in upward or downward direction.

     

Variability of photosynthesis-irradiance curves and ecosystem respiration in a small river

• 1 We investigated photosynthesis‐irradiance relationships (P‐I curves; P = oxygen production rate due to photosynthesis, I = light irradiance rate at the water surface) and ecosystem respiration in a 9 km long reach of a river that is characterised by light conditions favouring primary production, high ambient nutrient concentrations, a high re‐aeration rate, and frequent spates. We addressed the question of how disturbances (spates) and season influence photosynthesis and ecosystem respiration.
• 2 We used an oxygen mass‐balance model of the river to identify ecosystem respiration rates and the two parameters of a hyperbolic P‐I function (P_max = maximum oxygen production rate due to photosynthesis, α = the initial slope of the P‐I function). The model was fitted to dissolved oxygen concentrations quasi‐continuously recorded at the end of the reach. We estimated parameters for 137 three‐day periods (during the years 1992–97) and subsequently explored the potential influence of season and disturbances (spates) on P_max, α and ecosystem respiration using stepwise regression analysis.
• 3 Photosynthesis‐irradiance relationships and ecosystem respiration were subject to distinct seasonal variation. Only a minor portion of the variability of P‐I curves could be attributed to disturbance (spates), while ecosystem respiration did not correlate with disturbance related parameters. Regular seasonal variation in photosynthesis and ecosystem respiration apparently prevailed due to the absence of severe disturbances (a lack of significant bedload transport during high flow).

     

Importance of AOX pathway in optimizing photosynthesis under high light stress: role of pyruvate and malate in activating AOX

The present study shows the importance of alternative oxidase (AOX) pathway in optimizing photosynthesis under high light (HL). The responses of photosynthesis and respiration were monitored as O₂ evolution and O₂ uptake in mesophyll protoplasts of pea pre‐incubated under different light intensities. Under HL (3000 µmol m^?2 s^?1), mesophyll protoplasts showed remarkable decrease in the rates of NaHCO₃‐dependent O₂ evolution (indicator of photosynthetic carbon assimilation), while decrease in the rates of respiratory O₂ uptake were marginal. While the capacity of AOX pathway increased significantly by two fold under HL, the capacity of cytochrome oxidase (COX) pathway decreased by >50% compared with capacities under darkness and normal light (NL). Further, the total cellular levels of pyruvate and malate, which are assimilatory products of active photosynthesis and stimulators of AOX activity, were increased remarkably parallel to the increase in AOX protein under HL. Upon restriction of AOX pathway using salicylhydroxamic acid (SHAM), the observed decrease in NaHCO₃‐dependent O₂ evolution or p‐benzoquinone (BQ)‐dependent O₂ evolution [indicator of photosystem II (PSII) activity] and the increase in total cellular levels of pyruvate and malate were further aggravated/promoted under HL. The significance of raised malate and pyruvate levels in activation of AOX protein/AOX pathway, which in turn play an important role in dissipating excess chloroplastic reducing equivalents and sustenance of photosynthetic carbon assimilation to balance the effects of HL stress on photosynthesis, was depicted as a model.     

Three‐dimensional microscale modelling of CO2 transport and light propagation in tomato leaves enlightens photosynthesis

We present a combined three‐dimensional (3‐D) model of light propagation, CO₂ diffusion and photosynthesis in tomato (Solanum lycopersicum L.) leaves. The model incorporates a geometrical representation of the actual leaf microstructure that we obtained with synchrotron radiation X‐ray laminography, and was evaluated using measurements of gas exchange and leaf optical properties. The combination of the 3‐D microstructure of leaf tissue and chloroplast movement induced by changes in light intensity affects the simulated CO₂ transport within the leaf. The model predicts extensive reassimilation of CO₂ produced by respiration and photorespiration. Simulations also suggest that carbonic anhydrase could enhance photosynthesis at low CO₂ levels but had little impact on photosynthesis at high CO₂ levels. The model confirms that scaling of photosynthetic capacity with absorbed light would improve efficiency of CO₂ fixation in the leaf, especially at low light intensity.     

Influence of salt stress on C4 photosynthesis in Miscanthus sinensis Anderss.

• Miscanthus sinensis Anderss. is a good candidate for C₄ bioenergy crop development for marginal lands. As one of the characteristics of marginal lands, salinization is a major limitation to agricultural production. The present work aimed to investigate the possible factors involved in the tolerance of M. sinensis C₄ photosynthesis to salinity stress.
• Seedlings of two accessions (salt‐tolerant ‘JM0119’ and salt‐sensitive ‘JM0099’) were subjected to 0 mm NaCl (control) or 250 mm NaCl (salt stress treatment) for 2 weeks. The chlorophyll content, parameters of photosynthesis and chlorophyll a fluorescence, activity of C₄ enzymes and expression of C₄ genes were measured.
• The results showed that photosynthesis rate, transpiration rate, chlorophyll content, PSII operating efficiency, coefficient of photochemical quenching, activity of phosphoenolpyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK) and gene expression of PEPC and PPDK under salinity were higher after long‐term salinity exposure in ‘JM0119’ than in ‘JM0099’, while activity of NADP‐malate dehydrogenase (NADP‐MDH) and NADP‐malic enzyme (NADP‐ME), together with expression of NADP‐MDH and NADP‐ME, were much higher in ‘JM0099’ than in ‘JM0119’.
• In conclusion, the increased photosynthetic capacity under long‐term salt stress in the salt‐tolerant relative to the salt‐sensitive M. sinensis accession was mainly associated with non‐stomatal factors, such as reduced chlorophyll loss, higher PSII operating efficiency, enhanced activity of PEPC and PPDK and relatively lower activity of NADP‐ME.

     

Crassulacean acid metabolism enhances underwater photosynthesis and diminishes photorespiration in the aquatic plant Isoetes australis

? Underwater photosynthesis by aquatic plants is often limited by low availability of CO(2), and photorespiration can be high. Some aquatic plants utilize crassulacean acid metabolism (CAM) photosynthesis. The benefits of CAM for increased underwater photosynthesis and suppression of photorespiration were evaluated for Isoetes australis, a submerged plant that inhabits shallow temporary rock pools. ? Leaves high or low in malate were evaluated for underwater net photosynthesis and apparent photorespiration at a range of CO(2) and O(2) concentrations. ? CAM activity was indicated by 9.7-fold higher leaf malate at dawn, compared with at dusk, and also by changes in the titratable acidity (μmol H(+) equivalents) of leaves. Leaves high in malate showed not only higher underwater net photosynthesis at low external CO(2) concentrations but also lower apparent photorespiration. Suppression by CAM of apparent photorespiration was evident at a range of O(2) concentrations, including values below air equilibrium. At a high O(2) concentration of 2.2-fold the atmospheric equilibrium concentration, net photosynthesis was reduced substantially and, although it remained positive in leaves containing high malate concentrations, it became negative in those low in malate. ? CAM in aquatic plants enables higher rates of underwater net photosynthesis over large O(2) and CO(2) concentration ranges in floodwaters, via increased CO(2) fixation and suppression of photorespiration.     

Arbuscular mycorrhiza influences carbon‐use efficiency and grain yield of wheat grown under pre‐ and post‐anthesis salinity stress

• Soil salinity severely affects and constrains crop production worldwide. Salinity causes osmotic and ionic stress, inhibiting gas exchange and photosynthesis, ultimately impairing plant growth and development. Arbuscular mycorrhiza (AM) have been shown to maintain light and carbon use efficiency under stress, possibly providing a tool to improve salinity tolerance of the host plants. Thus, it was hypothesized that AM will contribute to improved growth and yield under stress conditions.
• Wheat plants (Triticum aestivum L.) were grown with (AMF+) or without (AMF?) arbuscular mycorrhizal fungi (AMF) inoculation. Plants were subjected to salinity stress (200 mm NaCl) either at pre‐ or post‐anthesis or at both stages. Growth and yield components, leaf chlorophyll content as well as gas exchange parameters and AMF colonization were analysed.
• AM plants exhibited a higher rate of net photosynthesis and stomatal conductance and lower intrinsic water use efficiency. Furthermore, AM wheat plants subjected to salinity stress at both pre‐anthesis and post‐anthesis maintained higher grain yield than non‐AM salinity‐stressed plants.
• These results suggest that AMF inoculation mitigates the negative effects of salinity stress by influencing carbon use efficiency and maintaining higher grain yield under stress.

     

Metabolism of <Superscript>14</Superscript>C-glycine as a substrate for photorespiration of the leaf at different developmental stages of ephemeroides

The metabolism of ¹⁴C-glycine (a substrate for photorespiration) was studied in the light and in darkness under natural CO₂ concentration (0.03%) in the leaves of ephemeroides Scilla sibirica Haw. and Ficaria verna Huds. at different developmental stages. Using one and the same sample, potential photosynthesis (at 1% CO₂), true photosynthesis (at 0.03% CO₂), and leaf respiratory capacity were measured by the radiometric and manometric methods, respectively. All measurements were performed at 15°C, an average temperature during ephemer growth. It was found that, in the white zone of the Scilla leaf, the rate of CO₂ evolution resulting from metabolization of exogenous ¹⁴C-glycine was similar in the light and in darkness. In the green zone of the Scilla leaf and in the green leaf of Ficaria, both ¹⁴C-glycine absorption and ¹⁴CO₂ evolution were lower in the light as compared with darkness, which is explained by CO₂ reassimilation. In all treatments of both plant species, a specific inhibitor of glycine decarboxylase complex (GDC), aminoacetonitrile (5 mM) suppressed CO₂ evolution by 20–40%. It was concluded that in ephemeroides mitochondrial GDC, responsible for CO₂ evolution in photorespiration, is formed at the earliest stage of leaf development. This indicates that photorespiration can occur simultaneously with the development of the leaf photosynthetic activity. On the basis of the assumption that carbon losses in the form of CO₂ evolved during photorespiration comprise 25% of true photosynthesis, it was calculated that, in ephemer leaves, the highest rates of photorespiration and photosynthesis were attained during flowering when the leaf area was the largest and the rate of dark respiration was reduced by 1.5–2.0 times. The highest rates of dark respiration were observed in the beginning of growth. In senescing leaves by the end of the plant vegetation, potential photosynthesis and true photosynthesis were reduced, whereas dark respiration remained essentially unchanged. It is concluded that the high rates of potential and true photosynthesis are characteristic of ephemeroides when they complete their short developmental program in early spring (at 15°C); theoretically, photorespiration also occurs at a high rate during this period, when this process provides for a defense against the threat of photoinhibition at low temperature and high insolation.     

Photorespiration and Glycolate Metabolism: A Re-examination and Correlation of Some Previous Studies

Some previous studies of photorespiration and glycolate oxidation were re-examined and correlated by infra-red CO₂ analysis. Data about rate of photosynthesis and oxygen sensitivity indicated that complete inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1 dimethyl urea (DCMU) allowed dark respiration to continue in the light. Photorespiration was also inhibited. The oxygen sensitivity of glycolate-stimulated CO₂ production was found to be compatible with the proposal that glycolate is a substrate of photorespiration. Both `in vivo' and `in vitro' studies of the alga Nitella flexilis have revealed a pathway of glycolate oxidation similar to that of higher plants. DCMU inhibition of photosynthesis by Nitella gave results similar to those for the monocotyledons tested. Under very low light intensity, carbon dioxide compensation in corn was measurable but was not sensitive to high oxygen concentration. It appears that the lack of photorespiration in this plant is not the end result of efficient internal recycling of CO₂ to photosynthesis.     

Variation in leaf respiration in relation to growth and photosynthesis of Lolium

Six Lolium genotypes with contrasting apparent photorespiration and CO_a compensation concentration, [C0₂]_c, were compared for net photosynthesis, dark respiration, leaf starch accumulation, rate of leaf expansion and shoot regrowth. Plants were grown in day/night temperatures of 15/10 and 25/20 ^oC. There were significant (P < 0–05) differences between the genotypes in all these parameters. At 25/20 ^oC apparent photorespiration was correlated with [CO₂]_c. Correlation coefficients, pooled from both temperature regimes, revealed that genotypes with high rates of net photosynthesis accumulated more leaf starch during light periods than genotypes with slow photosynthesis, but rates of leaf expansion and dry matter increase were only correlated, negatively, with dark respiration. Apparent photorespiration was negatively correlated with dark respiration. These findings suggest that attributes related to photorespiration such as [CO₂]_c and O₂ uptake from CO₂-free air in the light are unlikely to be useful selection criteria for growth of C₃ grasses, that net photosynthesis was probably not limiting growth and that maintenance respiration may have been an important determinant of genotypic differences in growth rate. Selections for slow and fast rates of dark respiration of mature leaves were therefore made at 8 and at 25 ^oC from within two different populations of L. perenne, S.23. This characteristic showed repeatabilities (broad-sense heritability) of from 0–41 to o-66. Six independent comparisons of simulated swards of the slow- and fast-respiring selections were made under periodic cutting regimes, either in a growth room at 25 ^oC or in a glasshouse from August to May. Growth of all plots of slow-respiring genotypes was consistently more rapid than that of the fast-respiring, at 25 ^oC in the growth room, and during autumn and spring in the glasshouse. There was no difference in winter growth. The implications of these results for the use of gas exchange measurements as selection criteria in plant breeding programmes are discussed.     

Short‐term acclimation to warmer temperatures accelerates leaf carbon exchange processes across plant types

While temperature responses of photosynthesis and plant respiration are known to acclimate over time in many species, few studies have been designed to directly compare process‐level differences in acclimation capacity among plant types. We assessed short‐term (7 day) temperature acclimation of the maximum rate of Rubisco carboxylation (V_cmax), the maximum rate of electron transport (J_max), the maximum rate of phosphoenolpyruvate carboxylase carboxylation (V_pmax), and foliar dark respiration (R_d) in 22 plant species that varied in lifespan (annual and perennial), photosynthetic pathway (C₃ and C₄), and climate of origin (tropical and nontropical) grown under fertilized, well‐watered conditions. In general, acclimation to warmer temperatures increased the rate of each process. The relative increase in different photosynthetic processes varied by plant type, with C₃ species tending to preferentially accelerate CO₂‐limited photosynthetic processes and respiration and C₄ species tending to preferentially accelerate light‐limited photosynthetic processes under warmer conditions. R_d acclimation to warmer temperatures caused a reduction in temperature sensitivity that resulted in slower rates at high leaf temperatures. R_d acclimation was similar across plant types. These results suggest that temperature acclimation of the biochemical processes that underlie plant carbon exchange is common across different plant types, but that acclimation to warmer temperatures tends to have a relatively greater positive effect on the processes most limiting to carbon assimilation, which differ by plant type. The acclimation responses observed here suggest that warmer conditions should lead to increased rates of carbon assimilation when water and nutrients are not limiting.     

Diaheliotropic leaf movement enhances leaf photosynthetic capacity and photosynthetic light and nitrogen use efficiency via optimising nitrogen partitioning among photosynthetic components in cotton (Gossypium hirsutum L.)

• Phototropic leaf movement of plants is an effective mechanism for adapting to light conditions. Light is the major driver of plant photosynthesis. Leaf N is also an important limiting factor on leaf photosynthetic potential. Cotton (Gossypium hirsutum L.) exhibits diaheliotropic leaf movement. Here, we compared the long‐term photosynthetic acclimation of fixed leaves (restrained) and free leaves (allowed free movement) in cotton.
• The fixed leaves and free leaves were used for determination of PAR, leaf chlorophyll concentration, leaf N content and leaf gas exchange. The measurements were conducted under clear sky conditions at 0, 7, 15 and 30 days after treatment (DAT).
• The results showed that leaf N allocation and partitioning among different components of the photosynthetic apparatus were significantly affected by diaheliotropic leaf movement. Diaheliotropic leaf movement significantly increased light interception per unit leaf area, which in turn affected leaf mass per area (LMA), leaf N content (N_A) and leaf N allocation to photosynthesis (N_P). In addition, cotton leaves optimised leaf N allocation to the photosynthetic apparatus by adjusting leaf mass per area and N_A in response to optimal light interception.
• In the presence of diaheliotropic leaf movement, cotton leaves optimised their structural tissue and photosynthetic characteristics, such as LMA, N_A and leaf N allocation to photosynthesis, so that leaf photosynthetic capacity was maximised to improve the photosynthetic use efficiency of light and N under high light conditions.

     

Drought responses,phenotypic plasticity and survival of Mediterranean species in two different microclimatic sites

• Climate models predict a further drying of the Mediterranean summer. One way for plant species to persist during such climate changes is through acclimation. Here, we determine the extent to which trait plasticity in response to drought differs between species and between sites, and address the question whether there is a trade‐off between drought survival and phenotypic plasticity.
• Throughout the summer we measured physiological traits (photosynthesis – A_max, stomatal conductance – g_s, transpiration – E, leaf water potential – ψl) and structural traits (specific leaf area – SLA, leaf density – LD, leaf dry matter content – LDMC, leaf relative water content – LRWC) of leaves of eight woody species in two sites with slightly different microclimate (north‐ versus south‐facing slopes) in southern Spain. Plant recovery and survival was estimated after the summer drought period.
• We found high trait variability between species. In most variables, phenotypic plasticity was lower in the drier site. Phenotypic plasticity of SLA and LDMC correlated negatively with drought survival, which suggests a trade‐off between them. On the other hand, high phenotypic plasticity of SLA and LDMC was positively related to traits associated with rapid recovery and growth after the drought period.
• Although phenotypic plasticity is generally seen as favourable during stress conditions, here it seemed beneficial for favourable conditions. We propose that in environments with fluctuating drought periods there can be a trade‐off between drought survival and growth during favourable conditions. When climate become drier, species with high drought survival but low phenotypic plasticity might be selected for.

     

Crassulacean acid metabolism in the Basellaceae (Caryophyllales)

• C₄ and crassulacean acid metabolism (CAM) have evolved in the order Caryophyllales many times but neither C₄ nor CAM have been recorded for the Basellaceae, a small family in the CAM‐rich sub‐order Portulacineae.
• 24 h gas exchange and day–night changes in titratable acidity were measured in leaves of Anredera baselloides exposed to wet–dry–wet cycles.
• While net CO₂ uptake was restricted to the light period in well‐watered plants, net CO₂ fixation in the dark, accompanied by significant nocturnal increases in leaf acidity, developed in droughted plants. Plants reverted to solely C₃ photosynthesis upon rewatering.
• The reversible induction of nocturnal net CO₂ uptake by drought stress indicates that this species is able to exhibit CAM in a facultative manner. This is the first report of CAM in a member of the Basellaceae.

     

End product feedback effects on photosynthetic electron transport

The inhibition of photosynthetic electron transport when starch and sucrose synthesis limit the overall rate of photosynthesis was studied inPhaseolus vulgaris L. andXanthium strumarium L. The starch and sucrose limitation was established by reducing photorespiration by manipulation of the partial pressure of O₂ and CO₂. Chlorophylla fluorescence quenching, the redox state of Photosystem I (estimated by the redox status of NADP-dependent malate dehydrogenase), and the intermediates of the xanthophyll cycle were investigated. Non-photochemical fluorescence quenching increased, NADP-dependent malate dehydrogenase remained at 100% activity, and the amount of violaxanthin decreased when starch and sucrose synthesis limited photosynthesis. In addition, O₂-induced feedback caused a decrease in photochemical quenching. These results are consistent with a downward regulation of photosynthetic electron transport during end product feedback on photosynthesis. When leaves were held in high CO₂ for 4 hours, the efficiency of Photosystem II was reduced when subsequently measured under low light. The results indicate that the quantum efficiency of open Photosystem II centers was reduced by the 4 hour treatment. We interpret the results to indicate that feedback from starch and sucrose synthesis on photosynthetic electron transport stimulates mechanisms for dissipating excess light energy but that these mechanisms do not completely protect leaves from long-term inhibition of photosynthetic electron transport capacity.