The Specific Density of a Leaf as a Characteristic of the Photosynthetic Apparatus

At early stages of ontogeny (up to 50–60% of the maximum leaf area) of wheat (Triticum aestivumL.), meadow fescue (Festuca pratensisHuds.), reed fescue (F. arindinaceaSchreb.), and sugar beet (Beta vulgarisL. var. saccharifera(Alef) Krass), there is a correlation between changes in the specific leaf density (SLD); photosynthetic CO₂assimilation rate; activity of the key photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39); and the concentrations of chlorophyll (Chl) a, Chl b, carotenoids, and soluble leaf proteins. However, the SLD does not correlate with the activity of phospho(enol)pyruvate carboxylase (EC 4.1.1.31). Senescence was accompanied by a decrease in the leaf SLD. Treatment with cytokininomimetics (6-benzylaminopurine and Metribuzin) caused an increase in the SLD. The specific leaf density is suggested to be a structural and functional characteristic of the photosynthetic apparatus of agricultural plants.     

Effect of preparations exhibiting cytokinin-like activity on the specific density of leaf in grasses

The effects of synthetic preparations exhibiting cytokinin-like activity (6-benzylaminopurine, Thidiazuron, and kartolin-2) on the specific leaf area (SLA) were studied in plants of the family Gramineae (wheat, Triticum aestivum L.; meadow fescue, Festuca pratensis Huds.; and reed fescue, F. arindinacea Schreb.). At the early stages of ontogeny (until the leaf area reached 50-60% of the maximum value), treatment of plants of the three species with cytokinin-like preparations caused an increase in SLA. The SLA value in these plants was correlated with the rate of photosynthetic assimilation of carbon dioxide and activities of carbon metabolism enzymes: ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), NAD-malate dehydrogenase (EC 1.1.1.37), and NADP-glyceraldehydrophosphate dehydrogenase complex, which includes phosphoglycerate kinase (EC 2.7.2.3) and glyceraldehydrophosphate dehydrogenase (EC 1.2.1.13). However, there was no correlation of SLA with the activity of phospho(enol)pyruvate carboxylase (EC 4.1.1.31), an anaplerotic carboxylation enzyme of grasses. SLA is suggested to reflect the state and activity of the photosynthetic apparatus and can be recommended as a characteristic of photosynthesis variability (e.g., caused by cytokinin-like preparations).     

Effect of Preparations Exhibiting Cytokinin-like Activity on the Specific Density of Leaf in Grasses

The effects of synthetic preparations exhibiting cytokinin-like activity (6-benzylaminopurine, Thidiazuron, and kartolin-2) on the specific leaf area (SLA) were studied in plants of the family Gramineae (wheat, Triticum aestivum L.; meadow fescue, Festuca pratensis Huds.; and reed fescue, F. arindinacea Schreb.). At the early stages of ontogeny (until the leaf area reached 50–60% of the maximum value), treatment of plants of the three species with cytokinin-like preparations caused an increase in SLA. The SLA value in these plants was correlated with the rate of photosynthetic assimilation of carbon dioxide and activities of carbon metabolism enzymes: ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), NAD-malate dehydrogenase (EC 1.1.1.37), and NADP-glyceraldehydephosphate dehydrogenase complex, which includes phosphoglycerate kinase (EC 2.7.2.3) and glyceraldehydephosphate dehydrogenase (EC 1.2.1.13). However, there was no correlation of SLA with the activity of phospho(enol)pyruvate carboxylase (EC 4.1.1.31), an anaplerotic carboxylation enzyme of grasses. SLA is suggested to reflect the state and activity of the photosynthetic apparatus and can be recommended as a characteristic of photosynthesis variability (e.g., caused by cytokinin-like preparations).     

水稻叶片生育过程中RubisCO活性与光合、光呼吸的关系

水稻生育过程中,ＲｕＢＰ羧化酶活性与光合速率、ＲｕＢＰ加氧酶活性与光呼吸速率、ＲｕＢＰ羧化酶活性与加氢酶活性以及光合速率与光呼吸速率之间是相关的。籼型品种与粳型品种间酶活性的高低及光合、光呼吸速率的高低基本一致,籼型三系杂交稻（Ｆ１）无明显的光合优势。酶的羧化活性的高低只在一定范围内与光合速率的高低平行。在正常生育条件下,酶蛋白的数量不是水稻光合速率的限制因子。     

Contrasting responses of photosynthesis and carbon metabolism to low temperatures in tall fescue and clovers

Growth, photosynthesis and carbohydrate metabolism in plants of two grassland species, clover ( Trifolium subterraneum L. cv. Areces and Gaitan) and tall fescue ( Festuca arundinacea Schreb.), shifted from 25 to 12°C for 1 day or developed at 12°C were compared with controls kept at 25°C. Cold development produced a larger inhibition of growth in fescue than in clovers. In contrast, transferring plants from high to low temperature inhibited photosynthesis to a lesser extent in fescue than in clovers, this difference being associated with an increase in the activation state of Calvin cycle enzymes in fescue, but not in the clovers, a decreased cytosolic fructose-1,6-bisphosphatase (cFBPase, EC 3.1.3.11) activity in clovers, and an accumulation of hexose phosphates only in fescue. Development at 12°C partly relieved the inhibition of photosynthesis in clovers, in contrast with fescue, which correlated with increases in total ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco, EC 4.1.1.39) activity only in clovers, and with greater increases in total stromal FBPase (sFBPase) activity in clovers than in fescue. The activity of sucrose synthesis enzymes was increased in the two clovers and fescue developed in the cold, while carbohydrate accumulation was much bigger in cold-developed fescue than in clovers because of a 5-fold increase in fructan contents in the former. The contents of phosphorylated intermediates increased in clovers but decreased in fescue grown at 12°C. Our results suggest that restricted ribulose-1,5-bisphosphate (RuBP) regeneration limited the recovery of photosynthetic capacity in cold-developed fescue.     

A protector effect of cytokinin preparations on the photosynthetic apparatus of wheat plants under water deficiency conditions

The protective effects of the cytokinin 6-benzylaminopurine and the compounds thidiazuron and kartolin, displaying a cytokinin activity, on the photosynthetic apparatus of young seedlings and leaves of adult plants of two wheat (Triticum aestivum L.) cultivars, Mironovskaya 808 (more drought tolerant) and Lutescens 758 (less tolerant to water stress), were compared on the background of an increasing water deficiency. At the stages of drought and subsequent rehydration, cartolin preparations were the most efficient protectors, enhancing a less pronounced decrease in the intensity of photosynthesis, carboxylating activity of the key enzyme of carbon metabolism--ribulose bisphosphate carboxylase/oxygenase (EC 4.1.1.39)-and the activity of NADP-glyceraldehyde phosphate dehydrogenase--the enzyme complex comprising phosphoglycerate kinase (EC 2.7.2.3) and glyceraldehyde phosphate dehydrogenase (EC 1.2.1.13). This effect also included an increase in the leaf specific density and plant productivity. The negative influence of water stress on the photosynthetic apparatus was more pronounced in a less tolerant cultivar Lutescens 758 and in the seedlings as compared with the adult plants.     

The photosynthetic irradiance-response of Norway spruce exposed to a long-term elevation of CO2 concentration

During an open-top chamber experiment performed in a mountain stand of young (12-year-old) Norway spruce (Picea abies [L.] Karst.), the trees were exposed to one of two CO² concentrations (ambient CO², AC, or AC + 350 μmol mol-1 = elevated CO₂, EC) continuously over three growing seasons. To evaluate the EC influence, measurements of the relations between the rate of net CO₂ uptake (P _N ) and incidental photosynthetically active photon flux density (PPFD), as well as the content of photosynthetic pigments and chlorophyll (Chl) a fluorescence were taken in the third growing season. The short-term response to EC was evident mainly on ribulose-1,5-bisphosphate carboxylase/oxygenase kinetics without any significant change to the utilization of radiant energy. The long-term effect of EC was responsible for a decrease in P _N , content of Chl a + b, Fv/Fm ratio, quantum yield of fluorescence, and photochemical quenching. Changes of stoichiometry between the electron transport, Calvin cycle and the end-product synthesis were confirmed for responses to the long-term import of EC and led to a definition of the photosynthetic acclimation to EC in Norway spruce. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of leaf age, nitrogen nutrition and photon flux density on the organization of the photosynthetic apparatus in leaves of a vine (Ipomoea tricolor Cav.) grown horizontally to avoid mutual shading of leaves

Effects of leaf age, nitrogen nutrition and photon flux density (PFD) on the organization of the photosynthetic apparatus in leaves were investigated in a vine, Ipomoea tricolor Cav., which was grown horizontally so as to avoid mutual shading of leaves. The plants were grown hydroponically at two nitrate levels under two growth light treatments. For one group of the plants, leaves were exposed to full sunlight. For another group, respective leaves were artificially shaded in a manner that simulated changes in the light gradient with the development of an erect herbaceous canopy: old leaves were placed under progressively shadier conditions with growth of the plants (canopy-type shading). In all the treatments, chlorophyll (Chl) content gradually decreased with leaf age. Photosystem I (PSI) per Chl was constant, independent of leaf age, nitrogen nutrition and/or PFD. Photosystem II (PSII) and cytochrome / per Chl, and Chl a/b ratio were independent of leaf age and/or nitrogen nutrition but decreased with the decrease in growth PFD. Ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39, RuBPCase) per Chl steeply decreased with decrease in PFD. When leaves grown at the same PFD were compared, RuBPCase/Chl was lower in the plants grown under lower nitrogen availability and also decreased with leaf age in the plants grown without shading. These decreases were attributed to the curvilinear relationship between RuBPCase and Chl in leaves grown at full sunlight, that was independent of nitrogen availability and leaf age. From these results, it is concluded that the composition of the photosynthetic apparatus is independent of leaf age but changes depending on the light environment and total amount of photosynthetic components of the leaf.Abbreviations Chl chlorophyll - cyt f cytochrome f - PFD photon flux density - RuBPCase ribulose-1,5-bisphosphate carboxylase The author thanks Drs. K. Sonoike, Y. Kashino, K. Okada, H. Hatanaka, Y. Suzuki and A. Aoyama for technical advise. The author also thanks Drs. I. Terashima, A. Makino (Tohoku University, Sendai, Japan), Dr. J.R. Evans (Research School of Biological Sciences, Australian National University, Canberra) and Prof. A. Watanabe for valuable suggestions.     

Biochemical Changes that Occur during Senescence of Wheat Leaves : I. Basis for the Reduction of Photosynthesis

Changes in activities of photosynthetic enzymes and photochemical processes were followed with aging of vegetative and flag leaves of wheat (Triticum aestivum L. cv Roy). Activities of stromal enzymes began to decline prior to photochemical activities. In general, total soluble protein and the activities of ribulose-1,5-bisphosphate carboxylase and NADP-triose-phosphate dehydrogenase declined in parallel and at an earlier age than leaf chlorophyll (Chl), leaf photosynthesis, and photosynthetic electron transport activity. Leaves appeared to lose whole chloroplasts as opposed to a general degradation of all chloroplasts based on three lines of evidence: (a) electron transport activity calculated on an area basis declined much earlier than the same data expressed on a Chl basis; (b) Chl content per chloroplast was similar for mature and senescent tissue; and (c) the absorbance at 550 nanometers (light scattering) per unit of Chl remained essentially constant until the end of senescence. Chloroplasts did, however, undergo some modifications before they were lost (e.g. loss of stromal enzyme activities), but the reduction in leaf photosynthesis was apparently caused by a loss of whole chloroplasts.     

A protector effect of cytokinin preparations on the photosynthetic apparatus of wheat plants under water deficiency conditions

The protective effects of the cytokinin 6-benzylaminopurine and the compounds thidiazuron and kartolin, displaying a cytokinin activity, on the photosynthetic apparatus of young seedlings and leaves of adult plants of two wheat (Triticum aestivum L.) cultivars, Mironovskaya 808 (more drought tolerant) and Lutescens 758 (less tolerant to water stress), were compared on the background of an increasing water deficiency. At the stages of drought and subsequent rehydration, kartolin preparations were the most efficient protectors, enhancing a less pronounced decrease in the intensity of photosynthesis, carboxylating activity of the key enzyme of carbon metabolism—ribulose bisphosphate carboxylase/oxygenase (EC 4.1.1.39)—and the activity of NADP—glyceraldehyde phosphate dehydrogenase—the enzyme complex comprising phosphoglycerate kinase (EC 2.7.2.3.) and glyceraldehyde phosphate dehydrogenase (EC 1.2.1.13). This effect also included an increase in the leaf specific density and plant productivity. The negative influence of water stress on the photosynthetic apparatus was more pronounced in a less tolerant cultivar Lutescens 758 and in the seedlings as compared with the adult plants.     

Effect of water deficit on photosynthetic and other physiological responses in grapevine (Vitis vinifera L. cv. Riesling) plants

The grapevine (Vitis vinifera L. cv. Riesling) plants subjected to water deficit were studied for changes in relative water content (RWC), leaf dry mass, contents of chlorophyll (Chl), total leaf proteins, free amino acids, and proline, and activities of ribulose-1,5-bisphosphate carboxylase (RuBPC), nitrate reductase (NR), and protease. In water-stressed plants RWC, leaf dry matter, Chl content, net photosynthetic rate (P _N), and RuBPC and NR activities were significantly decreased. The total leaf protein content also declined with increase in the accumulation of free amino acids. Concurrently, the protease activity in the tissues was also increased. A significant two-fold increase in proline content was recorded.     

Light‐induced systemic regulation of photosynthesis in primary and trifoliate leaves of Phaseolus vulgaris: effects of photosynthetic photon flux density (PPFD) versus spectrum

The objectives of this work using Phaseolus vulgaris were to examine whether the light spectrum incident on mature primary leaves (PLs) is related to leaf‐to‐leaf systemic regulation of developing trifoliate leaves (TLs) in photosynthetic characteristics, and to investigate the relative importance of spectrum and photosynthetic photon flux density (PPFD) in light‐induced systemic regulation. Systemic regulation was induced by altering PPFD and the spectrum of light incident on PLs using a shading treatment and lighting treatments including either white, blue, green or red light‐emitting diodes (LEDs). Photosynthetic characteristics were evaluated by measuring the light‐limited and light‐saturated net photosynthetic rates and the amounts of nitrogen (N), chlorophyll (Chl) and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39). Shading treatment on PLs decreased the amounts of N, Chl and Rubisco of TLs and tended to decrease the photosynthetic rates. However, we observed no systemic effects induced by the light spectrum on PLs in this study, except that a higher amount of Rubisco of TLs was observed when the PLs were irradiated with blue LEDs. Our results imply that photoreceptors in mature leaves have little influence on photosynthetic rates and amounts of N and Chl of developing leaves through systemic regulation, although the possibility of the action of blue light irradiation on the amount of Rubisco cannot be ruled out. Based on these results, we concluded that the light spectrum incident on mature leaves has little systemic effect on developing leaves in terms of photosynthetic characteristics and that the light‐induced systemic regulation was largely accounted for by PPFD.     

Distinctive Responses of Ribulose-1,5-Bisphosphate Carboxylase and Carbonic Anhydrase in Wheat Leaves to Nitrogen Nutrition and their Possible Relationships to CO(2)-Transfer Resistance

The amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total chlorophyll (Chl), and total leaf nitrogen were measured in fully expanded, young leaves of wheat (Triticum aestivum L.), rice (Oryza sativa L.), spinach (Spinacia oleracea L.), bean (Phaseolus vulgaris L.), and pea (Pisum sativum L.). In addition, the activities of whole-chain electron transport and carbonic anhydrase were measured. All plants were grown hydroponically at different nitrogen concentrations. Although a greater than proportional increase in Rubisco content relative to leaf nitrogen content and Chl was found with increasing nitrogen supply for rice, spinach, bean, and pea, the ratio of Rubisco to total leaf nitrogen or Chl in wheat was essentially independent of nitrogen treatment. In addition, the ratio of Rubisco to electron transport activities remained constant only in wheat. Nevertheless, gas-exchange analysis showed that the in vivo balance between the capacities of Rubisco and electron transport in wheat, rice, and spinach remained almost constant, irrespective of nitrogen treatment. The in vitro carbonic anhydrase activity in wheat was very low and strongly responsive to increasing nitrogen content. Such a response was not found for the other C₃ plants examined, which had 10- to 30-fold higher carbonic anhydrase activity than wheat at any leaf-nitrogen content. These distinctive responses of carbonic anhydrase activity in wheat were discussed in relation to CO₂-transfer resistance and the in vivo balance between the capacities of Rubisco and electron transport.     

Seasonal pattern of photosynthetic rate and its relationship with chlorophyll content,ribulose-1,5-bisphosphate carboxylase activity and biomass production

Net photosynthetic rate (P_N), ribulose-1,5-bisphosphate carboxylase (RuBPC) activity, chlorophyll (Chl) content and biomass production were estimated at monthly intervals inChukrasia tabularis, Dolichandrone atrovirens, Eugenia jambolana, Gmelina arborea, Lannea coromandelica, Terminalia arjuna andTerminalia bellerica from September 1990 to August 1991. The leaves of all the seven tree species showed significantly higher P_N during summer than in winter and these rates differed from one species to the other. A positive correlation was found between P_N of different tree species and their Chl content or biomass production. There was no significant correlation between ribulose-1,5-bisphosphate carboxylase activity and P_N when these were expressed on leaf area basis.     

Seasonal pattern of photosynthetic rate and its relationship with chlorophyll content,ribulose-1,5-bisphosphate carboxylase activity and biomass production

Net photosynthetic rate (P_N), ribulose-1,5-bisphosphate carboxylase (RuBPC) activity, chlorophyll (Chl) content and biomass production were estimated at monthly intervals inChukrasia tabularis, Dolichandrone atrovirens, Eugenia jambolana, Gmelina arborea, Lannea coromandelica, Terminalia arjuna andTerminalia bellerica from September 1990 to August 1991. The leaves of all the seven tree species showed significantly higher P_N during summer than in winter and these rates differed from one species to the other. A positive correlation was found between P_N of different tree species and their Chl content or biomass production. There was no significant correlation between ribulose-1,5-bisphosphate carboxylase activity and P_N when these were expressed on leaf area basis.     

Genetic Variability in Carbon Fixation, Sucrose-P-Synthase and ADP Glucose Pyrophosphorylase in Maize Plants of Differing Growth Rate

The net photosynthetic rate and the activities of ribulose 1,5 bisphosphate carboxylase (RubisCo), phosphoenolpyruvate carboxylase, sucrose-P-synthase, and ADP glucose-pyrophosphorylase, key enzymes of the leaf carbohydrate metabolism were compared in eight maize (Zea mays L.) genotypes presenting large differences in growth rate. The sucrose-P-synthase activity varied in the ratio 1 to 3 from the less active to the more active genotype and this variation was highly correlated with those in growth rate. ADP glucose pyrophosphorylase activity was not significantly different from one genotype to another whatever the basis for expression, leaf area, or soluble protein. The photosynthetic rate varied with similar amplitude (1:1) to the RubisCo activity or RubisCo quantity but the correlation with growth rate was highly significant for photosynthesis and nonsignificant for RubisCo or phosphoenolpyruvate carboxylase. So, in our series of genotypes the sucrose synthesis capacities as expressed by sucrose phosphate synthase activity seem to have a good predicting value for mean growth rate at a young stage.     

Photosynthesis and nitrogen relationships in leaves of C3 plants

Summary The photosynthetic capacity of leaves is related to the nitrogen content primarily bacause the proteins of the Calvin cycle and thylakoids represent the majority of leaf nitrogen. To a first approximation, thylakoid nitrogen is proportional to the chlorophyll content (50 mol thylakoid N mol^-1 Chl). Within species there are strong linear relationships between nitrogen and both RuBP carboxylase and chlorophyll. With increasing nitrogen per unit leaf area, the proportion of total leaf nitrogen in the thylakoids remains the same while the proportion in soluble protein increases. In many species, growth under lower irradiance greatly increases the partitioning of nitrogen into chlorophyll and the thylakoids, while the electron transport capacity per unit of chlorophyll declines. If growth irradiance influences the relationship between photosynthetic capacity and nitrogen content, predicting nitrogen distribution between leaves in a canopy becomes more complicated. When both photosynthetic capacity and leaf nitrogen content are expressed on the basis of leaf area, considerable variation in the photosynthetic capacity for a given leaf nitrogen content is found between species. The variation reflects different strategies of nitrogen partitioning, the electron transport capacity per unit of chlorophyll and the specific activity of RuBP carboxylase. Survival in certain environments clearly does not require maximising photosynthetic capacity for a given leaf nitrogen content. Species that flourish in the shade partition relatively more nitrogen into the thylakoids, although this is associated with lower photosynthetic capacity per unit of nitrogen.     

Photosynthesis and kinetic characteristics of rubisco in Hibiscus cannabinus L

Photosynthetic characteristics in kenaf (Hibiscus cannabinus L.), a C3 plant, were compared with Abelmoschus esculentus (L.) Moench, another member of Malvaceae. Kenaf leaves exhibited significantly higher rate of photosynthesis (40 mg CO2 dm(-2) hr(-1)) which was 24.6 mg dm(-2) hr(-1) in A. esculentus. Rate of photo and dark respiration was similar in both the species. Kenaf leaf photosynthesis had a higher optimum temperature (32 degrees C) than that of A. esculentus (26 degrees C). Photosynthesis in kenaf leaves required higher saturation irradiance (1,600 micromole m(-2) sec(-1)). There was a significant correlation between photosynthetic rate and biomass yield in these species. The primary product of photosynthesis after 5 seconds of 14C-assimilation was 3-PGA in both the species. The kinetic properties of RuBP carboxylase/oxygenase were determined in the leaf extracts. Higher carboxylase activities were recorded with kenaf leaf extracts (245 pmole mg chl(-1) hr(-1)). Km (CO2) for kenaf leaf carboxylase was significantly lower (7.8 microM) than A. esculentus (13.5 microM) and corresponding difference in Vmax values of carboxylase was recorded between the two species. The kinetic characteristics of oxygenase were similar in both the extracts. These results indicated the variation in carboxylase activity and its kinetic characteristics reflected a significant difference in CO2 assimilation in C3 plants.     

Mulberry Leaf Metabolism under High Temperature Stress

Effects of high temperature on the activity of photosynthetic enzymes and leaf proteins were studied in mulberry (Morus alba L. cv. BC2-59). A series of experiments were conducted at regular intervals (120, 240 and 360 min) to characterize changes in activities of ribulose-1,5-bisphosphate carboxylase (RuBPC) and sucrose phosphate synthase (SPS), photosystem 2 (PS 2) activity, chlorophyll (Chl), carotenoid (Car), starch, sucrose (Suc), amino acid, free proline, protein and nucleic acid contents in leaves under high temperature (40 °C) treatments. High temperature markedly reduced the activities of RuBPC and SPS in leaf extracts. Chl content and PS 2 activity in isolated chloroplasts were also affected by high temperature, particularly over 360 min treatment. Increased leaf temperature affected sugar metabolism through reductions in leaf starch content and sucrose-starch balance. While total soluble protein content decreased under heat, total amino acid content increased. Proline accumulation (1.5-fold) was noticed in high temperature-stressed leaves. A reduction in the contents of foliar nitrogen and nucleic acids (DNA and RNA) was also noticed. SDS-PAGE protein profile showed few additional proteins (68 and 85 kDa) in mulberry plants under heat stress compared to control plants. Our results clearly suggest that mulberry plants are very sensitive to high temperature with particular reference to the photosynthetic carbon metabolism.     

The expression of photosynthetic traits during and following severe chilling stress of European and tropical maize genotypes

Four genotypes of maize ( Zea mays L.), one from North-West Europe, two from tropical highlands and one from tropical lowlands were grown at 24°C until full expansion of the second leaf. Seedlings were then subjected to 2 days at 5°C, with or without a previous conditioning phase at 10°C for 4 days. After stress plants were allowed to recover at 24°C for 5 days. Previous conditioning protected the tropical lowland variety to some extent against losses in chlorophyll content and ribulose I,5-bisphosphate carboxylase (EC 4.1.1.39) activity by severe chilling stress. Without conditioning heavy losses occurred during the recovery phase. In all other genotypes the values of these traits decreased more with than without conditioning. Phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity was little affected during stress, but declined considerably during the recovery phase, with the exception of the North-West European genotype. Previous conditioning prevented or retarded losses of NADP⁺ malate dehydrogenase (EC 1.1.1.82) activity during the stress phase. However, at the end of the recovery phase losses were as high with as without conditioning, and activity was very low in one tropical highland genotype and in the tropical lowland genotype. In summary, the chilling stability of photosynthetic traits was better in the North-West European genotype than in the tropical ones.