The effects of 24‐epibrassinolide (EBR) spray application on gas‐exchange, chlorophyll fluorescence characteristics, Rubisco activity, and carbohydrate metabolism were investigated in cucumber (Cucumis sativus L. cv. Jinchun No. 3) plants grown in a greenhouse. EBR significantly increased the light‐saturated net CO₂ assimilation rate (A_sat) from 3 h to 7d after spraying, with 0.1 mg l^–1 EBR proving most effective. Increased A_sat in EBR‐treated leaves was accompanied by increases in the maximum carboxylation rate of Rubisco (V_c,max) and in the maximum rate of RuBP regeneration (J_max). EBR‐treated leaves also had a higher quantum yield of PSII electron transport (ϕ_PSII) than the controls, which was mainly due to a significant increase in the photochemical quenching (q_P), with no change in the efficiency of energy capture by open PSII reaction centres (F^′_v/F^′_m). EBR did not influence photorespiration. In addition, significant increases in the initial activity of Rubisco and in the sucrose, soluble sugars, and starch contents were observed followed by substantial increases in sucrose phosphate synthase (SPS), sucrose synthase (SS), and acid invertase (AI) activities after EBR treatment. It was concluded that EBR increases the capacity of CO₂ assimilation in the Calvin cycle, which was mainly attributed to an increase in the initial activity of Rubisco.

The carbon isotope composition (δ¹³C) of CO₂ produced in darkness by intact French bean (Phaseolus vulgaris) leaves was investigated for different leaf temperatures and during dark periods of increasing length. The δ¹³C of CO₂ linearly decreased when temperature increased, from −19‰ at 10°C to −24‰ at 35°C. It also progressively decreased from −21‰ to −30‰ when leaves were maintained in continuous darkness for several days. Under normal conditions (temperature not exceeding 30°C and normal dark period), the evolved CO₂ was enriched in ¹³C compared with carbohydrates, the most ¹³C-enriched metabolites. However, at the end of a long dark period (carbohydrate starvation), CO₂ was depleted in ¹³C even when compared with the composition of total organic matter. In the two types of experiment, the variations of δ¹³C were linearly related to those of the respiratory quotient. This strongly suggests that the variation of δ¹³C is the direct consequence of a substrate switch that may occur to feed respiration; carbohydrate oxidation producing ¹³C-enriched CO₂ and β-oxidation of fatty acids producing ¹³C-depleted CO₂ when compared with total organic matter (−27.5‰). These results are consistent with the assumption that the δ¹³C of dark respired CO₂ is determined by the relative contributions of the two major decarboxylation processes that occur in darkness: pyruvate dehydrogenase activity and the Krebs cycle.

The effects of ultraviolet-B (UV-B) radiation on water relations, leaf development, and gas-exchange characteristics in pea (Pisum sativum L. cv Meteor) plants subjected to drought were investigated. Plants grown throughout their development under a high irradiance of UV-B radiation (0.63 W m−2) were compared with those grown without UV-B radiation, and after 12 d one-half of the plants were subjected to 24 d of drought that resulted in mild water stress. UV-B radiation resulted in a decrease of adaxial stomatal conductance by approximately 65%, increasing stomatal limitation of CO2 uptake by 10 to 15%. However, there was no loss of mesophyll light-saturated photosynthetic activity. Growth in UV-B radiation resulted in large reductions of leaf area and plant biomass, which were associated with a decline in leaf cell numbers and cell division. UV-B radiation also inhibited epidermal cell expansion of the exposed surface of leaves. There was an interaction between UV-B radiation and drought treatments: UV-B radiation both delayed and reduced the severity of drought stress through reductions in plant water-loss rates, stomatal conductance, and leaf area.

Despite its relevance, protein regulation, metabolic adjustment, and the physiological status of plants under drought is not well understood in relation to the role of nitrogen fixation in nodules. In this study, nodulated alfalfa plants were exposed to drought conditions. The study determined the physiological, metabolic, and proteomic processes involved in photosynthetic inhibition in relation to the decrease in nitrogenase (N_ase) activity. The deleterious effect of drought on alfalfa performance was targeted towards photosynthesis and N_ase activity. At the leaf level, photosynthetic inhibition was mainly caused by the inhibition of Rubisco. The proteomic profile and physiological measurements revealed that the reduced carboxylation capacity of droughted plants was related to limitations in Rubisco protein content, activation state, and RuBP regeneration. Drought also decreased amino acid content such as asparagine, and glutamic acid, and Rubisco protein content indicating that N availability limitations were caused by N_ase activity inhibition. In this context, drought induced the decrease in Rubisco binding protein content at the leaf level and proteases were up-regulated so as to degrade Rubisco protein. This degradation enabled the reallocation of the Rubisco-derived N to the synthesis of amino acids with osmoregulant capacity. Rubisco degradation under drought conditions was induced so as to remobilize Rubisco-derived N to compensate for the decrease in N associated with N_ase inhibition. Metabolic analyses showed that droughted plants increased amino acid (proline, a major compound involved in osmotic regulation) and soluble sugar (D-pinitol) levels to contribute towards the decrease in osmotic potential (Ψ_s). At the nodule level, drought had an inhibitory effect on N_ase activity. This decrease in N_ase activity was not induced by substrate shortage, as reflected by an increase in total soluble sugars (TSS) in the nodules. Proline accumulation in the nodule could also be associated with an osmoregulatory response to drought and might function as a protective agent against ROS. In droughted nodules, the decrease in N₂ fixation was caused by an increase in oxygen resistance that was induced in the nodule. This was a mechanism to avoid oxidative damage associated with reduced respiration activity and the consequent increase in oxygen content. This study highlighted that even though drought had a direct effect on leaves, the deleterious effects of drought on nodules also conditioned leaf responsiveness.