Foliar metabolite levels, including myoinositol, show correlation with grain yield in tropical maize field trials during drought, heat, and simultaneous drought

While the responses of photosynthesis to water stress have been widely studied, acclimation to sustained water stress and recovery after re-watering is poorly understood. In particular, the factors limiting photosynthesis under these conditions, and their possible interactions with other environmental conditions, are unknown. To assess these issues, changes of photosynthetic CO₂ assimilation (A_N) and its underlying limitations were followed during prolonged water stress and subsequent re-watering in tobacco (Nicotiana sylvestris) plants growing under three different climatic conditions: outdoors in summer, outdoors in spring, and indoors in a growth chamber. In particular, the regulation of stomatal conductance (g_s), mesophyll conductance to CO₂ (g_m), leaf photochemistry (chlorophyll fluorescence), and biochemistry (V_c,max) were assessed. Leaf gas exchange and chlorophyll fluorescence data revealed that water stress induced a similar degree of stomatal closure and decreased A_N under all three conditions, while V_c,max was unaffected. However, the behaviour of g_m differed depending on the climatic conditions. In outdoor plants, g_m strongly declined with water stress, but it recovered rapidly (1–2 d) after re-watering in spring while it remained low many days after re-watering in summer. In indoor plants, g_m initially declined with water stress, but then recovered to control values during the acclimation period. These differences were reflected in different velocities of recovery of A_N after re-watering, being the slowest in outdoor summer plants and the fastest in indoor plants. It is suggested that these differences among the experiments are related to the prevailing climatic conditions, i.e. to the fact that stress factors other than water stress have been superimposed (e.g. excessive light and elevated temperature). In conclusion, besides g_s, g_m contributes greatly to the limitation of photosynthesis during water stress and during recovery from water stress, but its role is strongly dependent on the impact of additional environmental factors.

• The close rosette growth form, short petioles and small leaves of Arabidopsis thaliana make measurements with commercial gas exchange cuvettes difficult. This difficulty can be overcome by growing A. thaliana plants in ‘ice-cream cone-like’ soil pots. • This design permitted simultaneous gas exchange and chlorophyll fluorescence measurements from which the first estimates of mesophyll conductance to CO₂ (g_m) in Arabidopsis were obtained and used to determine photosynthetic limitations during plant ageing from c. 30–45 d. • Estimations of g_m showed maximum values of 0.2 mol CO₂ m⁻² s⁻¹ bar⁻¹, lower than expected for a thin-leaved annual species. The parameterization of the response of net photosynthesis (A_N) to chloroplast CO₂ concentrations (C_c) yielded estimations of the maximum velocity of carboxylation (V_{c,max_Cc}) which were also lower than those reported for other annual species. As A. thaliana plants aged from 30 to 45 d, there was a 40% decline of A_N that was entirely the result of increased diffusional limitations to CO₂ transfer, with g_m being the largest. • The results suggest that in A. thaliana A_N is limited by low g_m and low capacity for carboxylation. Decreased g_m is the main factor involved in early age-induced photosynthetic decline.