The agronomic meaning of mesophyll conductance - A comparison among different rice (Oryza spp.) cultivars subjected to rainfed or watered conditions

Sensitivity to drought was explored among seven rice cultivars. Especially, the study dealt with mechanistic links between agronomical productivity and physiological traits related to primary productivity. In order to achieve such insights, uplands and lowlands rice cultivars were compared under both well watered and rainfed conditions. Measurements of photosynthesis, chlorophyll a fluorescence and carbon stable isotopes ratios were performed and related to observations of yields, phenology and biometry of rice cultivars. The effects of mesophyll conductance on water-use efficiency and productivity were particularly focused. Two independent methodologies were adopted for the estimation of the constraints to CO2 diffusion from the intercellular airspaces to the carboxylation sites into the chloroplasts: namely, the "variable J" method and the "carbon isotope discrimination in recently synthesized sugars" method. A wide range of photosynthesis responses, observed at ambient conditions, was ascribed to both water availability and genotypic diversity. Diversities of flowering phenology, height morphometry and physiological responses in water-use efficiency, as estimated by carbon isotope discrimination analysis, were linked to the variability in grain yield, among the tested rice cultivars. A strong positive relationships between grain yield and mesophyll conductance suggests the influence on primary productivity of the CO2 diffusive processes across the cell wall and the other cellular compartments. Perspectives in breeding programmes for increasing the drought tolerance in rice are foreseen.