PLANT HYDRAULIC CONDUCTIVITY DETERMINES PHOTOSYNTHESIS IN RICE UNDER PEG-INDUCED DROUGHT STRESS

Photosynthesis (A) plays a key role in maintaining plant carbon balance, but it is sensitive to drought. Both A and plant hydraulic conductivity (K-plant) decrease under water deficit. It is not clearly whether the declined K-plant is more related to root or leaf, whether the decreased A is related to K-plant and/or leaf hydraulic conductivity (K-leaf) and diffusive alone or both diffusive and metabolic impairments decreased A. Two drought-tolerant (DW) contrasting rice genotypes were used to explore the relationship of A, K-plant and K-leaf under PEG induced drought stress (PEG-DS). The results showed that photosynthesis related parameters of A, stomatal conductance (gs), transpiration rate (Tr), maximum Rubisco carboxylation rate (Vcmax), maximum electron transport rate (J(max)), carboxylation efficiency (CE), K-leaf, K-plant and xylem sap flow rate (XSFR) were all decreased significantly under PEG-DS. These decreases were more severe in DW-sensitive genotype IR64 than DW-tolerant genotype Hanyou-3. However, both intercellular CO2 concentration (Ci) and CO2 concentration inside chloroplasts (Cc) were prominently increased in IR64 rather than in Hanyou-3 under PEG-DS. In addition, both g(s) and g(m) (mesophyll conductance to CO2) were strongly positively correlated with A (R-2=0.98 & 0.71). Photosynthesis of both genotypes were increased with increasing Ci under each treatment. Furthermore, A and gs were significantly correlated with K-plant (R-2 =0.94 & 0.96) but not with K-leaf, and K-plant was not related to K-leaf. K-plant rather than K-leaf determines photosynthesis in rice under drought conditions, which was mainly attributed to K-plant decreases the stomatal conductance and ultimately lead to decrease in photosynthesis.