Aquaporins are major determinants of water use efficiency of riceplantsriceplants in the field

tThis study aimed at specifying the reasons of unbalanced water relations of rice in the field at middaywhich results in slowing down photosynthesis and reducing water use efficiency (WUE) in japonica andindica rice under well-watered and droughted conditions. Leaf relative water content (RWC) decreasedin the well-watered plants at midday in the field, but more dramatically in the droughted indica (75.6 and71.4%) than japonica cultivars (85.5 and 80.8%). Gas exchange was measured at three points during theday (9:00, 13:00 and 17:00). Leaf internal CO2(Ci) was not depleted when midday stomatal depressionwas highest indicating that Ciwas not limiting to photosynthesis. Most aquaporins were predominantlyexpressed in leaves suggesting higher water permeability in leaves than in roots. The expression of leafaquaporins was further induced by drought at 9:00 without comparable responses in roots. The datasuggest that aquaporin expression in the root endodermis was limiting to water uptake. Upon removal ofthe radial barriers to water flow in roots, transpiration increased instantly and photosynthesis increasedafter 4 h resulting in increasing WUE after 4 h, demonstrating that WUE in rice is largely limited by theinadequate aquaporin expression profiles in roots.