Stomatal conductance in rice leaves and panicles responds differently to abscisic acid and soil drought.

Improvement of photosynthesis in non-foliar green tissues is beneficial for enhancing crop yield. Recently, we have demonstrated that panicle stomatal conductance is a major limiting factor for photosynthesis. However, mechanisms underlying the responses of panicle stomatal conductance (gs,panicle) and photosynthesis (Apanicle) to environmental stimuli remain unknown. In the present study, the responses of gs,panicle and leaf stomatal conductance (gs,leaf) to exogenous application of abscisic acid (ABA) and step-changes in vapor pressure deficit (VPD) were investigated at the anthesis stage in pot-grown rice plants. Furthermore, the effects of drought on Apanicle and leaf photosynthesis (Aleaf) were examined. Smearing and xylem feeding of ABA significantly decreased gs,leaf. In contrast, while smearing of ABA substantially increased gs,panicle, its xylem feeding dramatically decreased gs,panicle. In addition, both gs,leaf and gs,panicle effectively responded to step changes in VPD. Furthermore, both Aleaf and Apanicle were sensitive to plant dehydration; however, given the lower sensitivity of panicle water potential than that of leaf water potential to drought, Apanicle was less sensitive to soil drought than Aleaf. These findings indicate that gs,panicle is hydropassively regulated, while panicle photosynthesis is less sensitive to drought.