Rapid stomatal response to fluctuating light: an under-explored mechanism to improve drought tolerance in rice.

Light inside a canopy constantly fluctuates. Under fluctuating light (FL) conditions, stomatal conductance and photosynthetic rate constantly change. In this study, we explored whether this dynamics of stomata movements upon FL influenced the water use efficiency of rice in the field. We used a USDA-curated rice mini-core diversity panel consisting of 204 worldwide distributed accessions. A priori model on dynamic stomatal response to FL was utilised to identify kinetic parameters describing the stomatal delays during the closing (τcl) and the opening (τop) phase. Result showed that τcl had a larger variation than τop across the mini-core panel. τcl was negatively correlated with water use efficiency (WUE) related traits, stem diameter, grain weight per tiller and heading time, but positively correlated with maximum annual temperature, carbon assimilation related traits and biomass (P<0.05). We further showed a strong correlation of τcl with the relative decrease of biomass under drought in 14 accessions with different τcl. We discussed the adjustment of stomatal conductance under fluctuating light in light of the trade-off between optimising CO2 uptake and optimising water saving. This study suggests that stomatal dynamics under fluctuating light is closely related to drought resistance and hence detailed study is needed to enable its application in breeding drought tolerance in rice.