Increase rate of light-induced stomatal conductance is related to stomatal size in the genus Oryza

The rapid response of stomatal conductance (gs) to fluctuating irradiance is of great importance to maximize carbon assimilation while minimizing water loss. Smaller stomata have been proven to have a faster response rate than larger ones, but most of these studies have been conducted with forest trees. In the present study, the effects of stomatal anatomy on the kinetics of gs and photosynthesis were investigated in 16 Oryza genotypes. Light-induced stomatal opening includes an initial time lag (λ) followed by an exponential increase. Smaller stomata had a larger maximum stomatal conductance increase rate (Slmax) during the exponential increase phase, but showed a longer time lag and a lower initial stomatal conductance (gs,initial) at low light. Stomatal size was, surprisingly, negatively correlated with the time required to reach 50% of maximum gs and photosynthesis (T50%gs and T50%A), which was shown to be positively correlated with λ and negatively correlated with gs,initial. With a lower gs,initial and a larger λ, small stomata showed a faster decrease of intercellular CO2 concentration (Ci) during the induction process, which may have led to a slower apparent Rubisco activation rate. Therefore, smaller stomata do not always benefit photosynthesis as reported before; the influence of stomatal size on dynamic photosynthesis is also correlated with λ and gs,initial.