The impact of stomatal kinetics on diurnal photosynthesis and water use efficiency under fluctuating light

Dynamic light conditions require continuous adjustments of stomatal aperture. As stomatal conductance (g s ) kinetics are a magnitude slower than photosynthesis ( A ), they are hypothesized to be key to plant productivity and water use efficiency. Using step-changes in light intensity, we studied the diversity of light-induced g s kinetics in relation to stomatal anatomy in five banana genotypes ( Musa spp.) and modelled the impact on A and intrinsic water use efficiency ( i WUE). Banana generally exhibited a strong limitation of A by g s , indicating a priority for water saving. Significant genotypic differences in g s kinetics and g s -based limitations of A were observed. For two contrasting genotypes the impact of differential g s kinetics on A and i WUE was further investigated under realistic diurnally fluctuating light conditions and at whole-plant level. Genotype-specific stomatal kinetics observed at the leaf level were corroborated at whole-plant level, suggesting that despite differences in g s control at different locations in the leaf and across leaves, genotype-specific responses are still maintained. However, under diurnally fluctuating light conditions g s speediness had only a momentary impact on the diurnal i WUE and carbon gain. During the afternoon there was a setback in kinetics: the absolute g s and the g s responses to light were damped, strongly limiting A and the diurnal i WUE. We conclude that the impact of the differential g s kinetics on the limitation of A was dependent on the target light intensity, the magnitude of change, the g s prior to the intensity change and particularly the time of the day. One sentence summary Genotype-specific stomatal rapidity is for the first time validated at whole-plant level, but under fluctuating light the impact of stomatal dynamics depends on other factors like the time of the day.