Stomata of shade-adapted leaves of European beech are more responsive to changes in light conditions than light-adapted leaves

Yasin Gundesli, Emilie Joetzjer, Matthias Cuntz, Didier Le Thiec, David Combemale, Cyril Bure, Oliver Brendel
Université de Lorraine, AgroParisTech, INRAE, UMR Silva, 54000 Nancy, France

Stomatal conductance responds gradually to changing light taking several tens of minutes while photosynthesis responds almost instantaneously, leading to suboptimal water-use efficiency: slow stomatal opening limits CO2 uptake due to diffusion limitation and slow stomatal closure results in water loss.
We investigated the impact of different treatments on the temporal decoupling of photosynthesis and stomatal conductance in beech seedlings. Shade-adapted leaves and leaves from drought-stressed plants were more reactive to light changes and have smaller stomatal conductance amplitudes than sun-adapted leaves and leaves from well-watered plants. Stomatal conductance did not respond reactively if drought-stressed plants had also insufficient potassium supply.
Leaves that showed more reactive in closing stomata also showed more reactive when opening stomata, which hints on the same osmotic regulation for opening and closing.
Photosynthesis of shade-adapted leaves profited from more reactive stomatal responses while photosynthesis of drought-stressed plants did not profit from more reactive opening stomata because their low stomatal conductances limited CO2 supply even in the final steady-states. There was water loss during slow closing of stomata, but leaves also conserved water during slow opening of stomata, leading to a negligible influence of the stomatal response times on the water balance of the leaves.