Plant drought tolerance has a significant impact on survival in a changing environment. Atmospheric drying, i.e. raising vapour pressure deficit, has become an important driver of plant-water relations, as it increases both evaporation and transpiration. Plants respond to higher evaporative demand by closing their stomata, thereby lowering water loss by transpiration and maintaining turgor levels and hydraulic conductivity, but at the cost of a decreased carbon dioxide uptake and photosynthetic rate. To study the response of two oak species, pedunculate (Quercus robur) and sessile oak (Q. petraea), to harsh environmental conditions, an experiment with 30 seedlings was carried out. We focused on hot dry days with a significant increase of midday VPD values. Stomatal conductance, transpiration, VPDleaf and photochemical efficiency were measured on two days in July 2024. These stressful conditions led to low stomatal conductance, high VPDleaf and low photosynthetic efficiency, which indicated the stress the seedlings were under. No statistically significant differences in midday stomatal conductance between the species were found. To compare the results with values in milder conditions, another series of measurements was made on a less extreme day in August 2025. While the conditions were notably milder, no differences between the species were found that day either. To determine whether the two oak species differ in their responses to atmospheric drought, further studies would be needed.
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