The diurnal time course of leaf gas exchange parameters of pedunculate oak seedlings subjected to experimental drought conditions

The seedlings of pedunculate oak (Quercus robur L.) were exposed to different soil water contents (non-limiting and severe drought) for 30 days in a greenhouse in order to examine the effects of prolonged summer drought on diurnal fluctuations of gas exchange parameters. Results showed that, in control treatment, diurnal course of photosynthetic rate (A), transpiration rate (E) and stomatal conductance (g_s) was bimodal, showing clear midday depression. In stress treatment, the diurnal dynamics of A exhibited an atypical double-peaked pattern, whereas the diurnal change in E and g_s presented a single-peaked curve. A positive and significant correlation between A and g_s was observed in well-watered as well as in drought stressed plants, whereas intercellular CO₂ concentration (Ci) increased with decreasing A rate. Therefore, we might assume that in both treatments the largest part of the reduction in photosynthesis was caused by stomatal closure; however, this finding does not preclude the existence of alternative mechanisms that may also have affected photosynthesis. The diurnal dynamics of instantaneous water use efficiency (WUE) exhibited maximum values in the early morning, afterwards it decreased during late morning and throughout the afternoon with the decline being greater in the stressed than in the control plants. Comparing to the control, the plants subjected to permanent water deficit have increased intrinsic water use efficiency (WUEi) during the early morning, pointing out on minimized loss of water per fixed CO₂. Leaf temperature (T1), as an environmental factor, was in a significant negative correlation with WUE, and in a positive one with E, in both treatments. The results of the study point to the adverse effect of prolonged experimental drought on net photosynthesis and other gas exchange parameters in pedunculate oak leaves, allowing them to be used as an important indicator of environmental conditions and trends in climate change.