Features of photosynthesis and water regime of Quercus pubescens Willd

The ecophysiological response of Quercus pubescens Willd on the impact of progressive soil drought has been studied and the optimum zones and threshold values of soil moisture, temperature, illumination, limiting photosynthesis and transpiration have been determined. The dependences of the rate of net photosynthesis (Pn) and the intensity of transpiration (E) on stomatal conductance (g_s), leaf temperature (T_l) and soil moisture (Ws) are shown. The relationship between the speed of the xylem flow in the plant shoot (Sf, rel. units), the change in the diameter of this shoot (d, mm) and soil moisture has been determined. In the process of dehydration of the plant, a synchronous decrease in these parameters is observed, which serves as a signal for watering the plant. The obtained ratios of the processes of photosynthesis and respiration under the influence of soil drought made it possible to determine their optimal and limiting conditions. The optimal values are: Ws = 19 - 22%, Pn = 12 - 13 µmol/m²s. With such soil moisture, total respiration Pt = 1.2 - 1.3 μmol/m²s. Changes in the ratio between the components of total respiration (Pt, Rd, Rph) under the influence of soil drought were studied. Watering of plants has a significant effect on the studied parameters, and after repeated watering and subsequent soil drought, their general decrease is observed. The relationship between the rate of net photosynthesis (Pn), total respiration (Pt) and leaf temperature (Tl) made it possible to determine the temperature maximum of net photosynthesis (thermal compensation point). At this point, point Pn = Pt = 2.0 μmol/m²s at Tl = 37º<C. The fraction of respiratory costs from true photosynthesis (Pt /Pg) was determined, which is an integral indicator of the energy balance of the whole plant. It was found that in the absence of soil drought, the share of respiratory expenditure from true photosynthesis is 25-35%, and at soil moisture of 3-7% it increases to 58-62%. The obtained research results made it possible to determine the optimal and limiting conditions for the growth of this species and the possibility of its introduction to other regions.

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