Response to light of shade-grown beech seedlings subjected to different watering regimes.

The morphological and physiological responses (net photosynthesis, stomatal conductance, leaf chlorophyll concentration and leaf orientation) of shade-grown beech seedlings after transfer to a high-light regime were studied in well-watered plants and in plants subjected to water stress. Immediately after exposure to high irradiance, shade-grown seedlings displayed a lower photosynthetic rate, a higher leaf chlorophyll concentration and a more horizontal leaf orientation than light-acclimated plants (control plants). The chlorophyll concentration of shade-grown plants gradually declined throughout the period of exposure to high irradiance reaching the amount observed in control plants. Rates of photosynthesis of shade-grown plants began to recover after 8 days of exposure to high irradiance and recovery was nearly complete after 30 days. Leaf orientation of shade-grown plants also changed rapidly during the first 8 days of exposure to high irradiance and by the end of the experiment, there were no significant differences between shade-grown and control plants. Water stress caused significant declines in net photosynthesis and leaf conductance in both shade-grown and control plants. Net photosynthesis did not recover completely in the shade-grown water-stressed plants during the 30-day exposure to high irradiance. Reversible leaf paraheliotropism and chlorophyll bleaching were observed in response to soil drying followed by rewatering.