Deep shade alters the acclimation response to moderate water stress in Quercus suber L.

The interactive effects of shade and drought on different morphological and physiological traits were addressed on Quercus suber L. seedlings. In our experiment, limited light treatment (1 per cent) represented the main factor constraining cork oak seedlings growth. Maximal photochemical efficiency (Fv/Fm) with light ≥15 per cent exhibited a midday fall, but under deep shade (1 per cent), Fv/Fm remained constant (<0.8 values) throughout the day. The quantum efficiency of photosystem II (PSII) was lower under moderate drought only in deeply shaded plants. The drought also interacted with light through the increase of the soluble sugar content at 100 per cent light, but not under shade. Under deep shade, soluble sugar content tended to be even lower under moderate drought conditions. The chlorophyll content was not the highest under deep shade as could be expected, while the efficiency of PSII was the lowest under deep shade. We concluded that cork oak can acclimate to moderate shade (15 per cent light), but deep shade impairs some of the physiological responses to cope with low light conditions under moderate drought stress. Plants growing under deep shade were very sensitive to moderate water stress in terms of loss of carbon fixation capacity.