EFFECT OF SHADE ON STOMATAL CONDUCTANCE, NET PHOTOSYNTHESIS, PHOTOCHEMICAL EFFICIENCY AND GROWTH OF OAK SAPLINGS

The European oak species, pedunculate (Quercus robur) and sessile oak (Q petraea), both considered to be light demanding, were tested for th eir shade tolerance. Two- and 3-year-old nursery grown seedlings were planted either in the open field or with 50% reduction in sun irradian ce, in the spring of 1992. During the following 3 years, growth was mo nitored. In the third summer, the following ecophysiological parameter s were measured: stomatal conductance, net photosynthesis, pho tochemi cal efficiency of dark-adapted leaves, as well as carotenoid and chlor ophyll content. Stomatal conductance and photosynthesis were increased in the open field treatments, while the shaded plants had larger leav es with fewer stomates per unit leaf area, more chlorophyll per unit d ry weight and increased chlorophyll/carotenoid ratio. The photochemica l efficiency of photosystem II as measured on dark-adapted leaves was 3-4% lower in the open field plants as compared to the shade grown one s. During the day it exhibited a decrease at noon in plants of both tr eatments; this decrease recovered completely at the end of the afterno on. There was no difference in overall height of the plants between th e two treatments; however, the root collar diameter was significantly smaller in the shade grown plants. Thus, results of some other investi gations, according to which young oak plants grow better under shade, could not be confirmed.