Shade Shoot Structure of Conifers and the Photosynthetic Response to Light at Two CO 2 Partial Pressure

Abstract

The response of net photosynthesis to irradiance was measured for shade-adapted shoots of different conifer species. Shoots were illuminated unidirectionally or in a light integrating sphere to study the effects of shoot structure. Shoot structure was quantified as R maX , the ratio of the shoot-silhouette area to the leaf-silhouette area. The initial slopes and the convexities (rate of bending) of the light response curves were strongly affected by R maX during unilateral illumination. There was also a strong positive effect of R max on the maximum efficiency of net photosynthesis and a strong negative effect of R max on the light compensation point. Increasing atmospheric CO 2 partial pressure (C a ) from 35 to 70 Pa did not affect the convexity of the light response curves nor rates of dark respiration. Increasing C a affected the initial slope, the light compensation point, the maximum rate of photosynthesis and the efficiency of net photosynthesis. Except for the maximum rate of net photosynthesis, the responses to C a were controlled by shoot structure. Studies of the effect of atmospheric CO 2 on photosynthesis and growth in conifers need to consider variations in shoot structure.