Net photosynthesis and carbon allocation of loblolly pine (Pinus taeda L.) branches in relation to three levels of shade

Net photosynthesis, stomatal conductance, xylem pressure potential and volumetric soil moisture content were measured over two growing seasons (1988 and 1989) on branches of 10-year-old loblolly pine trees growing near Athens, Georgia. Six trees were used for the study, three were irrigated and three did not receive irrigation. Within the crown of each tree, shade treatments were applied at random to three branches at the mid-crown position. The shade treatments were 0% shade (open branch), 30% shade and 60% shade. Photosynthetic light response curves, chlorophyll content, specific leaf area and leaf weight ratio were determined for current year foliage on all study branches in 1989.; Shading reduced mean midday net photosynthesis in approximate proportion to the shade level in both irrigated and non-irrigated trees throughout the growing season. Mean daily stomatal conductance was similarly reduced by shading. Photosynthetic light response curves indicated that the maximum rate of photosynthesis was reduced by the 60% shade treatment. Dark respiration, light compensation point and apparent quantum efficiency were not significantly affected by shading. Specific leaf area, leaf weight ratio and chlorophyll content increased under the heavy shade treatment.; Carbon allocation patterns of the shaded branches were examined in 1989. Lateral branches immediately subtending the shaded branches were labeled with {dollar}sp{lcub}14{rcub}{dollar}CO{dollar}sb2{dollar} on two dates during the growing season. Total activities were determined for the labeled branches immediately after labeling and for the shaded branches seven days after labeling.; Most of the carbon fixed by the lateral branches was either retained in the foliage, respired or translocated basipetally. However, significant amounts of activity were detected in the second flush tissue of the terminal shoots of the 60% shade treatment. Additional experiments in which the shaded terminal shoots were labeled indicated that shading did not significantly increase photosynthate retention. Shoot and needle growth of the first flush was unaffected by the shading treatments, whereas growth of subsequent flushes was reduced by the 30% and 60% shade. Comparisons of total carbon accumulation and photosynthetic production suggested that shifts in carbon allocation patterns were induced by altering source/sink relations. Overall these results indicate that the relative autonomy of loblolly pine branches is largely dependent upon the stage of phenological development.