Effects of Defoliation in the Developing Leaf Zone on Young Populus X euramericana Plants. I. Photosynthetic Physiology, Growth, and Dry Weight Partitioning

Controlled-environment studies were used to test the effects of a single, partial defoliation on growth, dry weight partitioning, photosynthesis, and leaf conductance of young poplar plants. Rates of photosynthesis and leaf conductance were determined and biomass harvests made 24 hr, 3 weeks, and 5 weeks after defoliation in the developing leaf zone. Photosynthesis was calculated from uptake of a 20 s pulse of 14 CO 2 administered to a portion of a leaf by a portable gassing device. Leaf conductance was measured with a diffusion porometer. Insignificant differences in height and diameter growth were observed between control and defoliated plants, but the latter displayed reduced root dry weights. Defoliation stimulated a twofold increase in lateral branch development. Individual leaves developing subsequent to defoliation were up to 43 percent larger than complementary leaves on control plants. Leaves on defoliated plants had significantly higher rates of photosynthesis than complementary leaves on control plants. There was a stimulation of photosynthetic rates on mature leaves below the zone of defoliation within 24 hr of treatment. This effect continued up to 5 weeks. New leaves produced subsequent to defoliation were larger and had higher photosynthetic rates than new leaves on control plants. Leaf conductance was higher on defoliated plants and followed a pattern similar to that of photosynthesis. Specific leaf weight increased slightly on leaves below the defoliated zone, but was significantly reduced on leaves produced subsequent to defoliation. Forest Sci. 28:599-612.