Carbon and nitrogen assimilation in red oaks (Quercus rubra L.) subject to defoliation and nitrogen stress.

To examine how rates of net photosynthesis and N uptake of red oak seedlings respond to defoliation under contrasting conditions of N availability, nitrogen-deficient plants were grown in sand culture and subjected to partial defoliation and increased N availability under low light conditions. Both photosynthesis and N uptake rates were measured regularly before and after the treatments. Defoliation resulted in elevated rates of net photosynthesis in both low-N and high-N trees, but the high-N trees were able to maintain the high photosynthetic rates for a longer period of time. Nitrogen availability did not affect the photosynthetic rate of the undefoliated plants. Nitrogen uptake was not affected by the defoliation treatment, but was increased by increasing N availability in both the defoliated and undefoliated plants. Nitrogen uptake rates increased less than would be expected on the basis of N availability alone, but the uptake rates were apparently not limited by carbon supply in the short term. Suboptimal concentrations of N in plant tissues resulted in a strong sink for N even in the absence of refoliation.