Effects of light quantity and quality and soil nitrogen status on nitrate reductase activity in rainforest species of the genus Piper.

We studied nitrate reductase (NR) activity in six species of the genus Piper (Piperaceae) growing under a broad range of light availabilities. Field measurements were made on plants growing naturally in rainforest at the Los Tuxtlas Tropical Biological Preserve, Veracruz, Mexico at high- and lowlight extremes for each species. Foliar nitrogen on an area basis was positively related to the average daily photosynthetically active photon flux density (PFD) received by the leaf (r=0.76, p<0.01). In vivo NR activity was highly correlated with PFD (r=0.95, p<0.001) and less so with total leaf nitrogen (r=0.68, p<0.05). In vivo NR activity was always higher in high-light plants than in low-light plants within a species. Similarly, gap species such as P. auritum had much higher in vivo NR activities than shade species such as P. aequale. Soil NO 3- and NH 4+ pools and nitrogen-mineralization rates at Los Tuxtlas were similar between high- and low-light sites, indicating that the elevated NR activities in high-light plants were not the result of higher NO 3- availabilities in high-light microsites. We performed additional experiments at Stanford, California, USA on Piper plants grown at high- and low-light. Foliar NR was highly inducible by nitrate in the gap species (auritum) but not in the generalist (hispidum) or shade (aequale) species. Root NR activities were, in general, an order of magnitude lower than foliar activities. In total, these studies suggest that Piper gap species are inherently more competent to assimilate NO 3- and are better able to respond to sudden increases in NO 3- availability than are shade species.