NITROGEN PRODUCTIVITY DEPENDS ON PHOTOSYNTHETIC NITROGEN USE EFFICIENCY AND ON NITROGEN ALLOCATION WITHIN THE PLANT

The concept of plant nitrogen productivity was introduced at the end o f the 1970s to interpret the dependency of plant growth on internal ni trogen. It is defined as the increase in plant dry matter per unit tim e and per unit plant nitrogen content. Recently, plant nitrogen produc tivity has been expressed as the product of two terms: the leaf nitrog en ratio, which is the proportion of the plant's nitrogen present in t he leaves, and the leaf nitrogen productivity, which is defined as the increase in plant dry matter per unit time and leaf nitrogen content. In the present paper we use two data sets obtained from C-3 herbaceou s species to evaluate the relative importance of variation in leaf nit rogen ratio and leaf nitrogen productivity in determining interspecifi c variation in plant nitrogen productivity. Further, we analyse to wha t extent leaf and plant nitrogen productivities depend on photosynthet ic nitrogen use efficiency. Results show that in all cases, photosynth etic nitrogen use efficiency is a major determinant of both plant and leaf nitrogen productivities. A positive relationship between leaf nit rogen ratio and plant nitrogen productivity was found only when compar isons were made over broad taxonomic groups. (C) 1995 Annals of Botany Company