Photosynthesis of leaves is commonly observed to have a saturating response to increases in their nitrogen (N) content, while the response of plant maintenance respiration is more nearly linear over the normal range of tissue N contents. Hence, for a given amount of foliage, net primary productivity (NPP) may have a maximum value with respect to variations in plant N content. Using a simple analytically-solvable model of NPP, this idea is formulated and its broad implications for plant growth are explored at the scale of a closed stand of vegetation. The maximum-NPP hypothesis implies that NPP is proportional to intercepted radiation, as commonly observed. The light utilization coefficient (epsilon), defined as the slope of this relationship, is predicted to be epsilon = alpha Y-g(1 - lambda)(2), where alpha is the quantum yield, Y-g is the biosynthetic efficiency, and lambda is a dimensionless combination of physiological and environmental parameters of the model. The maximum-NPP hypothesis is also consistent with observations that whole-plant respiration (R) is an approximately constant proportion of gross canopy photosynthesis (A(c)), and predicts their ratio to be R:A(c) = 1 - Y-g(1 - lambda). Using realistic parameter values, predicted values for epsilon and R:A(c) are typical of C-3 plants, epsilon is predicted to be independent of plant N supply, consistent with observations that long-term growth responses to N fertilization are dominated by increased light interception associated with increased growth allocation to leaf area. Observed acclimated responses of plants to atmospheric [CO2], light and temperature are interpreted in terms of the model. (C) 1996 Annals of Botany Company
