From an analytical model it was shown that for a given total amount of nitrogen in the canopy, there exists an optimal leaf area index (LAI), and therefore an optimal average leaf introgen content, at which canopy photosynthesis is maximal. If the LAI is increased above this optimum, increased light interception will not compensate for reduction in photosynthetic capacity of the canopy resulting from reduced leaf nitrogen contents. It was further derived from the model that the value of the optimal LAI increases with the photosynthetic nitrogen use efficiency (PNUE) and decreases with the canopy extinction coefficient for light (KL) and incident photon flux density (PFD) at the top of the canopy. These hypotheses were tested on dense stands of species with different photosynthetic modes and different architectures. A garden experiment was carried out with the C4 monocot sorghum (Sorghum bicolor [L.] Moensch cv. Pioneer), the C3 monocot rice (Oryza sativa L. cv. Araure 4), the C4 dicot amaranth (Amaranthus cruentus L. cv. K113) and the C3 dicot soybean (Glycine max [L.] Merr. cv. Williams) at two levels of nitrogen availability.
The C4 species had higher PNUEs than the C3 species while the dicots formed stands with higher extinction coefficients for light and had lower PNUEs than the monocots. The C4 and monocot species were found to have formed more leaf area per unit leaf nitrogen (i.e., had lower leaf nitrogen contents) than the C3 and dicot species, respectively. These results indicate that the PNUE and the extinction coefficient for light are important factors determining the amount of leaf area produced per unit nitrogen as was predicted by the model.