Analysis of the distribution of light, leaf nitrogen, and photosynthesis within the canopy of Vicia faba L. at two contrasting plant densities

We analysed the spatial variation of irradiance, leaf nitrogen (N) concentration, specific leaf weight, and photosynthetic capacity of leaves within canopies of faba bean (Vicia faba var, minor) grown in the field at 2 contrasting densities (20 and 70 plants/m(2)), in order to assess whether plant density can affect the amount of irradiance transmitted, the distribution of N, and the rate of photosynthesis of leaves within the canopy. Nitrogen concentration per unit of leaf weight or per unit of leaf area (N(a)), and the rate of photosynthesis at saturating irradiance (A(max)), declined from the upper leaves to the lower, at both plant densities. Plant density had a strong effect on the extinction coefficients for total irradiance and for photon flux density (PFD), but little effect on the distribution of N(a). From the relationship between N(a) and the cumulative leaf area index, and the fraction of PFD transmitted through the canopy, it was determined that the coefficient of leaf N allocation was slightly lower in the high density crop. A similar curvilinear relationship between A(max) and N(a) was found at the 2 plant densities. Our results indicated that in faba bean a low plant density did not lead to a more uniform distribution of leaf N through the canopy compared with a high plant density, as was previously found in other species.