Profiles of Leaf Nitrogen and Light in Reproductive Canopies of Cotton (Gossypium hirsutum)

During vegetative growth, the vertical profile of leaf nitrogen (N) often parallels the profile of light distribution within the canopy. This is more advantageous in terms of canopy photosynthesis than a uniform distribution of leaf N. We investigated the influence of both reproductive growth and N supply on the profiles of N and light in canopies of irrigated cotton crops ( Gossypium hirsutum L.). Regular samplings were made from soon after the onset of reproductive growth until crop maturity. Every 2 weeks, a 1 m 2 sample of the canopy was cut in four successive vertical layers of equal thickness. Leaf area and N concentration (%) in each layer were measured. The vertical N gradient became more marked with ongoing reproductive development. It is hypothesized that because of the high rate of growth after the onset of reproductive development and the long duration of this phase compared to other species, the whole canopy photosynthetic benefit that would accrue from maintaining the N gradient is likely to be accentuated. The rate of decline in leaf N concentration in a layer was not related to either the initial concentration in the leaves nor the boll load within the layer.