Photosynthetic characteristics of field-grown cotton (Gossypium hirsutum L.) leaves were determined at several insertion levels within the canopy during two growing seasons. Net photosynthesis (Pn), stomatal conductance to CO₂ (gs(.)CO₂), leaf area expansion, leaf nitrogen and light intensity (PPFD) were recorded at 3-4 day intervals during ontogeny for the main-stem leaf and the first three sympodial leaves at main-stem nodes 8, 10 and 12. During early leaf development, Pn increased to a maximum of 16.5 µmol m^-2 s^-1 at 79-100% of full leaf expansion depending on leaf position within the canopy. Peak values of Pn were maintained for only a few days and were followed by a period of linear decline to negligible Pn at 60-65 days after leaf unfolding. Analysis of the parameters which contributed to the rise and fall pattern of Pn with leaf age indicated the primary involvement of leaf area expansion, gs(.)CO₂, leaf nitrogen and PPFD in this process.

The decrease in individual leaf Pn began shortly after anthesis and continued throughout the boll filling period. This suggested a possible inefficiency in the production of carbon (C) by leaves and the C requirement of bolls. Simulations of C allocation indicated that the C demand of the bolls were generally out of phase with the C production by the subtending leaves. The first three individual fruiting positions along the fruiting branch at main-stem node 10 required an average import of 47.3%, 25.7%, and 7.1%, respectively. Bolls at mainstem nodes 8, 10 and 12 required an average C import of 58.2%, 26.7%, and 4.4% to sustain dry weight development when analyzed across fruiting positions. The synchronization of C allocation from leaves to bolls at all fruiting positions was poor and as such poses a potential limitation to yield.