Photosynthesis of individual field-grown cotton leaves during ontogeny

Photosynthetic characteristics of field-grown cotton (Gossypium hirsutum L.) leaves were determined at several insertion levels within the canopy during the growing season. Single-leaf measurements of net photosynthesis (Pn), stomatal conductance to CO2 (gs·CO2), substomatal CO2, leaf area expansion, leaf nitrogen, and light intensity (PPFD) were recorded for undisturbed leaves within the crop canopy at 3-4 day intervals during the development of all leaves at main-stem nodes 8, 10, and 12. Patterns of Pn during leaf ontogeny exhibited three distinct phases; a rapid increase to maximum at 16-20 days after leaf unfolding, a relatively short plateau, and a period of linear decline to negligible Pn at 60-65 days. 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, leaf nitrogen, PPFD, and gs·CO2 in this process. The response of Pn and gs·CO2 to incident PPFD conditions during canopy development was highly age dependent. For leaves less than 16 days old, the patterns of Pn and gs·CO2 were largely controlled by non-PPFD factors, while for older leaves Pn and gs·CO2 were more closely coupled to PPFD-mediated processes. Maximum values of Pn were not significantly different for any of the leaves monitored in this study, however, those leaves at main-stem node 8 did possess a significantly diminished photosynthetic capacity with age compared to upper canopy leaves. This accelerated decline in Pn could not be explained by age-related variations in gs·CO2 since all leaves showed similar changes in gs·CO2 with leaf age.