Photon flux density versus leaf senescence in determining photosynthetic efficiency and capacity of Gossypium Hirsutum L. Leaves

Abstract

Within any agronomic crop, multiple processes contribute to the loss of carbon uptake with increasing leaf age. Leaves within the crop canopy experience increasingly lower insolation levels with increasing age due to development of leaves at higher canopy positions. This overshadowing occurs concurrently with declining photosynthetic activity due to physiological alterations with leaf aging. Research exploring the photosynthetic response to these changing light conditions and leaf senescence has failed to adequately separate environmental from physiological responses. Additionally, growth chamber grown plants often show very different photosynthetic responses to photon flux during the aging process than leaves grown under the highly variable photon flux levels in the natural environment. In this study, cotton ( Gossypium hirsutum, L.) leaves were held at various levels of full sun in an outside growing area and photosynthetic characteristics were determined with increasing leaf age. Photosynthetic efficiency and capacity decreased rapidly and substantially as the leaves aged; the decline in maximal photosynthetic capacity was more substantial than that of the photosynthetic efficiency. However, no statistically significant differences in leaf photosynthetic characteristics were observed due to the photon flux environment experienced during the aging process. The changes in leaf photosynthetic responses to light environment during leaf aging were solely the result of physiological changes within the senescing leaf and not the result of differences in PFD environment. Similar sun-to-shade adaptations in leaf protein and chlorophyll levels reported in previous studies were observed in response to photon flux environment, indicating an adjustment of the photosynthetic apparatus to incident light. These results have particular significance for leaves within crop canopies that often develop under near-full sun conditions, and gradually experience deeper levels of shade throughout the leaf life span as upper canopy foliage develops.