Photosynthetic capacity of cotton leaves on sympodial branches is maximal just prior to anthesis of the subtending flower and declines rapidly thereafter (Wullschleger and Oosterhuis, 1990a,b). This may be due at least in part to increased shading from upper leaves (Patterson et al., 1977). During the period of boll development, the leaf subtending the boll has very low photosynthetic activity (Wullschleger and Oosterhuis, 1990a,b) and receives low light (photon flux density, PFD) levels (Sassenrath-Cole and Heitholt, unpubl.). The loss of photosynthetic capacity limits photoassimilate supply to the developing boll, and under conditions of extreme shade, can result in ovule abortion and boll abscission. The photosynthetic capacity, the maximum photosynthetic activity under saturating light conditions, is the upper limit of leaf carbon uptake. Although this is a good measure of the health of the photosynthetic machinery within a leaf, a more realistic measure is the photosynthetic rate at limiting PFD, which is more commonly experienced by plants in a natural environment (Pearcy et al., 1990). The efficiency of photosynthesis under non-saturating PFD levels has been suggested to be a major limiting factor in crop performance (Ort and Baker, 1988) and has been observed to decline with leaf age (Constable and Rawson, 1980) and after exposure to deleterious growth conditions (Sassenrath and Ort, 1990b). A change in photosynthetic efficiency at low PFD would significantly limit carbon uptake in leaves lower in the canopy. In order to determine the impact of low growth PFD on photosynthetic performance, we grew leaves of cotton at various levels of low light and full sun. The photosynthetic light response curves for individual leaves were determined at frequent intervals after leaf unfolding. The maximum photosynthetic capacity, the photosynthetic efficiency, and the convexity of the light response curves were determined. The photosynthetic capacity and efficiency declined rapidly with leaf age. Only slight changes were observed in the degree of convexity of the light response curve over the leaf growing period. Very little difference was observed in the decline in photosynthetic characteristics as a function of growth PFD of the leaves, probably because the range of growth PFD levels were not different enough. The rapid and significant decrease in photosynthetic capacity and efficiency has particular relevance in determining the contribution of leaves within the canopy to total carbon uptake.

Constable, GA, Rawson, HM 1980 Aust J Plant Physiol 7:89-100. Ort, DR, Baker, NR 1988 Plant Physiol Biochem 26:555-565 Pearcy, RW, Roden, JS, Gamon, JA 1990 Agric For Meteorol 52:359-372 Sassenrath, GF, Ort, DR 1990 Plant Physiol Biochem 28:457-465 Wullschleger, SD, Oosterhuis, DM 1990 Photosyn Res 23:163-170