Response of leaf ontogeny and photosynthetic activity to reproductive growth in cotton

This study was conducted to determine if reproductive growth in cotton (Gossypium hirsutum L.) affects concurrent leaf development. Apparent photosynthesis (AP), stomatal conductance (Cs), soluble protein (SP), ribulose bisphosphate carboxylase (RuBisCO), and chlorophyll (Chl) were monitored in four main-stem cotton leaves which emerged at approximately 2 week intervals. The leaf which emerged during vegetative growth (48 days after planting) had higher AP, SP, and RuBisCO levels than that present in any leaves which emerged during fruit development. The last leaf studied (89 days after planting) was still present after boll maturation was completed and exhibited a rejuvenation in AP, SP, RuBisCO, and Chl starting at 30 days after leaf emergence. At 96 days after planting, the P700 Chl a-protein complex (PSI) was virtually absent from the leaves that emerged at 48 and 62 days after planting. The light harvesting Chl a/b complex was still present in these leaves, indicating greater degradation of PSI. The data emphasize the influence of developing fruit on concurrently developing leaves, an effect which was alleviated after boll maturation was completed. The declining AP per unit leaf area and smaller leaf size at the top of the plant results in a reduced photosynthetic potential of successively later emerging leaves. This reduction in leaf AP is consistent with earlier reported seasonal canopy photosynthesis patterns.