SINGLE LEAF AND CANOPY PHOTOSYNTHESIS RESPONSE TO PLANT-AGE IN COTTON

This study was conducted to determine the single leaf and canopy photosynthesis response to plant age in cotton (Gossypium hirsutum L.). Field experiments were conducted with different row spacings and leaf types to provide a range of plant and leaf distribution patterns. Single leaf gas exchange measurements were made on uppermost, fully expanded, main stem leaves throughout the growing season when air temperature was about 27-degrees-C and photosynthetically active radiation exceeded 1800-mu-mol m-2 s-1. Canopy photosynthetic rate per unit ground area was measured concurrently with the single leaf gas exchange rate. Leaf water potential and light interception were determined when the gas exchange measurements were made. Leaf area was measured to calculate canopy photosynthetic rate per unit leaf area. Single leaf photosynthetic rates declined 27 to 38% as plant age increased from 70 to 115 d after planting, the period of maximum dry matter accumulation. The canopy photosynthetic rate per unit ground area reached a maximum between 80 and 90 days after planting, as leaf area increased, but then declined rapidly. The decline in canopy photosynthetic rate per unit ground area was mainly due to the decline in single leaf photosynthesis rather than loss of existing leaf area as plant age increased, as indicated by a corresponding decline in canopy photosynthetic rate per unit leaf area. The decline in photosynthesis as plants aged was not caused by environmental limitations or carbon evolution from the soil. The rapidly developing fruit load and associated C evolution accounted for only 20% of the decline in canopy photosynthetic rate per unit ground area.