Soybean leaf photosynthesis in relation to maturity classification and stage of growth.

Leaf photosynthetic rates were measured on field-grown soybeans during the 1980 season. Comparisons were made between different cultivars and isolines representative of maturity groups I-IV. Mature, fully expanded leaves at different nodes on the plant were measured in high light to determine which had the highest potential photosynthetic rates at any one time. Successive leaves during the growing season had maximum rates which increased from about 22 μ mol CO2 m(-2) s(-1) on 25 June to a peak of 30-44 μ mol CO2 m(-2) s(-1) in early August.The persistency and eventual decline in the maximum rate was associated with the maturity group and related dates of flowering, pod fill and onset of senescence. Early maturing cultivars (groups I and II) had higher peak rates (38-44 μ mol CO2 m(-2) s(-1)) than later maturing cultivars (30-35 μ mol CO2 m(-2) s(-1), groups III and IV). However, the photosynthetic rates of early maturing cultivars declined rapidly after attaining their peak, whereas the leaves of later maturing cultivars maintained their photosynthetic activity for much longer.