Plant growth and development are both functions of sunlight interception and temperature. As a cotton plant develops, new leaves appear and expand, increasing sunlight interception. Initially the carbohydrates produced by the leaves are used to produce roots and more leaves. This production of new leaves causes the leaf area of the cotton plant to increase rapidly. Once reproductive structures begin to develop, carbohydrate supplies are slowly shifted to the developing fruit. As the fruit load on the plant increases and ages, the carbohydrate demand increases, and the development of new leaves steadily declines. Therefore, fruit development occurs with a leaf population that is steadily aging.

Leaf photosynthesis does not remain constant as the leaf grows and develops. A cotton leaf reaches its maximum photosynthetic capacity at about 20 days of age, after which it declines. Collectively, as the reproductive growth of the cotton plant is increasing, it is doing so with the support of a leaf canopy that is aging. Premature aging of the cotton leaf canopy due to water stress, low fertility and other stresses further reduces the photosynthetic capacity of the crop.

Figure 1. Photosynthetic capacity of a cotton leaf relative to leaf age. Leaves reach peak photosynthetic capability about 20 days after they unfold, after which their efficiency decreases over time.