Cotton responses to shade at different growth stages: Nonstructural carbohydrate composition

Photosynthetic irradiance is an important factor affecting cotton (Gossypium hirsutum L.) plant growth, development, lint yield, and fiber quality. A 2-yr study was conducted to determine the effects of an 8-d period of shade (63% reduction in photosynthetic photon flux density) at four growth stages [i.e., pinhead square (PHS), first flower (FF), peak flower (PF), and boil development (BD)I on leaf photosynthesis, chlorophyll concentration, and nonstructural carbohydrate (hexose, sucrose, and starch) concentrations in leaves, floral bracts, and floral buds of field grown cotton plants. At all four growth stages, shade caused a 43 to 55% decrease in leaf photosynthetic rate, and a 14 ton a leaf area basis) or 73% ton a dry weight basis) increase in total chlorophyll concentration, but did not affect leaf dark respiration rate. Starch concentration in leaves and floral bracts decreased sharply under shade, whereas only minor changes in hexose concentration occurred. Shade at the FF and PF stages did not, whereas shade at the ED stage did decrease, the concentration of total nonstructural carbohydrate (TNC) in 20-d-old floral buds. Averaged over the four growth stages during the 2 Sr, the TNC concentration of shaded cotton decreased 47 to 71% in the leaves, 37 to 48% in the bracts, and 5 to 20% in 20-d-old floral buds compared with unshaded control plants. Among the four growth stages, shade at the PF (1994) or ED (1993) stage caused the greatest decrease in leaf TNC concentration; shade at the ED stage resulted in the greatest decrease in the TNC concentrations of bracts and floral buds. Shade during plant reproductive growth significantly reduced leaf photosynthesis and Tr(TNC concentrations of field-grown cotton.