Cotton (Gossypium hirsutum L.): Physiological and morphological responses to water deficits and their relationship to yield

Abstract

With the aid of a rainout shelter and different irrigation treatments, a range of soil and plant water deficits were imposed on cotton (Gossypium hirsutum L.) in order to study the physiological and morphological responses to water stress as a step towards maximising yield per unit of available water. Diurnal changes in plant water relations, photosynthetic rate, leaf conductance and leaf expansion were measured on two or three treatments on five occasions during the growing season. Additionally, measurements of soil water content and detailed observations on the development of floral buds, flowers and bolls and the changes in leaf area and yield were made. As the soil water deficits increased, the leaves adjusted osmotically by 0.5 to 0.6 MPa at full turgor: diurnal changes in osmotic adjustment were about 0.1 MPa. The photosynthetic rate and leaf conductance at high quantum flux densities in the middle of the day decreased progressively with leaf water potential between −1.9 MPa and −3.5 MPa. Leaf expansion was more sensitive to soil and leaf water deficits than was leaf photosynthesis. Leaf size decreased during the day when the predawn leaf water potential decreased below −0.8 MPa. Water deficits reduced leaf and fruiting-site development, the number of floral buds and bolls retained, and final yield, but the proportion of flowers that set bolls was not affected except where substantial soil water deficits developed during early flowering. Large floral buds were retained at the expense of small floral buds. We suggest that soil water deficits reduced the capacity of the crop to carry fruit as a result of lower rates of leaf photosynthesis.