Gossypium hirsutum L. (upland cotton) and G. barbadense L. (Pima cotton) are two of the most important ®bre producing cotton species in cultivation. When grown side-by-side in the ®eld, G. hirsutum has higher photosynthetic and transpiration rates (Lu et al., 1997. Australian Journal of Plant Physiology 24: 693±700). The present study was undertaken to determine if the di