Two parental lines of Upland cotton (Gossypium hirsutum L.) plus their reciprocal F1, F2, and backcross populations were field-tested for environmental and genetic components of leaf diffusive resistance. Leaf resistance was measured with a diffusion porometer three times each day during 6 days in August. A large, consistent environmental component was present during the morning, afternoon, and evening, but the genetic component appeared mainly during the afternoon. Leaf water potential indicated that afternoon was the period of highest water stress. Genetic analyses revealed that leaf resistance was associated with both additive and dominance genetic variances, with an estimated narrow sense heritability of 25%. High leaf resistance was completely dominant to low resistance.
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