Circumstantial evidence suggests that the Musa balbisiana (B) genome confers greater drought tolerance to bananas and plantains than the Musa acuminata (A) genome. Hence the genetic makeup of bananas and plantains may affect the response of leaf gas exchange to the environment. Field data cannot be readily used to study the independent effects of environment but laboratory studies allow independent control of environmental parameters. We examined the independent effects of photosynthetic photon flux density, I, from 0 to 1450 mu mol quanta m(-2) s(-1), leaf temperature, T-1. from 21 degrees C to 43 degrees C, and leaf-air vapour pressure difference, Delta e, from 1.5 to 5.7 kPa on the stomatal conductance, g(s), transpiration, E-t, net photosynthesis, P-n, internal CO2 concentration, C-i, and instantaneous water use efficiency, E-w, of three Musa cultivars: cv Williams (AAA), cv Lady Finger (AAB), and cv Bluggoe (ABB). M, balbisiana genomes reduced the sensitivity of g(s) and P-n to Delta e more than M. acuminata genomes. Genomic composition did not affect the responses to T-1. As Delta e increased, g(s) and P-n declined linearly at the rate of approximately 10% of predicted maximum g(s) and P-n per 1 kPa increase in Delta e. This reduced stomatal aperture reduced C-i, which declined exponentially, thereby limiting P-n. Optimum temperatures for g(s) were 35 degrees C and 39 degrees C when Delta e was 1.5 and 3.0 kPa respectively. Optimum temperatures for P-n were about 29 degrees C when Delta e was 1.5 kPa and 33 degrees C when Delta e was 3.0 kPa. The predicted maximum temperature where P-n=0.0 would occur was 43 degrees C to 44 degrees C for all responses regardless of Delta e. The Williams cultivar was least sensitive to I showing less than 70% of predicted maximum photosynthesis and less than 50% of predicted maximum stomatal conductance at 1250 mu mol quanta m(-2) s(-1). We conclude that there are genetic differences in the response of leaf gas exchange to changing environment within banana and plantains. The mechanism underlying the response of leaf gas exchange is through an effect of Delta e and I on the stomata. rather than an effect or. T-1 on photosynthetic activity. Increasing proportions of B genomes decrease the sensitivity of stomata to Delta e but increases the sensitivity to I, especially at low photosynthetic photon flux densities. They also increase water use efficiency at the leaf level of organisation. The lower sensitivity of g(s) and P-n to Delta e of cultivars containing more B genomes is consistent with the view that the B genome contributes to drought tolerance in Musa sp. (C) 1998 Published by Elsevier Science B.V.