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 μmol quanta m-2s-1, leaf temperature, Tl, from 21°C to 43°C, and leaf-air vapour pressure difference, Δe, from 1.5 to 5.7 kPa on the stomatal conductance, gs, transpiration, Et, net photosynthesis, Pn, internal CO2 concentration, Ci, and instantaneous water use efficiency, Ew, of three Musa cultivars: cv Williams (AAA), cv Lady Finger (AAB), and cv Bluggoe (ABB). M. balbisiana genomes reduced the sensitivity of gs and Pn to Δe more than M. acuminata genomes. Genomic composition did not affect the responses to Tl. As Δe increased, gs and Pn declined linearly at the rate of approximately 10% of predicted maximum gs and Pn per 1 kPa increase in Δe. This reduced stomatal aperture reduced Ci, which declined exponentially, thereby limiting Pn. Optimum temperatures for gs were 35°C and 39°C when Δe was 1.5 and 3.0 kPa respectively. Optimum temperatures for Pn were about 29°C when Δe was 1.5 kPa and 33°C when Δe was 3.0 kPa. The predicted maximum temperature where Pn=0.0 would occur was 43°C to 44°C for all responses regardless of Δe. 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 μmol 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 Δe and I on the stomata, rather than an effect of Tl on photosynthetic activity. Increasing proportions of B genomes decrease the sensitivity of stomata to Δe 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 gs and Pn to Δe of cultivars containing more B genomes is consistent with the view that the B genome contributes to drought tolerance in Musa sp.