INDEPENDENT EFFECTS OF THE ENVIRONMENT ON THE LEAF GAS-EXCHANGE OF 3 BANANA (MUSA SP.) CULTIVARS OF DIFFERENT GENOMIC CONSTITUTION

Circumstantial evidence suggests that the Musa balbisiana (B) genome c onfers greater drought tolerance to bananas and plantains than the Mus a acuminata (A) genome. Hence the genetic makeup of bananas and planta ins may affect the response of leaf gas exchange to the environment. F ield data cannot be readily used to study the independent effects of e nvironment but laboratory studies allow independent control of environ mental parameters. We examined the independent effects of photosynthet ic photon flux density, I, from 0 to 1450 mu mol quanta m(-2) s(-1), l eaf 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 stomat al conductance, g(s), transpiration, E-t, net photosynthesis, P-n, int ernal 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 o f g(s) and P-n to Delta e more than M. acuminata genomes. Genomic comp osition 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 predic ted maximum g(s) and P-n per 1 kPa increase in Delta e. This reduced s tomatal aperture reduced C-i, which declined exponentially, thereby li miting P-n. Optimum temperatures for g(s) were 35 degrees C and 39 deg rees C when Delta e was 1.5 and 3.0 kPa respectively. Optimum temperat ures for P-n were about 29 degrees C when Delta e was 1.5 kPa and 33 d egrees C when Delta e was 3.0 kPa. The predicted maximum temperature w here P-n=0.0 would occur was 43 degrees C to 44 degrees C for all resp onses regardless of Delta e. The Williams cultivar was least sensitive to I showing less than 70% of predicted maximum photosynthesis and le ss than 50% of predicted maximum stomatal conductance at 1250 mu mol q uanta m(-2) s(-1). We conclude that there are genetic differences in t he response of leaf gas exchange to changing environment within banana and plantains. The mechanism underlying the response of leaf gas exch ange 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 increa ses 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 c ultivars containing more B genomes is consistent with the view that th e B genome contributes to drought tolerance in Musa sp. (C) 1998 Publi shed by Elsevier Science B.V.