Ds. Thomas et al., INDEPENDENT EFFECTS OF THE ENVIRONMENT ON THE LEAF GAS-EXCHANGE OF 3 BANANA (MUSA SP.) CULTIVARS OF DIFFERENT GENOMIC CONSTITUTION, Scientia horticulturae, 75(1-2), 1998, pp. 41-57
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.