Leaf age and environmental effects on gas exchange in leaves of bananas (cv. Williams) growing in a hot, arid environment

The effects of leaf segment, leaf position on the plant, leaf age and photosynthetic photon flux density (PPFD) at the leaf surface were examined on leaf gas exchange of cv. Williams banana. All measurements were made on irrigated plants at the end of the dry season (September to November) over three years in Kununurra, WA, (Lat 16 degrees S) a hot, arid region of North Western Australia. Net photosynthesis (P-n) did not differ between the segments on the leaf except when they received different PPFD. P-n reached a maximum of 20 to 25 mu mol CO2 m(-2)s-(1,) 9 d after the leaf had unrolled, that is when another new leaf had emerged and the measured leaf was in the second leaf position. Leaf chlorophyll concentration stabilized 7 d after unrolling but then increased slowly with time. The reduced rates of leaf gas exchange of older leaves are most likely a result of shading by younger leaves. The highest measured PPFD of 1800 mu mol quanta m(-2)s(-1) did not saturate P-n. Indeed, in a series of experiments, P-n measured at 1500 mu mol quanta m(-2)s(-1), was only 13 to 40% of the calculated maximum P-n at saturated values of PPFD, assuming P-n responds to PPFD in a hyperbolic function. In this study although P, was lower in older leaves, the calculated internal CO2 concentration did not increase even at high leaf temperatures and leaf-to-air vapour pressure differences. Therefore, the photochemistry of the chloroplasts did not constrain P-n. To compare gas exchange measurements among experimental treatments, care is required as leaf position and environmental effects can greatly influence results. Our data suggest that differences in P-n between treatments should take account of PPFD, especially in this environment where the maximum PPFD measured did not saturate P-n of individual leaves.