Leaf gas exchange of droughted and irrigated banana cv. Williams (Musa spp.) growing in hot, arid conditions

Leaf gas exchange (LGE) of banana plants cv. Williams growing in an arid tropical environment was measured on irrigated and droughted plants through an 8 d drying cycle and after re-irrigation. The relations between net photosynthesis (P-n), stomatal conductance (g(s)), transpiration (E-t) and internal CO2 concentration (C-i) and climatic factors (photosynthetic photon flux density (PPFD), leaf temperature (T-1) and leaf-to-air vapour pressure difference (Delta e) and plant and soil water status were explored. The highest P-n, of 23 mu mol CO2 m(-2) s(-1) and g(s) of 600 mmol m(-2) s(-1), occurred in early to mid morning but the highest E-t, of 10 mmol H2O m(-2) s(-1), occurred in the early afternoon. Water deficit significantly (P less than or equal to 0.05) reduced LGE when soil water potential (Psi(S)) at 30 cm soil depth was less than -30 kPa. The Psi(S) of irrigated plants was maintained above -20 kPa throughout the experiment. Water use efficiency was not significantly (P less than or equal to 0.05) different between treatments. Nor was the relation between the difference between leaf and air temperature (T-d) and Delta e. A unique relation existed between P-n and g(s) that was common to both irrigated and droughted plants. Soil drought, and in irrigated plants diurnal variation in Delta e and T-1, reduced P-n and g(s). C-i was positively correlated with both P-n and g(s) except when the plants had experienced more than 6 d of drought (Psi(S) lower than -50 kPa) when the relations became non-significant (P less than or equal to 0.05). After re-irrigation of the droughted plants these relations returned to being positive and statistically significant (P less than or equal to 0.05). Measurements of leaf water potential using either volumetric or pressure techniques rarely showed differences between irrigated and droughted plants. This was despite a large reduction in P-n, E-t and g(s). Mature banana leaves folded their laminae to a greater extent, and for longer periods during the day, in droughted plants but we suggest leaf folding may be too variable over the diurnal period to be used as a plant-based indicator of water deficit. The youngest leaf emerged up to 40% faster during the night than the day suggesting that day temperatures were above the optimum for leaf elongation. Drought significantly (P less than or equal to 0.05) reduced elongation of emerging leaves only when Psi(S) was less than -65 kPa. In irrigated plants P-n was reduced whenever g(s) declined due to diurnal changes in the environment. Psi(S) less than -30 kPa further reduced LGE in droughted plants but few associations with plant water status (either pressure or volume based measurements of leaf water status or plant morphology) could be linked with decreased LGE. We conclude that banana plants are well able to maintain their internal water status during drought and are sensitive to soil drying. Water status is maintained by reducing radiation load (leaf folding) and closing stomata. This is likely to bear strongly on the productivity of this crop in hot, arid environments.