Physiological responses of banana (MusaAAA; Cavendish sub-group) in the subtropics. VI. Seasonal responses of leaf gas exchange to short-term water stress

Abstract

SummaryA banana tissue culture block, cv. Williams, was divided in eight sub-plots each with independent irrigation control, to investigate effects of short-term water stress on the assimilation potential of vigorous plants never previously stressed. Supporting studies included recovery from short term water stress, validation of current crop factors of 0.8 × E0 (Autumn/Summer) and 0.5 × E0 (Winter), comparison of USDA class A Pan with tensiometer scheduling, and investigation of specific water demands of tissue culture plantings. Irrigation treatments during an autumn and winter study included a control (“daily” irrigation using A-Pan), a “water stress” treatment (water shortage until severe reductions in assimilation potential occurred) and a “normal” irrigation treatment using currently recommended crop factors. During a summer study, “normal” irrigation was scheduled physiologically, i.e. when plants showed a significant reduction in photosynthesis rate compared with the control. Soil moisture was monitored by tensiometer and neutron probe. The physiological responses of banana plants to different irrigation treatments were monitored by measurements of leaf gas exchange (photosynthesis rate, transpiration rate, stomatal conductance and internal CO2 concentration). During autumn, the first signs of water stress appeared after 4 d of water shortage. Twelve days of water shortage resulted in 79% reduction in photosynthesis, caused by a combination of stomatal limitations (84% reduction in stomatal conductance) and disturbed photosynthesis reaction (13% higher internal CO2 concentration). “Normal” irrigation at a crop factor of 0.8 showed no restricted leaf gas exchange indicating that soil water potential should be kept at – 15 to –20 kPa during warm seasons (summer to autumn) and that irrigation intervals should not be longer than three days. During winter, reductions in photosynthesis were only measured 8 d after initiation of water stress. A maximum reduction in photosynthesis of 32% occurred after 32 d of water shortage at a soil water potential of –69 kPa showing that water shortages during winter were less severe on assimilation than during warmer seasons. Irrigation at soil water potential of –30 kPa did not cause severe reductions in assimilation potential and a crop factor of 0.5 did not show long-term difference in gas exchange compared with “daily”. During summer, cloudy weather and reduced evaporative demand minimized symptoms of water stress until 7 d of water shortage at a soil water potential of –21.8 kPa. Physiological recovery of the plant from short term water stress took longer than 3 d in all three irrigation studies. In all measurements, irrigation scheduling with A-pan did not correlate with available soil water. Irrigation scheduling using soil water content (tensiometer or neutron probe) is a more effective indicator of actual water demand for bananas in the subtropics.