MEASUREMENTS OF PLANT AND SOIL-WATER STATUS AND THEIR ASSOCIATION WITH LEAF GAS-EXCHANGE IN BANANA (MUSA SPP.) - A LATICIFEROUS PLANT

The relationships between the water status of plant organs and their p hysiological functions are empirical. Measurements of the thermodynami c status of water in the tissues of laticiferous plants are difficult because the laticifers have positive hydraulic pressure and exude flui ds when cut. We set out to determine whether reduced soil water supply affected leaf water status, measured by a number of methods, and chan ged primary productivity. To do this we took pressure, volume and morp hological based measurements of leaf water status and recorded leaf ga s exchange of banana (AAA, Cavendish subgroup cv Grand Nain) plants gr owing in 251 pots during a drought cycle. After 5 days of drying, soil water potential fell from -1 to -60 kPa and net photosynthesis fell t o 1/10th of control values, despite few, small differences in leaf wat er status between irrigated and droughted plants. Net photosynthesis w as proportional to stomatal closure, both being influenced by photosyn thetic photon flux density (400-1400 mu mol quanta m(-2) s(-1)), as we ll as drying soil. Increasing the CO2 concentration in the ambient air to 1600 mu l l(-1) after 5 days of drying significantly (p less than or equal to 0.05) increased net photosynthesis from 10 to 15 mu mol CO 2 m(-2) s(-1) in irrigated plants and from 0.2 to 2.0 mu mol CO2 m(-2) s(-1) in droughted plants. Thus, the restriction in photosynthesis of droughted plants was stomatal with no detectable non-stomatal compone nt. Although changes in leaf water status were small, leaf water conte nt, root osmotic potential, and thermodynamic measurements based on th e exuding latex had stronger correlations with declining leaf gas exch ange than other methods of measuring leaf-water status. Soil water sta tus was more closely associated with changes in leaf gas exchange (r(2 )=0.76) than leaf water status (r(2)<0.40). Reduced leaf elongation wa s correlated with reduced stomatal conductance and transpiration. Diur nal leaf folding occurred in irrigated and droughted plants. Our exper iment supports the view that for the banana, water status measurements based on soil or root properties are more closely associated with lea f gas exchange than conventional techniques for measuring leaf water s tatus. The use of plant morphological characteristics in assessing pla nt water status, such as the rate of emergence of the youngest leaf, s hould not be ignored. (C) 1998 Elsevier Science B.V. All rights reserv ed.