Measurements of plant and soil water status and their association with leaf gas exchange in banana ( Musa spp.): a laticiferous plant

Abstract

The relationships between the water status of plant organs and their physiological functions are empirical. Measurements of the thermodynamic status of water in the tissues of laticiferous plants are difficult because the laticifers have positive hydraulic pressure and exude fluids when cut. We set out to determine whether reduced soil water supply affected leaf water status, measured by a number of methods, and changed primary productivity. To do this we took pressure, volume and morphological based measurements of leaf water status and recorded leaf gas exchange of banana (AAA, Cavendish subgroup cv Grand Nain) plants growing in 25 l pots during a drought cycle. After 5 days of drying, soil water potential fell from −1 to −60 kPa and net photosynthesis fell to 1/10th of control values, despite few, small differences in leaf water status between irrigated and droughted plants. Net photosynthesis was proportional to stomatal closure, both being influenced by photosynthetic photon flux density (400–1400 μmol quanta m −2 s −1), as well as drying soil. Increasing the CO 2 concentration in the ambient air to 1600 μl l −1 after 5 days of drying significantly ( p≤0.05) increased net photosynthesis from 10 to 15 μmol CO 2 m −2 s −1 in irrigated plants and from 0.2 to 2.0 μmol CO 2 m −2 s −1 in droughted plants. Thus, the restriction in photosynthesis of droughted plants was stomatal with no detectable non-stomatal component. Although changes in leaf water status were small, leaf water content, root osmotic potential, and thermodynamic measurements based on the exuding latex had stronger correlations with declining leaf gas exchange than other methods of measuring leaf-water status. Soil water status 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 was correlated with reduced stomatal conductance and transpiration. Diurnal leaf folding occurred in irrigated and droughted plants. Our experiment supports the view that for the banana, water status measurements based on soil or root properties are more closely associated with leaf gas exchange than conventional techniques for measuring leaf water status. The use of plant morphological characteristics in assessing plant water status, such as the rate of emergence of the youngest leaf, should not be ignored.