Sap flow, leaf gas exchange and chlorophyll fluorescence of container-grown cashew (Anacardium occidentale L.) trees subjected to repeated cycles of soil drying

Abstract

Summary. Cashew (Anacardium occidentale L.) is an emerging horticultural crop in tropical northern Australia. Supplementary watering is required during the dry season to achieve high yields but irrigation guidelines are not well defined. As an introduction to large-scale field experiments which will aim to define the irrigation requirements for cashew, this experiment was conducted on small, container-grown cashew trees to examine their response to drying soil and to evaluate a range of techniques for measuring tree water use and photosynthesis with possible application in the proposed field experiments. Measurements of sap flow, leaf chlorophyll fluorescence and leaf gas exchange were made on all trees throughout the experimental period.

The water use of trees in drying soil was measured using Granier’s sap flow system. Sap flux density (L/dm² sapwood area . h) of drying trees declined progressively over a 4-day period to a minimum level that was only 10% of the sap flow in the well watered trees. Measurements of leaf gas exchange showed similarly large reductions in photosynthesis and transpiration which were associated with a low (0.05 mol/m² . s) stomatal conductance in the drying trees. After rewatering, sap flow and leaf gas exchange recovered to the high levels of the well watered trees over 3–4 days. Similar behaviour was observed during the second drying period. Measurements of the ratio of variable to maximum fluorescence, F_v : F_m, an indicator of photoinhibition, were made on dark-adapted leaves before dawn and during the day. F_v : F_m was in the range 0.65–0.80 with no large or sustained differences between drying and well watered trees. When stomatal conductance and net photosynthetic rate progressively declined during the period following irrigation, the quantum yield of photochemical energy conversion in photosystem II, Φ_PSII, remained almost constant. It is possible that by providing a pathway for electron flow as an alternative to CO₂ assimilation during this period, photorespiration played an important role in avoiding photoinhibition.