WHOLE-PLANT WATER USE OF SOME TROPICAL AND SUBTROPICAL TREE CROPS AND ITS APPLICATION IN IRRIGATION MANAGEMENT

Abstract

Whole-plant water use provides a baseline for a plant's irrigation requirement. The measurement of whole-plant water use in tree crops is much more difficult than in field crops, owing mainly to their large size. The development of thermo-electrical methods for xylem sap flow measurement has provided tree physiologists with an accurate and convenient tool to study whole-tree water use and water relations. However, sap flow measurement in orchard grown trees is particularly challenging because of horticultural practices, such as grafting, training and structural pruning. Whole-tree water use on a range of tropical and subtropical tree crops, such as mango, cashew, mangosteen, banana and macadamia, was measured using a Granier sap flow measuring system. The Granier system was found suitable for tree crops with very diverse morphologies and xylem structures, e.g. mango, mangosteen and banana. For a given species, radial variation in water flux in the sapwood has a more or less regular pattern, but the circumferential (aspect) variation was substantial and random. To obtain a reasonable estimate of the whole-plant or orchard water use, a large number of sap flow sensors and several dataloggers are required. To assist in taking account of spatial variabilities, a modified Granier method was developed and proved to be effective. A summary of the quantitative estimates of the whole-plant water use of several tropical and subtropical crops was provided. Sap flow was found to be a very sensitive indicator of plant water status useful for irrigation scheduling for plants grown in containers or on soil with limited soil water holding capacity but was not so sensitive for plants with deep roots and/or on soil with high water holding capacity. In its current design the sap flow system should remain as a research tool for researchers rather than an irrigation scheduling tool for farmers.