Effects of brackish water and fresh water irrigation on growth and gas exchange parameters of acacia saligna grown in a runoff-catchment sytem

Abstract

Effects of runoff harvesting and supplemental irrigation (using fresh and brackish water) on growth and gas exchange parameters of two morphological types of a desert leguminous tree, Acacia saligna, under arid conditions of Turkana district were studied. The experiments were laid out as completely randomised design (CRD) with three replicates. Water harvesting was done by channeling the runoff water into a 'Liman' situated adjacent to an established runoff path. The brackish and fresh water used for the supplemental irrigation treatments was obtained from different boreholes. The liman was divided into 18 plots each measuring 10 m x 40 m. Trees of the two morphotypes (broad-leaved .and narrow-leaved) which were of similar size preselected (tagged) in each plot for data collection. Four sets of data were taken on these trees i.e, during i) the very dry period, ii) runoff period, iii) irrigation episodes and iv) the drying phase between ii) and iv). Supplemetal irrigation was done by applying 200 litres of water fortnightly into micro-limans constructed round previously identified trees. Diurnal trends of gas exchange parameters and leaf water potential for the two morphotypes were studied during the four distinct periods. Diurnal measurements of stomatal conductance, transpiration rates, leaf and ambient temperatures and determination of vapour pressure deficit (VPD) were done using a porometer (at intervals of two hours) twice every week, while that for leaf water potenial was done using a pressure chamber. In each case three leaves were sampled per plant. Plant growth was moni tored throughout the experimental period by taking measurements on leaf extension, shoot elongation and girth increaments. Plants which received supplemental irrigation with fresh or brackish water showed significantly higher rates (P = 0.05) of leaf elongation (of upto 3.7 cm/week), girth increaments and physiological parameters such as stomatal conductance ( 2.8 cms-l compared to 1.4 cms-l) and transpiration rates) than those which received runoff water only. Use of brackish water resulted in decreased levels of these parameters but the effects were small and non-significant when compared to the fresh water treatments. Significantly higher magnitudes were shown by the broad leaved morphotypes than the narrow leaved. Based on these findings use of runoff water and supplemental irrigation under arid conditions enhances biomass production.