Mitigating negative effects of long-term treated wastewater application via soil and irrigation manipulations: Sap flow and water relations of avocado trees (Persea americana Mill.)

Abstract

Recent studies have shown significant negative effects of long-term irrigation with treated wastewater (TWW) on performance of orchards planted on clay soil. The aim of this study was to evaluate water use and water relations of different mitigation measures to remedy the declining performance of a mature commercial fruit-bearing 'Hass' avocado orchard (Persea americana Mill.) growing in a clay soil irrigated with TWW since 2009. The mitigation measures, each in 6 replicates, included freshwater (FW), blended TWW:FW in a 1:1 ratio (MIX), low-frequency TWW irrigation (LFI), TWW irrigated tuff trenches (TUF) and TWW as the control treatment. The Penman-Monteith equation was used to determine reference evapotranspiration (mm day−1) and canopy conductance (mm s−1). A variety of climatic variables and tree physiological responses were monitored on continuous and seasonal bases. Measurements included sap flow (mm day−1), carbon isotope discrimination (δ13C), and trunk growth (cm). Whole plant hydraulic conductance (mm s−1 MPa−1) was estimated using the water potential gradient and sap flow. In summer when water use was highest sap flow values measured in 10 trees per treatment in FW, TUF and MIX were higher than TWW by up to 31 %, 26 %, and 24 %, respectively (p < 0.05). Treatment ranking based on sap flow (F), canopy conductance (gc), and whole plant hydraulic conductance (Kp) was FW> TUF > MIX > LFI > TWW. δ13C rankings were the same from low to high values, but with MIX > TUF. Most of the physiological parameters determined using different techniques have synergistically shown that using tuff trenches and mixing poor quality water (TWW) with fresh water are promising mitigating measures. Monitoring and evaluation should continue to determine if the alternatives contribute to sustainable yield increases in orchards on long-term bases.