Gas exchange and biomass responses of young citrus trees to partial rooting-volume irrigation

Partial-area irrigation has been proposed for decreasing water use in field-irrigated citrus. Since micro-irrigation wets only part of the soil surface, it may not always meet citrus tree water needs and thus lead to water deficits. The purpose of this experiment was to determine the impact of partial rooting-volume irrigation (RVI) on gas exchange, biomass, and root hydraulic conductivity of young citrus trees. Sunlit, controlled-environment chambers were used to measure evapotranspiration rate (ET), carbon dioxide exchange rate (CER), water-use efficiency (WUE = CER/ET), and biomass accumulation of trees exposed to partial RVI treatments. One-year old 'Hamlin' orange scions [Citrus sinensis (L.) Osbeck] budded on either 'Carrizo' citrange [Poncirus trifoliata Raf x C. sinensis (L.) Osbeck], 'Swingle' citrumelo (P. trifoliata Raf x C. paradisii Macf.), or sour orange [C. aurantium (L.)] rootstocks were established in 4-compartment, split-root containers filled with 6.8 L of coarse sand in each compartment. Trees with 1/4, 2/4, 3/4, or 4/4 RVI were grown in chambers from 15 June to 15 Sept. 1986 and were irrigated at 2/3 depletion of available soil water (ASW). Maximum CERs occurred before 1100 h EST in all treatments followed by mid-day depression of CER, whereas daytime ET remained nearly constant. Typically, CER was highest for 4/4 RVI and lowest for 1/4 RVI. Leaf area and dry-weight accumulations of leaves and stems were positively related to the fraction RVI, whereas root hydraulic conductivity did not differ among RVI. Growth limitations by 1/4 RVI were least for scions on Swingle citrumelo rootstock. These results indicate that, for citrus grown on a soil with low water-holding capacity, a progressive reduction of the irrigated portion of roots would likely lead to increasingly severe water stresses and, furthermore, decrease CER, WUE, and growth more severely than it would decrease ET.