Effect of the soil water-to-air ratio on water status, leaf gas exchange and biomass of avocado trees

Avocado (Persea americana Mill.) is one of the most sensitive fruit tree species to flooded or poorly drained soil conditions. In Chile, avocado orchards are often planted in poorly drained soils that are low in oxygen resulting in tree stress. Understanding the relationship between the water-to-air ratio of different soils and avocado tree physiology and growth, should be helpful for irrigation management of the crop. The objective of this study was to relate the water-to-air ratios in different soils to water status, leaf gas exchange and biomass of avocado trees. Avocado trees were grown in each of five soils each collected from a different area of the Chilean avocado growing region with different physical properties and hence different water to air ratios. Thus, there were five treatments (T1-T5) corresponding to each of the five soils. The experiment was conducted during the spring and summer of 2005-2006 and 2006-2007 starting with two-year-old ‘Hass’ avocado trees planted outdoors in containers filled with one of the five soil treatments. At field capacity, the two-season average soil water-to-air ratio (W/A) was 1.7, 1.3, 0.6, 0.4 or 0.3 for treatments T1, T2, T3, T4, or T5, respectively. In addition to determining soil physical characteristics and monitoring W/A, net CO2 assimilation (A), transpiration (Tr), stomatal conductance (gs), stem water potential (SWP), shoot and root fresh and dry weights, leaf area and leaf retention were evaluated for trees in each treatment. Although aerobic soil conditions were maintained in all treatments, trees in soil with lower W/A had higher A, Tr, gs, and SWP than trees in the treatments with higher W/A. Also, trees in treatments with lower W/A had more biomass and longer leaf retention than trees in treatments with higher W/A. The results of this study indicate that the soil water-to-air ratio significantly affects physiology and growth of ‘Hass’ avocado trees.