Alternate and Fixed Partial Rootzone Drying Save Water In Citrus

We evaluated effects of alternate and fixed partial root zone drying (PRD) on leaf photosynthesis and water use efficiency (WUE = photosynthesis per transpiration) of split-root ‘Carrizo’ citrange seedlings growing in autoclaved fine sandy soil in a green house. There were three treatments: 1) a well watered control where both root zone halves were well watered (WW) with 50% ET_c (total =100% ET_c), 2) one half of the root zone consistently irrigated with 100% ET_c while the other received no water (fixed PRD= FPRD), or 3) one half of the root zone irrigated with 100% ET_c while the other was allowed to dry but the wet and dry halves were regularly switched (alternated PRD = APRD). Shoot growth did not differ among treatments. APRD and WW plants maintained similar patterns of root growth whereas the FPRD wet side resulted in the highest root growth and length. There were no treatment effects on specific root length (m g^-1).  Root length remained at the same levels in WW and FPRD dry side. Transient drought stress tended to increase net assimilation of CO₂ (A_CO2) and leaf transpiration as midday gas exchange rates were higher before irrigation than after irrigation. In addition, leaves above the dry side of the FPRD and above the temporarily dry side of the APRD had higher net gas exchange than WW plants. Leaf WUE did not differ among treatments but APRD and FPRD seedlings used 1.8% to 8.5% less water than the WW seedlings.