Water Quality Effects on Grafted and Non-grafted Citrus

Citrus production in Texas, currently valued at approximately $75 million, is the third largest citrus industry in the U.S., with Hidalgo and Cameron Counties in the Lower Rio Grande Valley (LRGV) accounting for more than 98% of bearing acres.  In recent years, the citrus industry in the U.S. and the LRGV in particular, has been facing numerous challenges with emerging diseases and recurrent droughts. Increased water deficits and other drought-related issues such as increased soil salinity have prompted the need for efficient water management strategies, including alternative water sources, and adoption of stress tolerant varieties to mitigate the effects of drought, salinity, and to meet population needs along the U.S.–Mexico border. Currently, the Rio Grande River provides most of the irrigation and domestic water in the LRGV, but proximity to the Gulf of Mexico limits groundwater use for crop irrigation in this area due to salt-water intrusion.  Grafting can enhance plant yield, disease resistance, promote better quality plants, fruits, and salinity tolerance for certain crops.   Among the requirements for rootstocks in the LRGV are tolerance to calcareous soils, salinity, and resistance to Citrus Tristeza Virus (CTV). Sour Orange (SO) has been a standard rootstock used when soil conditions are alkaline and clayey; however, alternative rootstocks are being considered due to SO susceptibility to CTV.  Several newer rootstock varieties meet the requirements needed in the LRGV and are CTV resistant; C22 and C146 rootstocks are among the most promising.  The objective of this study was to assess salinity tolerance of these citrus rootstocks, and determine if low-quality water can be used for irrigation. This experiment utilized a simulated brackish water solution applied at five levels of electrical conductivity to assess salinity tolerance, physiological changes and general tree health of Olinda Valencia grafted on SO, C22, or C146 rootstocks.  At 1 dS/m, the average height of seedlings grafted on SO and C22 was increased by approximately 33%, but reduced by nearly 71% for seedlings grafted on C146.  However, at salinities higher than 5 dS/m, height was reduced by at least 36% for all grafted plants compared to the control. Non-grafted C22 and C146 plants irrigated with 5 dS/m water showed a 27% and 46% increase, respectively, while SO rootstocks showed a 54% height reduction.  The interaction between rootstock and salinity indicates that future research should focus on finding optimal rootstock–scion combinations and alternative incremental irrigation with low quality water.