Recently concluded research demonstrates the UCBI pistachio rootstock to be a viable option in saline situations.

Studies in the 2000s by UC Cooperative Extension Pomologist Louise Ferguson and other UC researchers suggested that the UCBI rootstock takes up less sodium and chloride than seedling PGI and sequesters both in the rootstock. These findings, according to Ferguson in a post on the San Joaquin Valley Trees and Vines blog, precipitated a series of investigations from 2012 to 2021 by UC Davis’ Georgia Drakakaki and Jessie Godfrey to further test the salinity tolerance of UCBI.

Drakakaki and researchers showed that sodium and chloride were being excluded at the root level in UCBI and that sodium was sequestered between vacuoles. According to Ferguson, they found a correlation between vacuolar sequestration of sodium in the root cortex and suberization of the exodermis and endodermis, which decreased sodium uptake. Additionally, researchers demonstrated the UCBI rootstock was intercepting sodium in the transport stream and storing it in the xylem parenchyma and recirculating chloride in the phloem, Ferguson said.

These results collectively confirm and explain earlier findings of Ferguson and UCCE Farm Advisor Emeritus Blake Sanden demonstrating seedling UCBI’s higher salinity tolerance compared to seedling PGI. The UCBI seedling rootstock is more salinity-tolerant because it excludes sodium and chloride at the root level, stores sodium in the vacuoles of the root’s cortex and retrieves sodium from the xylem stream, storing it in the xylem parenchyma and recirculating chloride in the rootstock trunk phloem, Ferguson said.

Ferguson noted that besides determining the relative salinity tolerance of the current commercially available rootstocks, these trials do not change what growers have already been doing. “Salinity cannot be eliminated; it can only be managed,” she said.

However, Ferguson did note that in the mid and long term, the results of this research are significant.

“In the midterm, the phenotypic and physiological biomarkers identified by Godfrey and Drakakaki can be used to identify seedling rootstocks for clonal propagation,” she said.

UC Davis Geneticist Patrick J. Brown is currently conducting ongoing field studies to determine whether salinity tolerance quantitative trait loci, locations in the UCB-1 genome that control sodium and chloride amounts in ungrafted UCBI seedling rootstock leaves, translate into yield differences in mature pistachio orchards, which can help in the long term.

“In the long term, understanding the molecular mechanisms behind these differences in rootstock performance can be used to further improve the salinity tolerance of pistachio rootstocks,” Ferguson said.