A comparative study on physiological responses to drought in walnut genotypes (RX1, Vlach, VX211) commercially available as rootstocks

Abstract

Walnut production in California depends on the performance of three clonal hybrid rootstocks, and this study provides evidence that roots of genotype RX1 exhibit unique properties to cope with water stress induced by drought. Three hybrid genotypes RX1 (Juglans microcarpa × J. regia), Vlach and VX211 (both J. hindsii × J. regia) are the most commonly used rootstocks for walnut production in California. These rootstocks provide various levels of disease resistance, but their performance under drought is unknown. Based on our findings on xylem stress physiology of native walnut species, we hypothesize that hybrid genotypes originating from wild species native to drier habitats will exhibit superior root performance under drought stress. Using a whole-plant experimental approach, we evaluated root and canopy physiological characteristics of 2-year-old tree saplings of RX1, Vlach and VX211 under two soil moisture treatments (‘control’ 70–90% and ‘drought’ 20–40% w/w soil moisture). In control saplings, root biomass was twofold smaller in RX1 as compared to VX211, but root system hydraulic conductance (Kro, predominantly cell-to-cell pathway) was more than 50% greater in RX1 and Vlach as compared to VX211. The relatively low Kro of VX211 was related to a larger number of root cortical cell layers and endodermis development. In drought-stressed saplings, root biomass was reduced by 27% (P < 0.05) in VX211 but no significant reduction in root biomass was detected in RX1 and Vlach. Maintenance of root biomass under drought in RX1 and Vlach was associated with an 80% decrease in Kro, a threefold increase in leaf intrinsic water-use efficiency, and maintenance of leaf turgor as compared to control conditions. Drought-induced reductions in Kro were linked to the formation of a multiseriate root endodermis in all three genotypes.