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

Authors: KNIPFER, T - University Of California, Davis; GROM, J - University Of California, Davis; REYES, C - University Of California, Davis; MOMAYYEZI, M - University Of California, Davis; McElrone, Andrew; Kluepfel, Daniel

Submitted to: Trees
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/24/2019
Publication Date: 1/9/2020
Citation: Knipfer, T., Grom, J., Reyes, C., Momayyezi, M., McElrone, A.J., Kluepfel, D.A. 2020. A comparative study on physiological responses to drought in walnut genotypes (RX1, Vlach, VX211) commercially available as rootstocks. Trees. 34(3):665-678. https://doi.org/10.1007/s00468-019-01947-x.
DOI: https://doi.org/10.1007/s00468-019-01947-x

Interpretive Summary:

Technical Abstract: Walnut production in California is increasingly dependent on the performance of the three hybrid clonal rootstock genotypes; RX1 (J. microcarpa X J. regia), Vlach and VX211 (both J. hindsii X J. regia). These commercially available genotypes provide various levels of disease resistance, but their performance under water limited conditions is unknown. Our goal was to evaluate the physiological characteristics of these three genotypes under two soil moisture treatments (‘control’ and ‘drought’) and identify root and canopy traits associated with enhanced root performance under drought. These experiments were performed using 2-year old trees of each genotype. The canopy phenotype was most similar between RX1 and VX211 even though both genotypes exhibited substantial differences in root hydraulic properties and root biomass production. In the control treatment, root biomass was more than 2-fold smaller in RX1 as compared to VX211, but root system hydraulic conductance (Kro) was more than 50% greater in RX1 and Vlach as compared to VX211. Under drought, no reduction in root biomass was observed for genotype RX1 and Vlach but it was reduced by 27% in VX211. Maintenance of root biomass production in genotype RX1 and Vlach was associated with a 83 % and 81 %, respectively, reduction in Kro (only 65% in VX211), a >3-fold increase in intrinsic water-use efficiency (WUE, only 1.2-fold in VX211), and a <15% change in leaf turgor (80% in VX211). Anatomical investigation of roots exhibited the development of a unique multiseriate endodermis under drought in all three genotypes that contributed to the drought-induced reduction in Kro. In conclusion, we categorized genotypes RX1 and Vlach as drought-tolerant and VX211 as drought-sensitive in terms of their root performance. Canopy traits of intrinsic WUE and leaf turgor were identified as potential targets for the selection of walnut genotypes with improved root performance under drought.