Soil salinization is an emerging problem in California vineyards. Research is needed to more fully understand the physiological response of grapevine roots to salt stress in order to develop cultural strategies that improve in-field management and to facilitate breeding of tolerance. Upon exposure to salinity, roots often exhibit a rapid decrease of water uptake capacity caused by inhibition of water-channel proteins called aquaporins. Aquaporins are found throughout fine root cellular membranes and can control the efficiency of water extraction from the soil. Prevention and/or alleviation of salinity-induced aquaporin inhibition have been demonstrated for some plants using calcium supplements in experimental conditions. Such a mechanism may contribute to the success of gypsum (i.e. calcium sulfate) applications used to lessen the detrimental effects of vineyard salinity.
In the original grant, we proposed to address the following short-intermediate term goals: 1) to quantify aquaporin response to salinity and the ameliorative effects of calcium in a suite of grapevine rootstocks using both hydraulic physiology and molecular probes under hydroponic and soil growth conditions; and 2) to investigate the role that aquaporins play in grapevine rootstock physiological responses to other abiotic factors (i.e. drought) and their contribution to vine vigor.
Our results indicate that aquaporins play a role in water uptake across numerous Vitis rootstocks. We documented significantly higher inherent aquaporin expression in high vigor and drought resistant rootstocks (e.g. 140Ru, 1103P and 110R) compared to those with low vigor and drought intolerance ratings (e.g. 420A and 101-14). These inherent differences may explain the known variation in vigor among these rootstocks, likely play a role in divergent patterns of drought tolerance, and represent potential target genes for breeding similar traits. In more recent efforts, we characterized anatomical, molecular, and biophysical aspects of fine roots impacting water uptake in Vitis, a woody perennial. This study provides one of the few quantitative analyses of tissues specific aquaporin expression in roots, and the first in a woody species. The study revealed strong parallels in developmental anatomy, distribution of aquaporins, and relationships with Lpr between herbaceous and woody fine roots within the meristematic/elongation and maturation zones. These similarities suggest a common foundation likely underlies the integration of root development and water uptake across plants. Fine root hydraulic permeability along the root length was positively correlated with aquaporin gene expression and negatively correlated with suberin deposition.
For the salinity response of aquaporins, we consistently found a dramatic upregulation of aquaporin gene expression for both the PIP1 and PIP2 aquaporin families. This initial response dampened over time as expression patterns returned to near pre stress conditions. Patterns of expression were similar across rootstocks (i.e. patterns of response were not clearly and consistently associated with resistance or susceptibility to salinity stress among rootstocks). Salinity experiments were done using a variety of experimental procedures and always showed similar results. Hydraulic conductivity of fine roots did not show a concomitant increase after salt stress initiation. This suggests that the increase in aquaporin expression increase following salt stress likely played a more local role on the cellular basis (i.e. by affecting local cell to cell water relations) rather than affecting the bulk tissue conductivity. Similar gene expression patterns were found in our newly developed tissue culture method when roots were transferred to saline media. This method was very successful in enabling us to track growth rate of individual roots over time and after exposure to salt. Fine root growth slowed abruptly upon transfer to saline media, but this effect was ameliorated if gypsum was present in either the establishment media or in the transfer media. These results suggest that gypsum applications commonly used in vineyards to lessen the effects of soil salinity are not only affecting the ion exchange of the soil column, but also having a direct impact on grapevine physiology by enabling the roots to maintain growth despite the saline conditions. More work is needed to explore this in detail and Drs. Walker and McElrone have recently initiated efforts to use the tissue culture system for evaluating Boron toxicity.