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:
- 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;
- To investigate the role that aquaporins play in grapevine rootstock physiological responses to other abiotic factors (i.e. drought, anoxia, nutrient status) and their contribution to vine vigor. We are on target to achieve these goals over a two year funding period. The awarded monies only became available in October 2008 through a Research Support Agreement (RSA) between USDA-ARS and UCDavis. This delayed the initiation of hydroponic studies and molecular analyses to address the first goal listed above. In the mean time, we initiated studies to address the second goal listed above. The current RSA expires 31 September 2009, and an additional year of funding would be used for a 2nd RSA (01 Oct 2009-31 Sept 2010) that would allow completion of all short-intermediate term objectives.
Our results from summer 2008 indicate that aquaporins play an integral role in water uptake across numerous Vitis rootstocks and respond rapidly to nutrient supplements. Aquaporin activity measured with chemical inhibitors was consistently high among several rootstocks, and responded rapidly to calcium nitrate for 110R. We documented significantly higher inherent aquaporin expression in high vigor and drought resistant rootstocks (1103P and 110R) compared to those with low vigor and drought intolerance ratings (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.
Since the start of the RSA in October 2008, we optimized methods to quantify aquaporin gene expression for a range of rootstocks subjected to salt stress in an ebb and flow system. Expression analysis in four rootstocks that vary in salt tolerance (Ramsey, Riparia, French Colombard, Thompson seedless) revealed that aquaporins are highly and variably responsive to salt stress among the rootstocks. Preliminary results are reported here. Fine roots for ten additional rootstocks (3309C, 5BB, 420A, 101-14, 110R, 039-16, 1103P, SO4, 140R, Schwarzman) under salt stress have been sampled, and these samples are currently being analyzed.