With our first year of funding from CGRIC, we made significant progress on this project and exciting new discoveries about grapevine fine root responses to drought that shed light on the differences among genotypes/rootstocks. The start date of the project was delayed by ~12months while we waited for the funds to become available, thus we couldn’t justify submitting a renewal in January 2016 for additional funds. We are now submitting a renewal to continue these efforts, which will cover the costs for the second year of the project. Our labs are continuing to work together closely on this and other related projects. In addition to the experimental results described below, we have collected and begun analyzing suberin responses using flurol yellow staining for the following 16 rootstocks/genotypes from a drought experiment run by Dr. Kevin Fort (Walker Lab): 140RU, 101-14, 110R, Riparia Gloire, St. George, 1103P, Dog Ridge, 1616C, Ramsey, Freedom, Schwarzmann, Chardonnay, Cabernet Sauvignon, Colombard, GRN-1, and GRN-2.
We originally hypothesized that embolism formation would be would of the primary limiting factors reducing fine root conductivity (Lpr) under drought stress. Instead, we discovered that embolism formation is preceded by cortical lacuna formation (i.e. the fine root cortex is tearing apart under mild drought stress for tissue just behind the root tip), which coincides with a precipitous drop in Lpr. In a series of follow-up experiments using neutron radiography, we compared the growth dynamic responses of 110R (drought resistant) and 101-14 (drought susceptible) during drydown and recovery and also linked this to lacuna formation and Lpr. Surprisingly 110R exhibited more lacuna under mild drought stress that was linked to lower Lpr for this tissue despite less suberization and greater root elongation rates after drought stress. A preliminary similar patterns has been found in a comparison of Ramsey and Riparia Gloire. We now suspect that slower suberization rates for fine roots of drought resistant rootstocks may contribute to their greater susceptibility to drought induced cortical-lacuna, but this response could provide the adaptive advantage of ensuring that the root tip is receiving adequate water and other resources (i.e. internal carbon supply). In other words, the dysfunction created by the cortical lacuna (not in the tip, but in tissues starting ~2cm behind the tip would reduce the competing surface area of the root cylinder and ensure that any water that is available is being dedicated to continued growth at the tip. We will continue to pursue this and other related hypotheses with the proposed renewal submitted along with this report.