2012 Annual Report
1a.Objectives (from AD-416):
Access currently available, newly released, and new parent rootstock materials for their physiological responses and tolerance to soil salinity and/or drought under greenhouse and field conditions. Quantify the effects of salinity and/or drought on scion physiological performance in terms of vine water relations, gas exhange, vine mineral nutrition, fruit yield, and quality under field and lab conditions.
1b.Approach (from AD-416):
The research will be a combination of field studies on cooperating farms, laboratory studies at UC Davis, greenhouse studies and sand tank studies at U.S. Salinity Laboratory.
This project supports objective 2 of the parent project. This research is being conducted in support of the NIFA-SCRI project “Developing Sustainable Vineyard Water Management with Limited and Impaired Water Supplies”. The deliverable being supported by this agreement is to “Develop and expand commercially available grapevine roots that better resist drought and tolerate salinity.” The information obtained from our recent study of salt and drought interactions has provided a number of important insights that contribute to this objective. A subset of our observations from this work follows: A. Fritted clay is unnecessary in the chloride exclusion assay. Despite favorable results obtained with this medium, it is relatively expensive and harder to obtain compared to UC Davis potting mix. Our results show that the potting mix also results in a rank order and relative uptake of chloride comparable to the known field performance of mature vines in an industry setting. This finding will streamline and reduce the cost of future screening efforts. B. Combined salt and drought stress strongly shift carbon allocation to roots. Grape rootstoch culitvar Ramsey in the ungrafted state exhibits extremely slow growth. Plants increase their biomass root-to-shoot ratio in response to drought stress. For the grapevine genotypes in our study, an absolute increase in root biomass was observed in response to the combined influence of salt and drought stress. Because grapevines occur almost exclusively in the presence of permanent, relatively shallow groundwater, this may reflect the grapevine’s adaptation to favor root growth until rooting near permanent groundwater is established. Ramsey, known for its drought resistance when used as a grapevine rootstock, was found to have very slow growth in both roots and shoot. Slow growth is a trait common to perennials adapted to abiotic stress, especially drought. It follows that slow, stress-resistant growth in the early stages of growth ultimately leads to greater root-length density in the soil and a relative invigoration of the scion in a mature vine from greater access to soil water. A future documentation of Ramsey compared to other genotypes over the course of development in the grafted state is planned. C. Drought reduces chloride translocation to the shoot. Daily watering optimizes the phenotypic separation of genotypes in a greenhouse assay. The relationship between the chloride exclusion capacity of Thompson and Ramsey over a gradient of drought seen in past work was repeated. A new insight was the observation that all genotypes tested showed a decline in chloride translocation to the shoot under drought, but that Ramsey was unique in rapidly reducing chloride translocation even under mild drought conditions. Increasing the severity of drought blurred the distinction in chloride exclusion capacity for the full set of genotypes, and therefore the optimal condition for chloride exclusion screening is daily application of water. If the elucidation of drought responsiveness is also a goal, a comparison of daily watering and 3rd-day watering intervals could be performed. D. Shoot biomass accumulation accounted for 40% of the variation in chloride translocation among all genotypes. At least part of the observed variability in chloride accumulation in the shoot over the gradient of drought appears to be mediated by the effect of drought stress on the growth rate of the shoot (r2 = 0.40, p < 0.001 There was no correlation between chloride uptake and root biomass. E. Combined salt and drought stress increased maximum rooting depth and root-to-shoot ratio nearly uniformly among genotypes. The whole plant response to salt and drought stress involved a clear alteration in root system architecture wherein shoot growth slowed and carbon diverted to roots was used to increase the depth of the root system. Surprisingly, the drought resistant genotypes Ramsey, 110R, and 140Ru did not have a stronger response than Riparia Gloire and Riparia-based rootstocks. In the well-watered state, the drought resistant rootstocks did have a slightly deeper root system. F. Deeper rooting did not come at the expense of reduced rooting near the soil surface. In the grapevine genotypes examined, surface rooting was not compromised to provide additional carbon for deep rooting. Instead, roots systems generally remained as wide as in the well-watered state, and the ratio of biomass in the upper and lower halves of the root system was unaltered. G. Maximum root depth per unit of shoot biomass distinguished drought resistant genotypes, especially Ramsey. An ideal drought-resistant genotype would have both deep roots to access deep soil water and a relatively small total leaf area to minimize soil water use. This ratio clearly distinguished the drought resistant genotypes Ramsey, 110R, and 140Ru from the V. vinifera genotypes, Riparia, and the Riparia-based rootstocks 101-14, 5C, and a weak chloride-excluding hybrid of Ramsey and Riparia that had growth characteristics similar to Riparia. The ratio of maximum root depth per unit of shoot biomass may be the most reliable character for future screening of potential drought resistant genotypes if grown in containers in a greenhouse environment for a duration similar to that used in this experiment. 6. Data set for drought resistance - root architecture assay completed. In 2011, a series of container studies were used to quantify root architecture for a small subset of rootstock genotypes. We are specifically interested to find if critical and readily measured architectural features of the root system related to performance under drought stress would hold true in the juvenile state when grown for a short period in a container or in the field. We used a series of pot sizes and geometries, rhizotron containers that allowed direct root system visualization, a hydroponic method, and excavated vines grown for single season in the field. We recently completed these experiments by examining the earliest growth stage from plantlets rooted from herbaceous cuttings. Two recent observations from this work follow: A. Major differences in root angle distribution can be detected and quantified at a very early stage in rooting. The surface rooting of Riparia and the deep rooting angle of Ramsey and 140Ru can be detected in a simple assay wherein herbaceous cuttings are rooted under mist in a perlite/vermiculite mixture, with virtually no time needed to wash media from the root system. Root scoring could also be performed on 20 replicates per genotype in a moderate time frame. The V. vinifera variety Colombard produced both surface roots characteristic of Riparia and deep roots characteristic of drought-resistant rootstocks. With the exception of rhizotron pots, an accurate assessment of root angle was generally not possible from plants grown for longer periods of time in a container, due to the loss of the natural rooting angle once the soil media was washed away. Roots at this stage are relatively long and have insufficient lignification to maintain their root angle without the supporting soil matrix. One year-old field-grown plants retained this angle when excavated during winter dormancy, because the roots had sufficiently lignified. B. A simple measure of root biomass from 4 week-old plants may serve as an index for total root length in specific cases. It has become evident in our root architecture investigations that a fundamental difference in the architecture of drought-resistant and drought-susceptible genotypes is the presence or absence of very dense fine rooting seen in the drought-susceptible genotypes. This character is not easily quantified in the very early rooting stage, but is easily measured as a root fresh weight after 4 weeks of growth in a container. The root biomass of nearly all of 22 Ramsey x Riparia hybrids showed an intermediate root density phenotype, perhaps indicating that this population is not segregating for this important trait.