Development of Next Generation Rootstocks for California Vineyards

2015 Screening of crosses for salt resistance – chloride exclusion in experimental hybrids – A sampling of individuals from 10 hybrid populations (Table 1) was screened for salt tolerance and chloride exclusion.  These hybrid populations were 1) crosses of wild genotypes earlier found to be strong chloride excluders, 2) crosses of commercial rootstocks, or 3) crosses of rootstocks to strong-excluding wild genotypes. Plants were assayed using components of the rapid screen method: herbaceous cuttings grown in fritted clay media in 1-gallon pots, and with a standard growth period prior to salt exposure. However, a higher (75 mM) NaCl solution was used rather than the standard 25 mM NaCl solution because of anticipated strong chloride exclusion derived from one or both parents and the accompanying need to distinguish between individuals using an unusually high chloride concentration. Parentage of hybrid groups is listed in Table 2.  Harvest date was based on the death of the most susceptible individuals, which in this case took nearly two months of high salt exposure.

Salt stress phenotypes at the time of harvest are presented in Figure 1. Although visual symptoms are less informative than chloride concentration in the leaves, which are currently being assayed, such symptoms do generally correlate with chloride concentration and so provide a rough preliminary estimate of results. Most notable from Figure 1 are the poor performance of all hybrids derived exclusively from commercial rootstocks: Ramsey x St. George and Dog Ridge x St. George, despite St. George being an established strong chloride excluder. This result implies that the seven wild genotypes used in this study are imparting a superior chloride exclusion relative to that found in commercial rootstocks. In support of this result is the very strong performance of V. girdiana x V. arizonica hybrids (Figure 1), several of which were completely asymptomatic. Surprisingly, V. vinifera cv. Thompson Seedless also performed relatively well. Several possibilities can account for this, including higher leaf succulence in Thompson Seedless that could mask the ordinary phenotypic effects of high chloride accumulation in the leaves, but the actual performance will not be known until the leaf chloride concentrations have been determined. It is also possible, though unlikely, that V. vinifera has weak chloride exclusion at low concentrations and strong chloride exclusion at high concentrations, perhaps in response to the high osmotic stress of 75 mM NaCl. Forthcoming data on leaf chloride concentration will provide a clarified and robust ranking of the genotypes in this study, and promises to provide direction for salt tolerance screening of hybrid populations in 2016.

2015 screening of seedling populations for resistance to nematodes and salt – Testing of recent seedling populations for nematode resistance (HarmA and HarmC, ring and dagger nematodes) and salt resistance continues. Progress here was held up a bit with the hiring of Becky Wheeler and departure of Liang Zheng. Liang retired with Howard Ferris last summer and we have been trying to ensure a smooth transition. We have completely revamped the greenhouse space for nematode screening and have well-established populations of three root-knot strains (HarmA, HarmC and Race3) and ring nematodes. We are also building our X. index populations too.

In 2015 we scored 41 populations (662 individuals) that were made to combine broad nematode resistance with salt and drought resistance, for horticultural appearance – lack of brushy growth, long internodes, long canes and good vigor. Of these 662 seedlings 18 were excellent and an additional 54 had good horticultural characters. These seedlings and others that passed horticultural screening this year were moved to rooting studies. Last year we tested 571 seedlings from crosses to combine broad nematode resistance with salt and drought resistance for rooting ability; 60 of these seedlings rooted well and will be advancing to nematode testing and root assays for depth and fibrosity