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104 WINES&VINES January 2016 GRAPEGROWING PRACTICAL WINERY & VINEYARD and biomass produced, it is more important to produce a plant that is dehydration-resistant than to attempt to find a plant that can maintain productivity once drought has begun to desic- cate plant tissues. Once leaf water potential begins to decline, sto- mata close more readily and car- bon assimilation is reduced. At a given point, this effect is pro- nounced enough to impact yield. The studies cited, and many oth- ers, are evidence of this principle. However, if leaf—and plant— hy- dration can be maintained, or at a minimum if dehydration can be reduced, yield losses can be buff- ered or even eliminated. There are two avenues to maintain leaf hydration: increased soil water access and the preven- tion of plant water loss. Effective soil water access is tied, at least in part, to root architecture and helps to explain how the leaf water potential of Ruby Seedless was minimally reduced when it was grafted on the deep-rooted 1103P rootstock. 5 We are cur- rently pursuing optimized meth- odologies for rapidly character- izing rootstock architecture. In a recent assay, 1103P had the steep- est average root angle among all the commonly available root- stocks in California. Other deep- rooted rootstocks in this assay included Dog Ridge, 99R, Ramsey and St. George. On the opposite end of the spectrum were rootstocks known to devigorate the scion, and these generally had shallow roots in our rapid assay: Riparia Gloire and 1616C. In addition to our rapid assay, we are characterizing root systems following excavation from greenhouse containers and from field plots. Although time-consum- ing both in the extended growth period and for data collection, this approach permits analysis of a more developed root system, al- lowing us to distinguish which defining architectural traits exist at the onset of root production and which require a greater degree of root system growth. How root sys- tem architecture changes in re- sponse to drought is a question that we are addressing through the excavations of vine roots. In an ideal scenario, it would be desirable to observe root growth and architecture in real time, which is ordinarily hin- dered by opaque media and soil. But methods do exist that per- mit this on some level, and our method of choice is a rhizotron: a relatively two-dimensional con- tainer with a 2-foot-high by 1-foot-wide plexiglass face. Our rhizotrons permit us four to six weeks of observations before the vine has outgrown the container, a period long enough to observe growth patterns that correlate to the behavior of a genetically identical mature vine, and also long enough for us to observe root system responses to drought and recovery from drought. To date we have characterized nearly all of the commercially available rootstocks in California using this system and also completed a drought treatment using these rhizotrons. It is possible to learn about the impact of a rootstock's roots with- out ever actually looking at the roots themselves. We grafted a common scion to seven rootstocks that span a range from very low to very high vigor potential. The resulting vines were planted in a test plot at the University of Cali- fornia, Davis, where a variety of irrigation methods will be used. Not only will we see the effect of the irrigation methods on such variables as yield, fruit quality and measures of plant stress, we will also observe the effect of various rootstocks. We have produced and are studying hybrid populations of rootstocks and various grape species with the goal of under- standing the genetic basis of drought resistance and identifying more drought-resistant individu- als from these populations. Future directions As is true in most scientific en- deavors, much work remains to be done. Currently, effects of irriga- tion on yield far outweigh effects ERROIR