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January 2016 WINES&VINES 103 PRACTICAL WINERY & VINEYARD GRAPEGROWING close relationship between tran- spiration and yield might lead to the premature and false conclu- sion that there really is nothing to be done: When water becomes scarce, yields will decline. Cer- tainly this can be true. If nothing is changed other than applying less water, some loss in yield— even large losses in yield—should be expected. Fortunately, how- ever, few issues in biology are ever simple, because simple trends are often components of a much larger, more complex system. The first issue to address is water-use efficiency, or the yield obtained per total volume of water applied. The conversion to drip irrigation in virtually all California vineyards has been a major step forward in water conservation. As noted earlier, transitioning from sprinkler to drip irrigation in- creases water-use efficiency, spar- ing water that would otherwise be lost to weed growth, evaporation and soil wetting beyond the reach of the grapevine root system. 2 Water-use efficiency was further improved in the above Sonoma County field demonstration by determining appropriate irrigation volumes with the use of soil mois- ture probes and plant stress mea- sures to determine the timing of irrigation initiation and irrigation intervals. The very meaning of drought is itself a complex issue. An equiv- alent seasonal precipitation could lead to water stress at one site with shallow soil, and therefore limited capacity for soil water stor- age, when compared to an adja- cent site with a deeper soil profile. A moderately deep soil that is pre- dominantly sand can have the same water storage limitations as a shallow loam. In reality, most sites are heterogeneous mixes of soil depth, texture, impenetrable layers, slope, aspect, stoniness and other factors, such that water stress can occur in patches even within the same block. The timing of drought is important. Flowering and set are particularly vulnerable to water stress in most crops, with early and late-season water defi- cits having relatively small impacts on final yield. In a study using Cabernet Franc that imposed water deficits pre-véraison, post-véraison and throughout the growing season, post-véraison water deficit re- duced yield a mere 2%, while pre- véraison deficit reduced yield 28%. 8 The degree of water limita- tion is critically important, in that the modifications plants make to buffer yield losses when the yield potential is reduced by 5% to 40% are very different from the charac- teristics necessary to buffer yield potential reductions of 50% or more. 3 Other factors muddying the picture are: the economics of pumping water when it is avail- able, fruit quality benefits from i m p o s i n g m i l d t o m o d e r a t e drought stress, longer term effects of soil salinization from repeated irrigations and expense of manag- ing canopies when vine growth potential is high. Breeding rootstocks for drought tolerance in grapes Can a grapevine rootstock be bred that reduces or eliminates yield loss when water supplies are lim- ited? If one approaches this ques- tion first through an examination of native vines that exist from the desert Southwest to the wet east- ern United Sates, it is clear that multiple plant traits do exist that optimize plant growth across this large gradient of precipitation. Volumes have been written ex- ploring these traits in detail, pro- viding thorough explanations why, for example, a high-yielding sunflower would be doomed in the Mojave Desert without irriga- tion. The role of plant breeding for drought tolerance in grape- vines then becomes a search for variability of these same traits within Vitis species, and combin- ing multiple, favorable character- istics within the same plant to make a difference. Because of the strong relation- ship between total transpiration Cherish every drop Reduce water usage by up to 85% with our tank and barrel cleaning devices By implementing our rotary impingement cleaning devices to their process, our customers have experienced: • 85% decrease in water and chemical usage • 80% reduction in time spent cleaning • 20% boost in productivity Alfa Laval Tank Equipment Inc. www.gamajet.com customerservice.exton@alfalaval.com The references for this article are available online at winesandvines.com