Water and nutrients exist together in close association because plant-available nutrient ions are dissolved in the soil solution and nutrient uptake by grapevines depends on water flow through the soil-root-shoot pathway. Leaf transpiration generates the tension necessary for the roots to absorb this essential solution, but in a drying soil, uptake of water and nutrients becomes progressively more difficult for grapevines. In addition, application of nitrogen fertilizer can increase the vine’s susceptibility to drought, because nitrogen favors shoot growth over root growth. However, because growth is more sensitive than photosynthesis to both water and nitrogen shortage, deficit irrigation may be used in conjunction with limited nitrogen application to control canopy development, yield, and fruit composition. Growth is the "pacemaker" for nutrient uptake by the vine, hence the growth reduction induced by water deficit also decreases vine nutrient requirements. Nevertheless, reducing water or nitrogen supply can be perceived as a stress by the vine, and its response depends on developmental status. For instance, water deficit applied before fruit set may reduce cluster and berry numbers, especially if combined with nitrogen shortage. Properly regulated deficit irrigation in combination with low to moderate rates of nitrogen application between bloom and veraison reduces canopy size, berry size, and yield, accelerates ripening, improves fruit color, and reduces disease incidence. However, this strategy also reduces yeast-assimilable nitrogen in the fruit, thereby increasing the risk of sluggish or stuck fermentation. Moreover, if the water or nitrogen deficit becomes too severe, fruit quality suffers from both limited assimilate supply and excessive fruit exposure to sunlight. The relationship between vine nutrition and deficit irrigation clearly requires careful guidance to make it a happy one. We recommend that you consult the full text of this article, which was published in the American Journal of Enology and Viticulture 56: 267-283
