The influence of deficit and dryland irrigation on root distribution, root water relations and fruit quality and yield decline

This research focuses on the influence deficit and dryland irrigation on root proliferation, temporal and spatial patterns of water and nutrient availability, fruit quality, and yield loss. The overall goal of this project is to develop a basic understanding of temporal and spatial patterns of root survivorship in the drip irrigation zone, and in dry soils outside the wet-up zone, and to assess how such heterogeneity in available water influences vine performance in a wine grape vineyard. The project is ongoing and is being carried out in an established 11-year-old Merlot vineyard at the UCDavis Research Station in the Oakville Region of Napa Valley and the vines have been subjected to deficit and dryland irrigation treatments for four years. The response is being assessed for two rootstocks (Vitis berlandieri x V. rupestris cv 1103P and V. riparia x V. rupestris cv 101-14 Mgt) that are popular in upland (1103P) and valley (101-14) sites in the North Coast and other regions. The project has been a collaborative effort between U.C. Davis (D.R. Smart) and the Pennsylvania State University (D.M. Eissenstat).

During the first three years following the initiation of the study, we have achieved a better understanding of root growth and survival through pursuit of several objectives. These objectives and the insights they are providing are of great value to wine grape, table grape and raisin grape growers. In this investigation, for example, we have documented that water can be transported, through flow reversal, throughout woody tissues of the vine by moving it axially across the high resistance transport pathways of stems. These findings provide tremendous insight into water transport processes that may be of fundamental importance for other high priority research issues in California viticulture like berry shrivel, where one hypothesis is that reversal of water flow leads to loss of berry turgor.

Our results concerning root survival are providing further insights into the role root system health, or sustainability, plays in yield decline under deficit irrigation. The two rootstocks have had diametrically opposite responses to irrigation deprivation. We have found that rootstock 101-14 has diminished capacity for roots to absorb nitrogen (N), and is not very responsive to N fertigation treatments (15 to 40 kg N ha-1). One major consequence of this difference results in lower yeast assimilable nitrogen contents in musts of 101-14, and this N deficiency has had a strong influence on fermentations and wine quality. We have documented that phylloxera plays a major role in the longevity of fine roots of the two rootstocks, even though they are known for good phylloxera resistance. The project is supporting efforts in other laboratories to understand how phylloxera feeding influences root health and the general longevity of these rootstocks.

The project has produced no less than five peer-reviewed publications, each breaking new ground in viticulture and the information concerning rootstock behavior under the irrigation deprivation treatments is brought regularly to the organizations such as the Clear Lake Winegrowers Association, the North Coast Viticulture Technical Group, the Sonoma County Viticulture Tech group as well as the regular meetings of the American Society of Enology & Viticulture.