A study was conducted over a three year period to evaluate traditional (measurement of leaf water potential (Ψleaf)) and new techniques (sap flow sensors and petiole ‘dendrometer’ sensors) to monitor vine water status in the field. Seventeen red, wine grape cultivars grafted onto 1103P and grown in a replicated trial at the Kearney Agricultural Research and Extension Center were evaluated to determine the appropriateness of Ψleafas a means to determine vine water status. Four of the cultivars, Cabernet Sauvignon, Grenache noir, Petite sirah and Syrah, were used to evaluate sap flow sensors to measure grapevine transpiration. Lastly, petiole ‘dendrometers’ were attached to Cabernet Sauvignon and Thompson Seedless grapevines to compare daily petiole diameter variations with measures of Ψleaf and grapevine water use measured with a weighing lysimeter.
One of the criteria for classification as a grape cultivar being isohydric vs. anisohydric is that midday Ψleafbetween a well-watered and water-stressed plant should be similar or at least not significantly different. If this were the case, Ψleaf could not be used to assess vine water status. Based upon the clear differences in Ψleafmeasured mid-morning, midday and mid-afternoon between irrigated and non-irrigated vines across years all the cultivars measured in this study would be classified as anisohydric. Others have suggested that the Ψleaf of a near-isohydric cultivar will decrease in response to soil water deficits but only to a certain level (~ -1.5 MPa for Grenache) while that of an anisohydric cultivar will continue to decrease (values < -1.5 MPa for Syrah). Midday Ψleaf of water stressed vines before and after veraison of all cultivars were not significantly different from one another and were more negative than -1.8 MPa as would be expected for an anisohydric cultivar. Therefore the use of midday Ψleaf would be appropriate for use in an irrigation management program.
The sap flow technique used in this study was able to discern volumetric sap flow and sap flow velocity differences among cultivars differing in canopy size with the exception of one case (Cabernet Sauvignon in 2010). The technique was also able to differentiate between the two irrigation treatments on a daily and diurnal basis. These differences in sap flow between treatments were also reflected in grapevine Ψleaf measurements. Lastly, sap flow volume/velocity varied from one day to the other in response to a reduction in evaporative demand across cultivars and irrigation treatment. However, the values of daily volumetric sap flow in 2011 for the irrigated vines were less than those estimated from the product of the crop coefficient, derived from the shaded area of each cultivar, and daily ETo. In addition, the diurnal sap flow velocity across cultivars and irrigation treatments appeared to increase rapidly early in the morning and thereafter leveled off. They did not follow the diurnal pattern of solar radiation as shown in other research. Therefore, the sap flow technique as measured under the conditions of this study would not provide a reliable, quantitative measure of grapevine transpiration. As such, it would also not be a stand-alone technique to assist in an irrigation management program, but has some utility in tracking relative changes in water use associated with stress.
A bend or deflection sensitive transducer was used as a means to monitor vine water status, measuring the diameter of a leaf’s petiole and/or the cluster’s peduncle. Daily maximum and minimum petiole diameters and the daily shrinkage were correlated with vine water status or vine water use. Petioles started to shrink as soon as water use increased as the sun came up in the morning and continued to shrink until the midday peak of vine transpiration (petiole diameter increased as the sun went down in the afternoon). The daily petiole diameter contractions remained fairly constant when grapevines were irrigated daily at full ETc or numerous times a week when being deficit irrigated. When irrigations were terminated, daily max and min petiole diameters decreased and the daily contractions increased. Monitoring peduncles provided the extra benefit of assessing growth rate of that tissue. Peduncle diameter growth rate was reflective of irrigation treatment when measurements were taken between veraison and fruit maturity. Further research is needed for this promising technique to assess vine water status to include measurements taken between berry set and veraison.
Berry weight at veraison was a linear function of mean midday Ψleaf measured between the initiation of irrigation and veraison across cultivars. The change in berry weight between veraison and September 9 was also a linear function of mean midday Ψleaf when measured between those two times across cutivars. There was a significant interaction between cultivar and irrigation treatment on yield. Vines with the highest yields that were not stressed between May and veraison or irrigated at 50%of estimated ETc during that time period did not always have the greatest yield when stressed between May and veraison. These results indicate that cultivars may respond differently to the imposition of water stress at various times during the growing season. Tempranillo had the greatest water use efficiency (water used per ton of fruit produced) of all cultivars while Freisa and Malbec the least across irrigation treatments. It is interesting to point out that Tempranillo was least affected by the irrigation treatments imposed in this study during 2012. These results indicate that wine grape growers may be able to choose cultivars that are inherently more water use efficient.