|Williams, Larry - UC DAVIS|
Submitted to: American Journal of Enology and Viticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 2, 2005
Publication Date: October 1, 2006
Citation: Williams, L.E., Trout, T.J. 2005. Relationships Among Vine- and Soil-based Measures of Water Status in a Thompson Seedless Vineyard in Response to High-Frequency Drip Irrigation. American Journal of Enology and Viticulture. Vol 56:4 (2005) pp 347-366. Interpretive Summary: An important tool to reduce irrigation water use is to schedule irrigations to match the requirements of the plant. This is especially important when the crop can be under-irrigated to improve quality. The most commonly-used plant indicator of water status is the tension or suction of the water in the leaves. Three methods have been used to measure the tension, called water potential, in plants - pre-dawn leaf water potential, mid-day leaf water potential, and mid-day stem water potential. In this study, all three methods were used in a grape vineyard where water applications ranged from 0.2, to 1.4 of the baseline water requirements measured with a weighing lysimeter. The water potential methods were compared to water applications, measured soil water contents, stomatal conductance, and CO2 assimilation rates. The 3 year study concluded that mid-day leaf and stem water potential were the best measurements to indicate the plant's need for water. With the benchmark values determined in the study, grape growers can confidently use these measurements to precisely schedule grape irrigations to maximize the value of their wine grapes and minimize irrigation water use.
Technical Abstract: A study was conducted in the field on Vitis vinifera L. (cv. Thompson Seedless) to compare various measurements of vine water status under high-frequency drip irrigation. Water use at 100% of vine evapotranspiration (ETc), was determined with a weighing lysimeter. Vines in the vineyard were irrigated at 0, 0.2, 0.6, 1.0, or 1.4 times the amount of water used by the lysimeter vines. Water applications occurred each time the lysimeter lost 16 L of water (2 mm depth; 8 L vine-1). Soil water content was measured in the 0.2, 0.6, 1.0, and 1.4 irrigation treatments. Predawn, midday leaf, and midday stem water potentials were measured at the ends of the 1991 and 1992 growing seasons and almost monthly during 1993. Soil water content (SWC)in 1993, remained constant throughout the growing season for the 1.0 irrigation treatment, increased in the 1.4 treatment, and decreased in the 0.2 and 0.6 treatments. Both midday leaf and stem water potential measurements detected differences among irrigation treatments to a greater extent than did predawn leaf water potential until very late in the 1993 growing season. There was a linear relationship between midday leaf and stem water potential. All three measurements of water potential were related to soil water content (using a quadratic function); however, the relationship between SWC and pre-dawn leaf water potential had the lowest R2 value, 0.52 compared to 0.90 and 0.94 for midday leaf and stem. Results indicated that pre-dawn leaf water potential would not be useful in accurately determining vine water status under high-frequency deficit irrigation.