Differential Thermal Analysis of Deacclimating Chardonnay and Cabernet Sauvignon Grape Buds as Affected by Evaporative Cooling

Abstract

The major production risk to grapes on the Texas High Plains is freeze injury prior to, as opposed to after, budbreak. Due to mild temperatures in mid-winter, buds begin to deacclimate early and are vulnerable to freezes in late March, weeks before budbreak for Cabernet Sauvignon, and at the onset of budbreak for Chardonnay. A delay in deacclimation by reducing bud heat summations during late winter could enhance the probability of a successful crop. Chardonnay and Cabernet Sauvignon grapevines were evaporatively cooled by intermittent microjet sprinkling (25 sec on every 4 min) from January to April any time air temperatures exceeded either 10°C or 15°C. Effects of cooling on bud cold hardiness prior to budbreak, time of budbreak, shoot development, yield, and juice composition were compared to non-cooled vines. Excised buds were subjected to differential thermal analysis (DTA) weekly from January through March during the deacclimation period. In DTA, low temperature exotherm (LTE) peaks from cooled buds occurred at 3°C to 5°C lower temperatures (more hardy) than from check buds in the weeks preceding budbreak. There were no differences in bud LTE freezing points between the 10°C and 15°C vine cooling treatments. A freeze (-3.5°C) on day of year (DOY) 83, killed emerging primary buds on non-cooled Chardonnay as evidenced from physical examination soon after the freeze. This resulted in fewer buds breaking, fewer shoots and clusters, and a tendency toward lower yields than for cooled vines. Although the freeze occurred 15 days before budbreak for Cabernet Sauvignon, non-cooled vines were injured as evidenced by fewer buds breaking, fewer shoots, and clusters and lower yields. DTA indicated that the cooled buds would survive lower temperatures than the check buds as was also demonstrated in the vineyard. Cumulative bud heat summation (degree days, base 10°C) was reduced by 49% and budbreak delayed by seven to eight days by the 10°C cooling treatment. Evaporative cooling had no effects on fruit quality.