Lynn J. Mills

Dynamic thermal time model of cold hardiness for dormant grapevine buds

BACKGROUND AND AIMS Grapevine (Vitis spp.) cold hardiness varies dynamically throughout the dormant season, primarily in response to changes in temperature. The development and possible uses of a discrete-dynamic model of bud cold hardiness for three Vitis genotypes are described. METHODS Iterative methods were used to optimize and evaluate model parameters by minimizing the root mean square error between observed and predicted bud hardiness, using up to 22 years of low-temperature exotherm data. Three grape cultivars were studied: Cabernet Sauvignon, Chardonnay (both V. vinifera) and Concord (V. labruscana). The model uses time steps of 1 d along with the measured daily mean air temperature to calculate the change in bud hardiness, which is then added to the hardiness from the previous day. Cultivar-dependent thermal time thresholds determine whether buds acclimate (gain hardiness) or deacclimate (lose hardiness). KEY RESULTS The parameterized model predicted bud hardiness for Cabernet Sauvignon and Chardonnay with an r(2) = 0·89 and for Concord with an r(2) = 0·82. Thermal time thresholds and (de-)acclimation rates changed between the early and late dormant season and were cultivar dependent but independent of each other. The timing of these changes was also unique for each cultivar. Concord achieved the greatest mid-winter hardiness but had the highest deacclimation rate, which resulted in rapid loss of hardiness in spring. Cabernet Sauvignon was least hardy, yet maintained its hardiness latest as a result of late transition to eco-dormancy, a high threshold temperature required to induce deacclimation and a low deacclimation rate. CONCLUSIONS A robust model of grapevine bud cold hardiness was developed that will aid in the anticipation of and response to potential injury from fluctuations in winter temperature and from extreme cold events. The model parameters that produce the best fit also permit insight into dynamic differences in hardiness among genotypes.

Rootstock Effects on Deficit-Irrigated Winegrapes in a Dry Climate: Vigor, Yield Formation, and Fruit Ripening

A rootstock field trial was conducted in the Yakima Valley, southeastern Washington, with three Vitis vinifera cultivars (Merlot, Syrah, Chardonnay). Vines were grown on their own roots or field-grafted to the rootstocks 5C, 99R, 140Ru, 1103P, 3309C, and an unnamed rootstock from Cornell University (here termed 101CU) that is a likely sibling or seedling of 101-14 Mgt. Repeated scion dieback due to cold injury to 99R led us to abandon this rootstock. Vine phenology, vigor, water status, yield formation, and fruit ripening and composition were evaluated during three years beginning in the vineyard’s ninth year. Own-rooted Merlot and Chardonnay grew more shoots than grafted vines, and 140Ru and 1103P tended to reduce pruning weights. However, 3309C was the rootstock associated with the highest pruning weights in Syrah and the lowest in Chardonnay. Rootstocks usually did not impact vine phenology, fruit set, and plant water status, although there was a trend for stem water potential to be highest with 3309C and lowest with 5C. The rootstock effect on yield formation depended on the scion cultivar, and variations in different yield components often cancelled out each other, but 3309C (Merlot and Syrah), 5C (Merlot and Chardonnay), and own roots (Chardonnay) were often associated with high yields. Nevertheless, the rootstocks had only minor effects on fruit ripening and did not consistently alter soluble solids, TA, K+, or anthocyanin pigments, but the pH was higher in fruit from own-rooted vines compared with grafted Merlot and Chardonnay. Overall, scion effects and differences due to yearly climate variation far outweighed any differences due to rootstock.

Modeling Dormant Bud Cold Hardiness and Budbreak in Twenty-Three Vitis Genotypes Reveals Variation by Region of Origin

Cold injury is a key environmental challenge in many grape-producing regions, especially those at high latitudes. Although grapevines acclimate to cold temperatures in fall and deacclimate when warm temperatures return in spring, cold hardiness varies with species, cultivar, phenology, ambient weather, photoperiod, and plant organ, which hampers implementation of effective mitigation practices. Using long-term data sets of lethal temperatures and spring phenology for primary buds of Vitis vinifera and Vitis labruscana, we parameterized and evaluated a discrete-dynamic model that simulates cold hardiness from early fall through budbreak of 23 genotypes. The model uses mean daily temperature as the sole input variable to drive daily changes in hardiness. Genotype-specific parameters, such as initial and maximum hardiness, temperature thresholds, acclimation and deacclimation rates, and chilling and heating requirements, were optimized through an iterative process. The model predicted cold hardiness with 0.89 ≤ r2 ≤ 0.99, depending on genotype. Because it simulates hardiness at budbreak, the model can also be used to predict the time of budbreak. Optimized model parameters revealed a north/inland-south/coastal gradient for genotype origin in terms of initial and maximum cold hardiness, and time of budbreak. Budbreak occurred earlier in hardier genotypes, consistent with more rapid deacclimation of genotypes originating from colder climates, paradoxically making these genotypes more vulnerable to spring frost in warmer environments. The current model of grapevine bud cold hardiness has uses in both climate modeling and risk assessment.

Fruit Ripening has Little Influence on Grapevine Cold Acclimation

This four-year study tested whether the physiological demand of fruit ripening may interfere with grapevine cold acclimation in autumn or with midwinter hardiness. Three harvest time treatments were established in a mature vineyard of own-rooted Cabernet Sauvignon vines: clusters were removed after fruit set, at veraison, or after the first fall frost. Average yield of the late harvested vines varied from 4.2 to 5.1 kg/vine (7.5 to 9.2 t/ha) among years, and soluble solids varied from 23.4 to 25.6 Brix. The presence of fruit during ripening delayed leaf senescence, measured as chlorophyll decline. The fruit also tended to delay the senescence-associated decrease in photosynthesis. All vines showed typical patterns of autumn cold acclimation, midwinter hardiness, and spring deacclimation. Cold hardiness of buds, cane phloem, and cane xylem varied during winter depending on prevailing temperature, and during the coldest winter reached levels of −27°C for 50% bud damage and −28°C for the onset of xylem injury. Early fruit removal had no effect on cane nonstructural carbohydrates and, with few exceptions, failed to enhance cold hardiness. Depending on the year, early fruit removal improved bud hardiness on 5 to 15% and xylem hardiness on 8 to 38% of all measurement dates. On those dates, the early harvested vines tended to be 1 to 2°C hardier than the late harvested vines, irrespective of the time of crop removal. No trend was found for phloem hardiness. These results indicate that cropping, at least within commercially acceptable limits in regions with sufficiently long or warm growing seasons, rarely impacts cold acclimation and maximum hardiness. Grapevines appear to adjust seasonal leaf physiology to meet their carbon demand for both fruit ripening and cold acclimation.

‘Sunbelt’ as a Possible ‘Concord’ Juice Grape Partner in a Warming World

The juice grape (Vitis labruscana) cultivar Sunbelt has been reported to ripen more uniformly than the cultivar Concord in warm climates; thus, ‘Sunbelt’ might be useful as either a blending partner with or replacement for ‘Concord’ as global climate change intensifies. We conducted a 4-year field trial to evaluate ‘Sunbelt’ alongside ‘Concord’ in arid southeastern Washington. ‘Concord’ yields were on the average 57% higher than ‘Sunbelt’ yields because ‘Concord’ vines produced more shoots of higher fruitfulness and consequently had more clusters. The 31% larger berries of ‘Sunbelt’ were insufficient to compensate for its lower cluster number. Conversion from hand pruning to minimal (machine) pruning had no consistent influence on yield in either cultivar. Juice soluble solids, titratable acidity (TA), red color intensity, and color huewere significantly higher in ‘Sunbelt’ than ‘Concord’, whereas pH and potassium were often similar. Both cultivars cold acclimated in autumn and deacclimated in spring, but hardiness varied during winter depending on prevailing temperatures. With some exceptions, the two cultivars had similar bud, phloem, and xylem hardiness. When differences were significant, ‘Sunbelt’ was 1 to 4 C less hardy than ‘Concord’ and also tended to deacclimate more readily in spring. The results from this study indicate that ‘Sunbelt’ shows promise as a blending partner with or an alternative to ‘Concord’ for warm vineyard sites or growing seasons even in regions with cold winters.