The Use of a Bud Freezing Technique to Determine the Hardiness of 20 Grape Genotypes

Abstract
Pencil thick canes (~8 mm) from 20 different grape cultivars were harvested in October and in November, 2000. These cultivars had previously been classified as hardy (A), semi-hardy (B) or tender (C). Single-bud cane sections were placed in a refrigeration unit and subjected to sequential freezing temperatures 19 °F, 5 °F, -8 °F and –22 °F (-7 °C, -15 °C, -22 °C, and -30 °C) and held for 24 h at each temperature before being removed for evaluation. October samples had almost 100 % bud kill once they were subjected to –8 °F (-22 °C) and colder. There was a marked separation among the three cultivar groups at 5 °F (-15 °C), being 46, 68 and 94% mortality for A, B, and C, respectively. By the November sampling, most of the hardy cultivars (A) were relatively little affected at –8 °F (-22 °C), whereas, groups B and C suffered 25% and 83% mortality, respectively.

KEYWORDS. Vitis, cold hardiness, winter injury

Introduction

Cold susceptibility or the lack of winter hardiness is one of the most important cultural constraints for Canadian grape production, particularly in Quebec (Dubois and Deshaies, 1997). The majority of Quebec commercial vineyards are concentrated between 45° and 47° north latitude where the winter minimum temperatures can reach -22 °F (-30 °C) regularly and -31 °F (-35 °C) occasionally (Jolivet et al., 1999). Under these extreme conditions, cold injury is expected in mid-winter, but it also often occurs in late fall or early spring. Autumn frosts cause premature defoliation and limit the duration of the normal vegetative cycle, making it difficult to follow a normal harvest season (Galet, 1993). Severe yield losses have also been attributed to late spring frost, which kill or damage the developing flower buds. Thus, the utilization of cultivars possessing adequate levels of cold tolerance through out the year would contribute greatly to yield stabilisation in the commercial Quebec vineyards. The dormant bud is usually considered the most susceptible part of the grape vine. Frequently exhibiting injury when other tissues of the vine survive the cold event (Ahmedullah, 1985; Quamme, 1986; Jolivet et al., 1999).
Several reports showed that the buds of grapevines supercool as a mechanism of freezing tolerance (Pierquet and Stushnoff, 1980; Quamme, 1986; Xiu et al., 1998). Cultivars resistant to the cold withdraw interstitial cellular water and modify the molecular structures to prevent ice crystal formation (Pierquet et al., 1977; Audran et al., 1993).
Significant differences in cold hardiness have been observed among the Vitis species and cultivars within each species. The tolerance of hybrids (American and French-American) varies from 5 °F to -31 °F (-15 to -35 °C) (Vandal, 1986; Galet, 1988). It has also been shown that the buds of many Vitis vinifera L. cultivars freeze between 5 and -6 °F (-15 °C and -20 °C) (Galet, 1993), whereas the hardier hybrids derived from Vitis riparia Michx and other native American species can tolerate temperatures as low as -31 °F (-35 °C) and still produce fruit (Vandal, 1986). Grape production for wine making is still a young industry in Quebec. Information is lacking on the influence of cultural practices on performance and hardiness of some of the new cultivars recently introduced to the Quebec Industry. The aim of this study was to evaluate the primary bud hardiness of 20 cultivars thought to be of value to the Quebec viticultural industry, using a visual expression of necrosis as described by Stergios and Howell (1977).

 

Materials and Methods

This study was conducted using 20 grapevine cultivars–American hybrids, French-American hybrids and vinifera cultivars- all growing on the Agriculture Canada sub-station at at Frelighsburg, Quebec (latitude 45o N, longitude 72o W). These cultivars had previously been classified by their field performance under cold climate conditions as hardy, semi-hardy and tender (Reisch et al, 1979; Odneal, 1983; Vandal, 1986; Bordelon et al., 1997; Dubois and Deshaies, 1997; Plocher and Parke, 2001) (Table 1).
Pencil thick, present-year canes (8 mm) were collected on October 12 and November 14, 2000, sealed in plastic bags and stored at 34 °F (1 °C) until required. Canes from each cultivar were cut to 1-bud pieces and 20 buds (1 bud from each cultivar) were placed in each of 25 plastic bags. These 25 bags were divided into 5 groups and each group placed in a sealed plastic container, also including a temperature sensor. The five plastic containers were placed in a temperature controlled circulating ethylene-glycol (E-G) bath (Neslab, model LT-50DD) at 34 °F (1 °C) for 48 h. To adapt the buds to cold conditions, the E-G bath was programmed to decrease temperature by 2 °C/h (Khanizadeh, 1991) until the first test temperature was reached (19 °F or -7°C). The E-G bath then remained at this temperature for 24 h, after which the first sample was removed for evaluation. The Cycle was repeated, with a gradual cooling until the next temperature was reached (19 °F, 5 °F, -8 °F and –22 °F corresponding to -7 °C, -15 °C, -22 °C, and -30 °C), held 24 h, samples removed for evaluation. For evaluation, five buds were harvested randomly from each cultivar at each temperature. Individual buds were sectioned through the tip of the primary meristem with razor blade and examined under a binocular microscope, checking for necrosis of the primary bud (Stergios and Howell, 1977). The primary bud that appeared bright and green was considered alive, and that appearing dull, straw coloured or black/brown was considered dead. The experiment was repeated 3 times for each sampling date.
Statistical analysis. The arc-sin transformed data on survival of primary bud were used to perform the analysis of variance by SAS (SAS, 1989). The Least Significant Difference (LSD) test (5%) was used to separate means.

Results and discussion
Significant differences in the survival of primary buds were observed among cultivars within each sampling date (P£ 0.001). Hardiness of the buds increased during the sampling period, with overall average of killed buds among all cultivars of 2.8, 2.9 and 3.5 for October and 1.3, 1.6 and 2.8 for November for groups A, B and C, respectively (Table 1). One of interesting things about the data is that the hardiest in October were not necessarily the hardiest in November. For October, the highest survival levels were for ‘Mitchurinetz’, ‘ES-6-12-28’ and ‘St. Pepin’ and the lowest for ‘Siegerrebe’ followed by ‘Vidal blanc’. For November, the highest survival rates were for ‘Sabrevois’ and ‘St. Pepin’ and lowest were ‘Siegerrebe’ and ‘SV-18307’, already characterized as cold susceptible (Reisch et al., 1979). November survival ratings were for higher in general than for October.
In the October samples, almost 100% of the buds of all cultivars were killed at –8 °F (-22 °C) or lower, whether they were in group A (‘Mitchurinetz’ or ‘St. Pepin’), B or C (‘Siegerrebe’, ‘Chancellor’ or ‘SV-18307’). The critical temperature was 5 °F (-15 °C), at which there was a marked separation in survival among the 3 groups cultivars (Fig 1). At this temperature, in contrast, there was high survival of primary buds for those already exhibiting high levels of cold resistance such as ‘Sabrevois’, ‘Mitchurinetz’, ‘St. Pepin’, ‘Prairie Star’ and ‘ES-4725’. The cultivars ‘Lucie-Kuhlmann’ and ‘GR-7’ were intermediate compared to the other cultivars in both, October and November sampling. In the November sampling, most of the hardy cultivars were little affected by the lethal temperatures in October, -8 °F (-22 °C), with survival of primary buds remaining high. The tender cultivars were still severely damaged at – 8 °F (-22 °C) in November (Table1).
Previous findings based on field observations show some agreement and some contrast with the above observations. Hemstad and Luby (2000) evaluated 15 cultivars for winter hardiness in Minnesota after experiencing –36 °F (-38 °C), and found ‘Mitchurinetz’, ‘St. Croix’ and ‘Kay Gray’ among the hardiest cultivars, while ‘St. Pepin’ had the most damage.
Bordelon et al. (1997) also evaluated percent survival of primary buds following -26°F (-32 °C) in January 1994 at 2 locations in Indiana and 6 locations in Ohio. These authors rated ‘St. Pepin’ as very hardy, ‘Chancellor’ as moderately hardy and ‘Vidal blanc’ as the most winter tender. However, great differences were observed within the same cultivars grown in different locations. All of the above authors agree that bus survival differences can be extremely variable and are not only affected by the vineyard conditions that predispose the vines to good acclimation or susceptibility to cold injury (location/aspect, vine health, soil drainage and fertility, and particularly crop load) but climatic patterns prior to and the timing of cold incident that also predispose the vines to good acclimation injury.
In general our results are in agreement with other reports on cold hardiness (Cahoon, 1973; Clore et al., 1974; Stergio and Howell, 1977; Vandal, 1986; Galet, 1988, Bourne and Moore, 1991; Hamman, 1993; Howell et al., 1998; Wolf and Warren, 2000, Gu et al., 2000). It is very clear that complex American hybrids such as ‘Sabrevois’, ‘Prairie Star’, ‘St. Pepin’, ‘St. Croix’ and ‘Kay Gray’, derived from many American species (Table 1), were generally the most hardy, having a higher tolerance for low temperatures, even in the early fall season. ‘Mitchurinetz’, with amurensis in its genealogy, was also extremely hardy, having been selected for cold tolerance in the former Soviet Union. French–American hybrids were less hardy, being crosses with a lot of vinifera in their history and being originally selected for productivity and disease resistance in the south of France. The full vinifera cultivar, ‘Siegerrebe’ was expectedly tender.
Although absolute hardiness is critical to cultivar choice for cold viticultural like Quebec, cultural and geographical factors can have a large effect on the ability for a vine to acclimate prior to potential cold events-location/aspect, local microclimate, vine age, crop load, shoot exposure to sunlight, node position on a given shoot (Ahmedullah, 1985; Reisch et al., 1979, Gu et al., 2000). In spite of the poor results in primary bud hardiness of the French-American hybrids in these tests, their use in Quebec viticulture may be justified because of their superior wine quality. Many of these hybrids are fruitful in the secondary buds most of which are incrementally hardier than the primary buds (Reisch et al., 1979; Pool, 1990).
Furthermore, the increase in their bud survival while ensuring a satisfactory and a stable production is manipulated by cultural practices like traditional covering of the vine by soil during the winter or the use of novel technique such as geotextile to cover the rows (unpublished data).

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