Effect of Gibberellic Acid and Paclobutrazol on the Incidence of Primary Bud Necrosis in cv. Shiraz (Vitis vinifera L.)

Materials and Methods

An experimental trial was established in October 2003 at the Nuriootpa Research Station (Barossa Valley, Australia) to assess the incidence of PBN on vines following application of gibberellic acid (GA₃) and paclobutrazol. Vines (cv. Shiraz) were planted in September 1988. Ten vines were used for each treatment and at three different application times: one week before flowering, at flowering, and one week after flowering. Vines in one row were randomly allocated to receive either no treatment (control: water) or one treatment of GA₃ (100 mg/L⁻¹) or paclobutrazol (100 mg/L⁻¹). Two methods of application were used to assess the translocation along the shoot. On each vine, either GA₃, paclobutrazol, or water was applied by either drop or paint method. In the drop method a pipette was used to apply 10 μL of either the control (water), GA, or paclobutrazol solution onto the first bud of each of the randomly selected shoots and was subsequently wrapped in Parafilm for 24 hr to assist bud penetration. The second method used a paintbrush to apply either GA or paclobutrazol solution to buds 1 to 10 on the selected shoot. For each treatment, shoot length and cane diameter were recorded for each shoot after leaf fall, and a total of 20 shoots per treatment were assessed for the incidence of PBN and bud fruitfulness (number of inflorescence primordia in the compound bud) in June 2004 by microscopic assessment. At this time, buds 1 to 10 on each shoot were dissected to determine mean percentage PBN and mean fruitfulness. When the primary bud was necrotic the secondary buds were dissected and the number of inflorescence primordia was recorded. Results were analyzed using analysis of variance (ANOVA) assuming a normal distribution with constant variance. Fisher’s least significant difference means comparison test (p ≤ 0.005) was performed to determine which means were significantly different.

Results

Overall incidence of PBN was 42% (control). Exogenous application of GA₃ to buds by both the drop and paint methods significantly increased the incidence of PBN when applied before flowering (Table 1⇓). Application of GA₃ before flowering caused PBN in 60% of buds under both application methods. Levels of PBN also increased when GA₃ was applied at flowering with the drop method. A significant increase in the incidence of PBN (59%) was observed when GA₃ was applied at flowering using the paint method, yet only marginally (47%) by the drop method. Drop application of GA₃ after flowering caused an increase in PBN (53%); however, application of GA₃ by the paint method did not have a significant effect at this time.

The growth retardant paclobutrazol significantly decreased PBN (27%) when applied by the drop method before flowering (Table 1⇑), but no effect was observed at other times. Paintbrush application of paclobutrazol before and at flowering caused no significant difference in PBN; however, after flowering there was a decrease (32%) in the incidence of PBN. Overall, buds treated with paclobutrazol had a lower incidence of PBN than those treated with GA₃. Regardless of treatment, the vines had high levels of PBN. Levels greater than 20% in a vine-yard are considered to have a significant impact on fruit-fulness and therefore final yield (Pool 2000).

Mean shoot length and cane diameter was recorded for all treatments at the various application times. No significant differences were observed between the drop and paint methods for these variables and hence results have been combined (Table 1⇑). Shoots that received GA₃ one week before and after flowering had significantly longer shoots with greater cane diameter. GA₃ applied at flowering caused no significant differences. Mean shoot length and cane diameter was significantly reduced following treatment with paclobutrazol one week before flowering. Mean cane diameter was also significantly less when applied at flowering, although no significant differences were observed after flowering. Neither treatment had a significant effect on mean fruitfulness.

Distribution of PBN along the shoot was similar, whereby incidence of PBN increased along the shoot to bud 6. Drop application of GA₃ before flowering resulted in higher levels of PBN in most buds along the shoot compared with the control (Table 2⇓). With the exception of buds 1 and 10, more than 40% of buds treated with GA₃ before flowering and at flowering were necrotic. Buds 1 to 2 showed consistently lower levels of PBN than other buds assessed, with the exception of some buds at node 10. The highest recorded incidence of PBN was 100% following GA₃ paintbrush application before flowering. With both drop and paint application, a reduction in the incidence of PBN was only observed at different node positions when paclobutrazol was applied one week after flowering. When paclobutrazol was applied one week before flowering and at flowering the incidence of PBN on average was nearly as high as those treated with GA₃.

Discussion

The application of GA₃ increased the incidence of PBN, with its effect most significant when applied before flowering. Stage of development appears to be critical in the response of buds to GA₃ application. Flowering and initiation of inflorescence occur at a similar time in the latter stage of the grand period of shoot growth (Coombe 2000). At this time, the undifferentiated anlagen (uncommitted primordia) develop either as inflorescence or tendril primordia and, later, the latent buds enter dormancy (Mullins et al. 1992). The process of flowering is controlled by naturally produced gibberellins (Stephan 1999), with high levels transferred to developing buds at the time of bud differentiation (Lavee 1987). Gibberellin activity is sensitive to temperature change and a reduction in activity has been observed with higher temperatures (Jones 1973, Hiller et al. 1979). In this study, treatments were applied before, during, and after the flowering period and average daily temperatures in November 2003 ranged from 18 to 20°C. Fluctuating temperatures are common during spring in the Barossa; however, temperature differences observed in this season between the different application times were minimal and were unlikely to have had a great impact on gibberellin activity.

The processes involved in floral initiation appear to coincide with the commencement of PBN (Lavee et al. 1981, Dry 1986, Morrison and Iodi 1990, Collins and Rawnsley, unpublished data, 2004). In vigorous grapevine cultivars such as Shiraz, the rise in GA levels may have an impact on the development of buds, as high GA levels have been associated with excessive cell elongation (Ziv et al. 1981). The imbalance of hormones and changes in cell development may lead to an increase in necrosis of the primary bud. The role of gibberellin in flower initiation may explain why PBN occurs at the onset of flowering.

There is genetic evidence that gibberellin inhibits flowering in grapevine (Weaver and McCune 1959, Weaver et al. 1966, Snir and Kessler 1975, Srinivasan and Mullins 1980). Gibberellin produced in seeds (such as those in grape berries) can influence the development of uncommitted primordia into tendrils and subsequently inhibit floral development. The inhibition of flowering by GA₃ is normally associated with stimulation of vegetative growth. Natural levels of gibberellins have been shown to suppress flowering in seed plants, such as apple (Unrath and Whitworth 1991) and avocado (Salazar-Garcia and Lovatt 1995). Gibberellins applied exogenously to table grapes can also significantly reduce bud fruitfulness in the following season (Biscay and Badr 2001). This study did not identify any changes in fruitfulness of the healthy buds remaining.

Different methods of gibberellin application were used with varying results. In the present study, drop application led to the most significant response. In another study, application of GA₃ to the leaves of grapevines increased the incidence of bud necrosis compared with direct application to the buds through smearing or petiole feeding (Ziv et al. 1981). However, at lower concentrations of GA₃, petiole feeding produced the highest levels of PBN. The direction of translocation of gibberellins may affect the position of necrotic buds along the shoot. GA₃ was applied to every bud from nodes 1 to 10 on treated shoots using the paint method and PBN increased progressively along the shoot until buds 6 and 7 when applied one week before and at flowering. When the drop method was applied to bud 1, PBN increased progressively along the shoot until bud 5. For both methods of application, the incidence of PBN was greater than the control, which indicated that GA application increased PBN and, in the case of the drop method, was translocated along the shoot. As described elsewhere, the incidence of PBN can increase along the shoot (Vasudevan et al. 1998b, Collins and Rawnsley, unpublished data, 2004). Bud 1 showed the lowest incidence of PBN compared with other buds, suggesting that gibberellins may have been translocated along the shoot from the lower buds. It may be that lower buds did not create a metabolic sink for GA₃ or were already differentiated at the time of treatment and, therefore, the incidence of PBN was not as high.

This investigation confirms previous reports that paclobutrazol can reduce shoot length (Chaney 2003, Christov et al. 1995) and, furthermore, reduce the incidence of PBN in Shiraz (Wolf and Warren 1995). Application of paclobutrazol retards vegetative growth through inhibition of production of gibberellin, which controls cell elongation (Lever 1986). When gibberellin production is inhibited, cell division occurs, but new cells do not elongate (Chaney 2003), leading to reduced shoot extension, internode length, and smaller leaf area than untreated plants (Christov et al. 1995, Wolf and Warren 1995). The application of paclobutrazol may produce changes in photosynthetic activity of chloroplasts, resulting in thicker leaves and increased photosynthetic capacity (Christov et al. 1995).

In this study, shoot vigor was assessed by measuring shoot length and cane diameter. Vine and shoot vigor has been strongly correlated with PBN (Ziv et al. 1981, Dry and Coombe 1994). Thicker shoots had up to 40% more necrotic primary buds than thinner shoots and a positive correlation was found between vigor and PBN incidence (Dry and Coombe 1994). Another study found that the most rapidly elongating shoots had the highest number of necrotic buds in the cultivar Riesling (Wolf and Warren 1995). Elevated GA₃ levels have been associated with vigorous shoots (Lavee et al. 1981). High vigor has also been correlated with low carbohydrates levels in grapevine buds prone to bud necrosis, and it was proposed that a reduction in carbohydrates may impair cell growth and development leading to a higher incidence of physiological disorders such as bud necrosis (Vasudevan et al. 1998b).

Vigorous shoots with high GA levels contributed to a greater incidence of PBN compared with less vigorous shoots with lower gibberellin levels (Lavee 1987). The added vigor induced by an application of GA₃ may also increase the sink strength of the shoot apex at the expense of the weaker sinks such as the buds. Our findings support the hypothesis that increasing the levels of gibberellins in developing buds can increase shoot vigor and the incidence of PBN. This was further demonstrated by the application of paclobutrazol, which decreased shoot vigor and the incidence of PBN. The timing of treatment application appears to be critical and warrants further investigation for the adoption of growth retardants in the winegrape industry.

Conclusions

With escalating pressure on grapegrowers to produce target yields, bud dissection analysis is likely to become a common viticultural practice in early yield estimation. Knowledge of potential influences and timing of PBN will improve our ability to manage PBN and therefore bud fertility in the vineyard. Application of GA₃ and paclobutrazol influenced the incidence of PBN and shoot vigor in the vineyard. This study supports previous assumptions that the incidence of PBN is related to shoot vigor, which is closely associated with changes in gibberellin concentration. Paclobutrazol and other growth retardants can inhibit the synthesis of gibberellins. Further investigations are needed to identify if the use of growth retardants in vigorously growing cultivars such as cv. Shiraz will be an effective means of controlling PBN.