Introduction

Poor fruit set may limit grapevine (Vitis vinifera L.) yield under certain environmental conditions, such as low temperature, overcast sky or rain, which seem to reduce fruit set, likely because they limit carbon assimilation (Caspari et al., 1998). Moreover, fruit set in grapevines is source-limited, and, consequently, if leaf area decreases, fruit set and bud fertility may be reduced (Coombe, 1959; Candolfi-Vasconcelos and Koblet, 1990; Poni et al., 2006a, 2008). In addition, treatments that increase carbohydrate availability, such as girdling and removal of competing sinks, usually improve fruit set (Coombe, 1959).

However, there is insufficient knowledge about the specific period of time shortly before and after bloom when grapevine is most sensitive to low carbohydrate supply and how this would affect the setting of fruits. For instance, this knowledge is important for modelling carbon balances in grapevines where fruit set routines have not been included (Poni et al., 2006b). The current study aimed to determine with more precision the period around flowering and fruit set when a reduction in carbon supply due to artificial shading would have a large effect on fruit set. Shoot girdling was used in conjunction with the shading to differentiate within-shoot supply alone versus potential additional support from the vine.

Materials and Methods

1. Experimental plot and plant material

The experiment was carried out during the 2007 growing season at the Cornell University NY State Agricultural Experiment Station in Geneva, NY, USA (42N 77W). An own-rooted ‘Concord’ (Vitis labruscana L.) vineyard planted in 1997 at a spacing of 2.74 by 2.44 m (1496 vines ha^-1) and a ‘Riesling’ (Vitis vinifera L.) vineyard planted in 2005 on 101-14 rootstock at a spacing of 2.74 by 2.1 m (1738 vines ha^-1), both oriented in the North-South direction, were used in the current study. These cultivars represent two commercial Vitis species and one is grown to obtain lower yields for wine quality (‘Riesling’) whereas the other one is grown for obtaining high yields of acceptable maturity (‘Concord’).

‘Concord’ vines were trained to a high (1.6 m) bilateral cordon with short canes of 5-6 buds and a single pendant curtain pruned during the winter to retain about 80 nodes. ‘Riesling’ vines were pruned by leaving two canes; shoot thinning was performed pre-bloom in early June to retain an average of 24 shoots per vine (11 shoots m^-1). Yields of these vineyards were 14.4 and 20.8 tons per ha for ‘Riesling’ and ‘Concord’, respectively.

The soil within the vineyards corresponds to the Lima series (fine-loamy, mixed, active, mesic Oxyaquic Hapludalf) and it is very deep (> 2 m), moderately well drained, with a fine loamy structure (Soil Survey Staff, 2018). In both vineyards, cultural practices of irrigation, fertilization and pest management were appropriate to maintain healthy vines with minimal stress. Rainfall was 317 mm over the growing season (May to October), as recorded by a weather station located close to the ‘Concord’ vineyard. Data on temperature, rainfall, solar radiation and wind speed were collected from this weather station (available at http://weather.nysaes.cals.cornell.edu/) during the study period (Figure 1).

Figure 1 - Weather conditions during the study period (13th June – 10th July 2007): daily rainfall, solar radiation and mean air temperature.

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Results

In the ‘Concord’ control vines, the position of the clusters within the shoot affected the number of flowers and berries per cluster and berry weight, decreasing with the upper clusters, but not the percentage of fruit set (Table 2). Basal and second clusters had more flowers and berries than third clusters. Berry weight changed similarly in this species.

In ‘Riesling’ control vines, basal and second clusters showed a greater number of flowers than the third cluster. In contrast, berry weight was greater in the second and third than in the basal clusters. Moreover, no differences in the number of berries per cluster were detected as well as in the percentage of fruit set (Table 2). We looked for other internal correlations that may have had effects, but found no significant correlations among number of flowers and fruits, fruit set and berry weight with shoot length and yield for any of the two species (data not shown).

Within each species, different letters in the row indicate significant differences at p < 0.05, according to Tukey HSD test.

^yBold values indicate significant (p < 0.05) effects.

The effect of the individual vine was significant for ‘Concord’ (Table 3). In this species, berries per cluster and fruit set were significantly altered by shading treatments. Girdling exerted a significant effect on all the variables measured in ‘Concord’, except for the number of flowers, which was determined before the treatments. The interaction between vine and shading was significant for fruit set and berry weight in ‘Concord’ (Table 3).

The effect of the individual vine was not significant for ‘Riesling’ (Table 3). Berries per cluster and fruit set were significantly altered by shading treatments. Girdling exerted a significant effect on berry weight but not on berry set in ‘Riesling’. Finally, the interaction between vine and shading was not significant (Table 3).

The influence of shading and girdling was expressed as the percentage of fruit set with respect to that of the control treatment (Figure 2). In the case of ‘Concord’, shading of girdled shoots (i.e. only support from within the shoot) had the strongest reduction of fruit set by about 30-40% when applied in the first 12 days after bloom, while having less effect after that period. When the shoots were not girdled, allowing import from other exposed shoots, shading reduced the fruit set by about 40% at 9-12 days after bloom, while having less effect immediately after bloom or after about 2 weeks. The difference between the girdled and ungirdled shoots was greatest just after bloom and then later at 21-24 days after bloom.

In ‘Riesling’, the reduction of fruit set was significant when shading was applied starting 5 and 13 days after flowering. However, the shade effect was very mild, 10-15% (Figure 2). Girdling did not significantly change the effect of shade on fruit set in ‘Riesling’.

Figure 2 - Relative effect of shading and girdling on fruit set for ‘Concord’ and ‘Riesling’ as a function of the days after flowering. The points are situated on the first day of the shade period. The significance of the shade effect refers to the shading versus the unshaded control, whereas the significance of the girdling effect refers to the girdled treatment versus the ungirdled control. Asterisks indicate significant differences: * p < 0.05; ** p < 0.01; *** p < 0.001; n.s. not significant. Bars indicate standard errors.

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Shading combined with girdling produced different results in ‘Concord’ (Table 4). The number of berries and the percentage of fruit set were significantly lower in the treatments shaded between 5 and 8 days (S5-8) and between 9 and 12 days after bloom (S9-12). However, when shading from day 13 after flowering onwards, ungirdled shoots showed numbers of fruits and fruit set percentages similar to those recorded in the control treatment. In general, berry weight was lower in girdled shoots.

In the case of ‘Riesling’, girdling had no significant effects on any of the variables considered. Moreover, neither the number of flowers nor fruit set have been significantly affected by the shading treatment (Table 5). However, a slight trend to lower fruit set percentages was observed from 5 days after bloom (S5-8). Moreover, the number of berries was lower for the treatment shaded between 5 and 8 days after bloom (S5-8). In addition, this treatment presented the highest value of berry weight.

Discussion

The current study confirmed that basal and second clusters appeared to show a better differentiation capacity, namely produce more flowers, than third clusters, as previously reported for ‘Concord’ by Goffinet (2004). This finding was suggested by the significantly lower values of flowers and berries observed on third clusters of both species. The lower size and weight of these third clusters and their later development might be a feasible explanation for their weaker action as sinks.

The experiment was not specifically designed to explore different responses between the two studied species, considering the fact that different vineyards were used. Because of this, the different response to the applied treatments among the two species cannot be explained simply by the genetic material employed. Furthermore, ‘Concord’ vines had 50% higher crop level than ‘Riesling’ vines, suggesting a stronger carbon limitation that may partly explain the stronger response to shading in ‘Concord’.A previous study by Caspari et al. (1998) showed that girdling without leaf removal apparently raised the source:sink ratio (by eliminating the woody stem and roots as sinks). In our case, girdling reduced fruit set because of the assumed limitation on carbohydrate availability due to shading and the absence of photo-assimilate transport from unshaded shoots. Fruit abscission occurs shortly after bloom and determines the amount of crop, which in turn affects carbon distribution between clusters and vegetation for the rest of the season. Previous findings (Intrigliolo and Lakso, 2009) showed that fruit drop in ‘Concord’ mainly occurred during the first 10 days after flowering, whereas in ‘Riesling’ it was closer to 14.

The current study clearly indicated that, for both species, the period from 5 to 12 days after flowering is when they are most sensitive to suffer reductions in fruit set caused by factors that reduce the carbohydrate supply. This sensitivity could have been caused by a coincidence of this time-span with the period when most flowers were going through pollination and fertilization. In the case of ‘Concord’ vines, final berries per cluster were also reduced at 21-24 days after bloom, likely due to the approximately 30% reduction in ambient radiation in combination with shading lowering the total light. Nevertheless, compensation by photo-assimilates transported from the whole vine can overcome an eventual reduction in carbohydrate supply of a single shoot, as suggested by our observations and previous studies (Vanden Heuvel et al., 2002).

Isolating shoots from the rest of the vine through girdling modified the behaviour of the shoots with respect to the shading treatments. Specifically, on girdled shoots in ‘Concord’, reductions in fruit set caused by shading were evident even when shoots were shaded three weeks after flowering, whereas ungirdled shoots were only sensitive until two weeks after flowering. The number of berries and their weight followed a similar behaviour in this species. These findings suggest that translocation and partitioning of photo-assimilates from unshaded to shaded shoots occurred in ‘Concord’ and were able to partially or fully counteract the negative effects of shading in fruit set. Previous research on potted grapevines was able to identify these translocation patterns (Vanden Heuvel et al., 2002). In field-grown grapevines, Hunter and Ruffner (2001) observed that translocation of photo-assimilates among vine organs was highly dependent on girdling, as early as at berry set.

Girdling did not significantly alter the behaviour of shaded shoots in ‘Riesling’ and the effects of the shading treatments on fruit set were similar for both girdled and ungirdled shoots in this species. As observed by Goffinet (2004), carbon reserves in ‘Concord’ vines tend to decline until bloom or two weeks after bloom, then stop declining and begin to recover. In this sense, the difference between girdled shoots and the ungirdled control at bloom until 9-12 days after bloom may represent the declining availability of reserves to import for the fruit. Then, the increasing difference in the 2-3 week period may reflect the opposite, namely, stopping carbon export from the shoot to other organs thus retaining more carbon for the fruit. This decay in carbon reserves at flowering has been observed in Vitis vinifera (L.) varieties such as ‘Chasselas’ (Zufferey et al., 2015).

Furthermore, although the shading and girdling were constant, the weather and varying development imply that there is a variable baseline of carbon relations that we superimposed to our treatments. Therefore, solar radiation and temperature over the period when each treatment was applied likely had a strong influence on the results, making data interpretation more difficult. For instance, solar radiation and temperature were much lower 9-12 days after bloom than 13-16 days after bloom; consequently, the effect of shading was different because the amount of total radiation received by shaded clusters was much lower for the 9-12 days after bloom treatment. Previous research (Holzapfel and Smith, 2012) proved that developmental stage or climatic factors had a stronger influence on carbohydrate mobilization and storage than the manipulation of reserve accumulation through cultural practices. In the current study, the combined effect of lower solar radiation and lower temperatures on certain days, added to the shading treatment, seemed to reduce flowers and berries per cluster, and fruit set.

It is not clear if the growth of sinks such as fruits and their related leaves on shoots behave as independent units or if the sink development responds to the whole plant source-sink status (Sprugel et al., 1991). The results from the current study were dependent on the vine, suggesting that a low level of branch autonomy is reached in ‘Concord’ and ‘Riesling’, which might have implications for modelling plant productivity. In fact, this lack of autonomy would allow for simpler “big leaf” or “big fruit” modelling approaches (Lakso et al., 2001). Nevertheless, this does not exclude the possibility of local effects within a vine because different light microclimates between portions of a vine affect fruit composition (Friedel et al., 2015).