Evaluation of carbon balance and carbohydrate reserves from forced (Vitis vinifera L.) cv. Tempranillo vines

Abstract

Elevated temperatures during berry ripening have been shown to affect grape quality. The crop forcing technique (summer pruning that ‘force’ the vine to start a new cycle) has been shown to improve berry quality by delaying the harvest date. However, yield is typically reduced on forced vines, which is attributed to vine low carbon availability soon after forcing and likely incomplete inflorescence formation. The present study aims to estimate the carbon balance of forced vines and evaluate vine responses to changes in carbon patterns due to forcing. Three treatments were studied on Tempranillo cultivar: non-forced vines (Control), vines forced shortly after fruit set (CFearly) and vines forced one month later at the beginning of bunch closure (CFlate). Whole canopy net carbon exchange was modelled and validated using two whole canopy gas exchange chambers. In addition, non-structural carbohydrate reserves at budburst, forcing date and harvest, were analysed. Yield, yield components and vegetative growth were also evaluated. Harvest date was delayed by one and two months in the CFearly and CFlate, respectively, which increased must acidity. However, yield was lower in the forced treatments compared to the Control (49% lower for CFearly and 82% for CFlate). In the second year, at the time when CFearly and CFlate dormant buds were unlocked (forced budburst), forced vines had significantly lower non-structural carbohydrates than Control vines at budburst. Although the time elapsed from budburst to reach maximum net carbon exchange was longer for the Control treatment (80 days) than for the forced treatments (about 40 days), average daily net carbon exchange until harvest was comparable between Control (60.9 g CO2/vine/day) and CFearly (55.9 g CO2/vine/day), but not for CFlate (38.7 g CO2/vine/day). In addition, the time elapsed from budburst to harvest was shorter in forced treatments (about 124 days) than for the Control (172 days). As a result, the cumulative net carbon exchange until harvest was reduced by 35% (CFearly) and 55% (CFlate) in the forced treatments. However, no differences in carbon reserves at harvest were observed between treatments partly helped by the higher source:sink ratio observed in forced than Control vines.

Long-Term Study of the Crop Forcing Technique on cv. Tempranillo (Vitis vinifera L.) Vines and Suggested Irrigation Strategies to Improve Water Use Efficiency of Forced Vines

“…We did not observe a reduction in the number of bunches per vine in F1 when compared to F2. This observation is contrary to other short-term (maximum 2-year trial) studies using similar forcing pruning dates, in which it was observed that the number of bunches per vine increased when forcing was carried out later [5,6,13]. No clear effects of the forcing date on the number of bunches per vine was observed in long-term studies [10].…”

“…This last observation is rather controversial. Contrary to our observations, some studies have reported that forced vines require more GDD to reach veraison than unforced vines [6,10], whereas studies realized in the same region as the present studies are in accordance with our results [7,13]. These contradictory results suggest that GDD should be taken with caution when comparing data from different climatic regions and with different irrigation strategies and when comparing unforced with forced vines, since other parameters besides temperature (including photoperiod, carbohydrate availability and periods of extreme high temperatures) may alter phenology [20].…”

“…In addition, forced treatments required more GDD to reach fruit set than unforced treatments, which may be related to the competition of plant development with fast-growing organs for a reduced carbohydrate reserve availability [6]. The previously cited observation is consistent with the minimum of carbohydrate reserve availability observed at the forcing pruning date [13,19]. However, GDD from fruit set to veraison was reduced in forced treatments, which equalized for F1 or even reduced for F2 the total (from budburst) GDD required to reach veraison when compared to unforced vines.…”

“…In non-forced vines, the number of bunches per vine is dependent on the carbohydrate reserves at budburst [26]. Therefore, our hypothesis is that the number of forced bunches per vine may be linked to the state of the carbon reserves at non-forced budburst and therefore, to the carbon balance and competence for carbon between the refilling of the carbohydrate reserve and other sink organs of the previous year, as well as the carbon balance and sink activity during the pre-forcing period of the same year [13].…”

“…Due to requirements of the plot owners to facilitate differentiated disease and pest control and to reduce experimental costs, plots were not randomly distributed in Experiment 1. Based on a high-resolution Normalized Difference Vegetation Index (NDVI) map generated in 2016 of the same field and previously described in [13], in an area as homogeneous as possible, three parallel and adjacent plots with four rows of 300 m (182 vines per row) per plot were established. In each plot, one forcing treatment was assigned randomly.…”