Study on the use of source-sink-reserves relations to manipulate grapevine nitrogen use efficiency, aroma and grape maturity and its relationship to yields and wine composition

Canopy management and fruit load control aim to keep a balance between vine’s sources and sinks. In fact, balanced vines may produce more consistent yields and have a more even ripening. This study aims to study the relationship between source–sink ratios and important parameters for production logistics and grape quality, such as progress of ripening and grape composition at harvest. After homogenizing all vines by removing laterals and adjusting the number of shoots to 20, we tested three levels of canopy density and fruit load combined in a factorial design (3 by 3). This is, 3 canopy levels, 100%, 66% and 33% of the leaves combined with 3 fruit loads, 100%, 66% and 33% of the fruit corresponding to 30, 20 and 10 clusters per vine, respectively. Carbon fixation rates were transiently higher in plants with a 33% of the canopy mediated by higher chlorophyll content, although this did not compensate for their smaller leaf area. Onset of ripening was sequentially delayed in 66% and 33% canopy treatments. The progress of ripening, accumulation of soluble solids and loss of acidity (increase in pH and decrease in total acidity), also occurred slower in 66% and 33% canopy treatments compared to 100% of the canopy. In fact, the time to reach commercial maturity (>25°Brix) was delayed 6 weeks for the 33% canopy level. Surprisingly, fruit load did not have a significant effect on the progress of ripening. When comparing all treatments at commercial maturity, the treatment maintaining 100% of the canopy had the highest total acidity and lowest pH. Contrarily, Anthocyanin content was slightly lower in this treatment. These results provide the basis for the control of the speed of ripening, aiming to coalesce variability within a vineyard or optimize the tank capacity through sequential ripening.

Fruit thinning is an important cultural practice in a premium vineyard as it directly affects yields. From fruit set through veraison, wine (especially red) grape growers can remove up to 2/3 of the initial crop borne by the plant at fruit set. This practice is performed aiming for a vine balance between leaf area (sources) and fruit load (sinks). A vine with a higher leaf area-to-fruit ratio (LA/F) is able to ripen (increase °Brix) the crop faster (Kliewer and Dokoozlian 2005; Parker et al. 2014), and presumably, achieve better quality. Contrarily, when there is too much fruit for a given leaf area, ripening (increase in °Brix) can be slower and even stop when is pushed into late October. Under normal circumstances, grape berry accumulates sugars and anthocyanins, while losing astringency (mainly proanthocyanins) and green aromas throughout ripening. Therefore, lack of ripening can be assessed as a slower accumulation of TSS and anthocyanins or retaining high amounts of proanthocyanidins and green aroma compounds. It is well understood that although a balanced wine is the target, there is greater penalization for under ripe wine characteristics than over ripe (Bindon et al. 2014), and in poor sugar accumulation is intimately related to vintage failure (Jones and Davis 2000). Therefore, there is a risk associated with carrying more crop in the grapevine as this can result in an insufficient ripening and quality loss. As grape price or wine retail price is a greater multiplier of revenue than yield in wine grape, dropping fruit is always the decision taken by premium wine grape growers. In addition, when there is a contract between grape growers and wineries, there are great economic interest plead for or against fruit thinning, without any certainty of the true pros and cons of fruit thinning. Given the stability of 2 summer weather in California, a better understanding of how source-sink-reserves affect plant fitness, growers could reduce the amount of fruit that they need to drop and still safely achieve an adequate ripening.

Nitrogen applications are a delicate matter as in their deficiency may result into poor growth and in excess may inhibit the production of grape anthocyanins (Soubeyrand et al. 2014). This study aims to determine the impact of nitrogen use efficiency and make recommendations about the application of nitrogen in combination with canopy and fruit load management decisions. Nitrogen is the most frequently deficient nutrient in vineyards, and thus the most often supplemented by fertilizers. Grapevines lose approximately 2.76 lbs of N per ton of grapes harvested, and this value serves as an indication of the minimum amount of N that should be replenished annually. However, that value is an estimate based on the average of data from several studies showing a range from 4.2 to 1.8 lbs of N per ton of grapes (see references in Mullins et al., (1992)). In addition, a great amount of nitrogen is invested in the growth of trunk, roots, leaves and shoots. Although a fraction of this nitrogen is reabsorbed into permanent structures when plants go dormant, a significant amount is lost with leaf fall and dormant pruning, leading to additional demand. Given the significant storage of nitrogen perennial crops in permanent structures, the use of a fertilizer containing labeled nitrogen (15N) allows an estimation of the fertilizer use efficiency, and therefore, determine if the amount of nitrogen prescribed needs to be modified according to cultural practices canopy or crop load management.