Investigating the effects of source-sink relationships on yield components, fruit development and composition of grapevines : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Viticulture practices (e.g., pruning, leaf removal, cluster thinning etc.), temperature and other climate conditions all have an influence on grapevine (Vitis vinifera L.) phenology, berry maturation and composition. Leaf removal as a common management practice alters the Leaf Area to Fruit Weight (LA: FW) ratio and may influence the timing of key phenological stages, berry maturation, photosynthesis activity, carbohydrate allocation and yield components. The outcome of this practice depends on the time and degree of leaf removal, the position and area of the removed leaves, the cultivar, and the climate. Winegrape growers need to decide where and when canopy management strategies are required. Therefore, understanding the relationship between different times and positions of leaf removal and yield components, carbohydrate allocation and berry composition could be beneficial for grape growers, particularly commercial vineyards and wineries. The outcomes of these practices on final yield, berry and wine composition are also important. Furthermore, research in two different environments, a vineyard and a glasshouse, could help to a better understand the interaction between source restriction and temperature. This study involved Pinot noir vines, both in outdoor vineyard conditions and within a moderated glasshouse environment. During various stages of the growing season, out of a total of twelve leaves on each shoot, six leaves were systematically removed, while the remaining six were retained at different positions along the shoot. The primary goals of this research were 1) to characterise the influence of apical versus basal leaf removal at three key phases of vine development: flowering, fruit set and bunch closure on the time of veraison, berry maturation parameters (total soluble solids, acidity and berry weight), berry components (seed, skin and pulp), berry composition (phenols, tannin and anthocyanins), vegetative growth (leaf, shoot and root), carbohydrate reserves and yield components; 2) to evaluate the real effect of source limitation achieved by apical and basal leaf removal at flowering and bunch closure on vegetative and reproductive parameters (as stated in aim 1), when the effects of environmental factors, especially temperature, are minimised under a controlled and moderated glasshouse environment; and 3) to investigate the effect of apical and basal leaf removal at flowering and bunch closure on leaf area, net photosynthesis, transpiration, stomatal conductance, and chlorophyll content over a period of time (from flowering to harvest). For the first and second aims, two similar experiments were conducted in a vineyard and a glasshouse. The experiment in the vineyard included two leaf removal positions (apical and basal) and three times (flowering, fruit set and bunch closure). A potted vine experiment with two positions of leaf removal (apical versus basal) at flowering and bunch closure was conducted in the glasshouse. The timing of veraison was determined in both experiments, and key components of berry composition including total soluble solids, pH, titratable acidity, malic acid, phenols, tannin, and anthocyanins were assessed from veraison to harvest. The seeds and skins of the berries were separated, and their fresh and dry weights were measured from veraison to harvest. Seed variability was also assessed by weighing each seed. At harvest(s), fruit set percentage, berry weight, bunch weight, yield, and vegetative growth (leaf, shoot, root) were evaluated. To achieve the third aim, a potted vine experiment was carried out in a glasshouse. Leaf removal treatments were applied at flowering and bunch closure as follows: twelve main leaves retained, six basal leaves retained, six apical leaves retained, leaves 2-7 (counting up the shoot) retained. Before treatments, the leaf area removed and retained were measured, and at harvest, the leaf area of all vines was evaluated. Vine net CO2 gas exchange was monitored from treatment to harvest on selected leaves from the top, middle, and bottom positions, depending on the treatment. Concurrently, leaf chlorophyll content and soil moisture were determined. Fruit set% and harvest bunch weight, vegetative growth and berry maturation were evaluated. Fruit set percentages ranged from 45 to 75% in the vineyard experiment and from 80 to 100% in the moderated glasshouse conditions. The observed result may be attributed to stable environmental conditions that can impact the success of fruit set. Another possibility is that potted (young) vines having only one inflorescence each, thus allowing for a more focused distribution of nutrients and carbohydrates to the flower cluster. In all three experiments, basal leaves retained vines delayed 50% veraison, however, the glasshouse experiments contributed to a greater delay than the vineyard trial. Keeping six apical leaves resulted in a shorter delay than keeping basal leaves.. Source limitation slowed the rate of total soluble solids accumulation in all experiments. Following the delay in veraison, keeping six basal leaves at flowering, fruit set and bunch closure resulted in a reduction in total soluble solids concentrations in the vineyard and potted vine trials. Apical leaves retained vines had lower total soluble solids than non-defoliated vines at the start of veraison due to a few days delay in reaching veraison; however, this delay was compensated during ripening, and there was no difference in total soluble solids between apical leaves retained vines and control at harvest in all three experiments. The hypothesised compensatory mechanisms identified for defoliation stress in basal leaves retained vines were increased chlorophyll content, stomatal conductance, transpiration, and maintenance of soil moisture, all of which enabled an increase in net photosynthesis over a period of time. These treatments, however, were unable to increase leaf area since they were the oldest leaves on the plant and had probably reached their maximum size. On the other hand, the compensatory mechanisms observed for the vines that retained apical leaves involved keeping a high rate of photosynthesis, transpiration and stomatal conductance as high as non-defoliated vine, while also increasing leaf size from the time of treatment application until harvest. As these leaves were young and immature they were still able to increase leaf size from the time of treatment application until harvest. In the vineyard trial, the compensatory strategy of basal leaves retained vines was insufficient, resulting in reduced yield components. Although basal leaves retained vines had no detrimental effect on yield components in the glasshouse experiments, the analysis of vegetative growth and carbohydrate reveals that all basal leaves retained treatments had reduced shoot diameter, shoot weight, and root weight/carbohydrate in all three experiments. This means that in basal leaves, transferring energy to sink organs by increasing photosynthesis (resulting in a delay in veraison) was possibly inadequate and came at the expense of perennial organs. In the vineyard experiment, keeping apical leaves at flowering reduced fruit set percentage and vine capacity (an estimate of the above ground dry weight of the vine), while the other times (fruit set and bunch closure) in all three experiments had no effect on yield components, carbohydrate reserves and vegetative growth when compared to control vines. Skin total phenols and tannin increased when six apical leaves were retained at flowering in the vineyard experiment, whereas the same treatment in the glasshouse had no effect on phenols or tannin, suggesting that the increase in phenols and tannin in the vineyard is most likely due to changes in the microclimate after removing the basal leaves, and defoliation has no significant effect on tannin and phenols in a control environment. Basal leaves retained treatments had reduced skin anthocyanins in both vineyard and glasshouse studies, which was not only because of the delay in veraison since basal leaves retained vines had lower anthocyanins concentration than non-defoliated vines at the same total soluble solids. While reduced carbohydrate availability is a primary factor affecting anthocyanin production in defoliated grapevines, there are other factors such as hormonal balance and light environment, and environmental factors that can also play a role in determining the final anthocyanin concentration in grape skins.