Wilmé Brown (Plant Physiologist) and Dr. Elmi Lötze (Head of ITEST™CARBOHYDRATES, ITEST™LEAF and Research and Development)
Carbohydrates and viticulture
The vine needs carbohydrates to survive the winter (the time during which leaves are absent and active photosynthesis cannot occur) and also to support vegetative growth during the coming season until new leaves emerge. While the postharvest period is not the only important time to build carbohydrate reserves, this period is also essential for addressing the accumulation of starch in the stem, roots and shoots before leaf senescence.
Prior to leaves reaching maturity and becoming sugar exporters (sources), new leaves initially act as sinks for the available carbohydrate reserves from the roots and shoots (Martínez-Lüscher and Kurtural, 2021). It is therefore important to do early carbohydrate analyses, 2 to 4 weeks before bud break, for sufficient time to make adjustments on selective blocks from flowering. These analyses are available as the ITEST™CARBOHYDRATES service.
Time to build up reserves
During photosynthesis, the essential energy source i.e. sugar, is produced in the green tissues of the vine, mainly the leaves.
During the season, sugars are consumed by various sinks on the plant. Sugars are also translocated to the shoots, stem and roots, the primary reserve organs, where it is converted into starch which is the reserve carbohydrate source (Smith and Holzapfel, 2014).
The conversion of sugars to starch occurs throughout the season in different parts of the plant. However, the conversion percentage, phenological stage during which it occurs and the organ within it occurs, differ and relate to the respective sinks that are present at the specific time of conversion. As such, the translocation of sugar from the leaves to the roots occurs during active root growth.
During harvesting the primary sink, the clusters, is removed, which impacts favourably on the photosynthesis of the vine. At this point the vine functions optimally due to the complete canopy and the environmental conditions (Zufferey et al. 2012). This results in additional energy being available in the plant for consumption by secondary sinks, such as shoots and roots. It is thus the ideal time to address the build-up of carbohydrate reserves for this deciduous crop.
The vine will during dormancy, as well as the following season, be reliant on carbohydrate reserves in its storage organs for all metabolic actions until flowering, when the newly formed leaves mature and become the primary source of sugars. Thus, it is important to attend to the health of the vine to ensure that the leaves continue to photosynthesize before leaf drop occurs.
Pruning
For effective management of crop load and cluster quality in viticulture, it is standard practice to prune shoots during winter. However, by removing the shoots, reserve carbohydrate sources are also removed. It is therefore important that the shoots retained on the vine must be strong and thick to be able to store the as much reserves as possible (Pellegrino et al., 2014). During the post-harvest period therefore, the role of the roots as reserve carbohydrate source is indispensable for provision of adequate support during bud break.
Carbohydrate analyses during the early post-harvest period can provide important information regarding the energy status of the vine for the coming season, thus enabling the producer to make timely adjustments regarding the intended crop load for the next season, as well as the intensity of pruning.
Bud break
Before bud break in spring, starch is converted into sugar and translocated from the permanent structures such as roots and shoots to support bud break and flowering. Carbohydrate levels can be determined through carbohydrate analyses of the shoots and roots as early as 2 to 4 weeks before bud break.
In viticulture, evenness of bud break and flowering events are important with regard to the planning of management practices such as spraying. By early intervention, this variation can be managed to impact the rest of the season i.e., flowering and berry maturation will occur more uniformly and lead to optimal sugar levels at harvest, which has a positive influence on the alcohol content and quality of the wine.
Bud break percentage versus shoot and root sugar levels
Carbohydrate analyses were carried out by Agri Technovation in different viticultural regions of South Africa.
Shoots
Results showed that sugar levels in vine shoots decrease 2 weeks before bud break, but that the bud break percentage increases. This relationship indicates that metabolic processes occur even before bud break and that the first organs to supply carbohydrates for development are the shoots.
Thus, the conversion from starch to sugars occurs even before bud break is noticeable (Duchene et al., 2003). Thereafter, carbohydrates from more distal organs (roots) are utilised as vascular bundle connections between the new shoots and the mature wood develop.
Roots
The ratio between the sugars in the roots and bud break percentage indicates that higher sugar levels lead to higher bud break percentages. This differs distinctly from that of the shoots, as the conversion of starch to sugar in the roots occurs only at a later stage. Again, this indicates the importance of available root carbohydrates during bud break, which can be achieved through good management during the season and an optimal post-harvest strategy.
Summary
ITEST™CARBOHYDRATES analyses 2 to 4 weeks before bud break provide insights into future bud break- and flower potential in wine grapes. It creates the possibility to apply various actions and specialised Agri Technovation products to manage suboptimal carbohydrate levels in time to address potential limitations of poor bud break and flowering.
Contact your nearest Laeveld Agrochem agent or Agri Technovation specialist to perform carbohydrate sampling on wine grapes.
References:
- Duchene et al. 2003. Effects of ripening conditions on the following season’s growth and yield components for pinot noir and gewurztraminer grapevines (vitis vinifera l.) In a controlled environment. J. Int. Sci. Vigne vin 37, n°l, 39-49
- Martínez-Lüscher and Kurtural. 2021. Same season and carry-over effects of source- sink adjustments on grapevine yields and non-structural carbohydrates. Front Plant Sci 12:695319.
- Pellegrino et al. 2014. Management practices impact vine carbohydrate status to a greater extent than vine productivity. Frontiers in plant science.
- Smith and Holzapfel. 2014. Post-harvest care of grapevines: Irrigation and nutrition.
- Zufferey et al. 2012. Carbohydrate reserves in grapevine (Vitis vinifera L. ‘Chasselas’): the influence of the leaf to fruit ratio. Vitis 51 (3), 103–110.