Technical Yearbook 2024

DECEMBER

The vine’s reserve bank By Hennie Visser

Carbohydrate reserves in the grapevine play a critical role in various physiological processes. It affects the vine’s overall health, development, growth and ultimate yield. These reserves are primarily stored in the permanent structure of the plant, like the roots, trunk and canes. Carbohydrate reserves are important during periods when the vine is not actively photosynthesising, like early in the spring and during winter dormancy, but also when conditions outside the limits of optimal physiological activity occur due to environmental factors. The importance of carbohydrate reserves Budding and initial growth Carbohydrate reserves stored in the roots, trunk and canes during the previous season, are critical for even budding and initial shoot growth in the spring. Sufficient reserves provide the necessary energy for optimal initial growth to create the canopy before the new mature leaves are capable of contributing photosynthetic product to the system. Sufficient reserves are therefore extremely important for the development of a healthy canopy and fruit-bearing structures. Flowering and fruit set This stage of development presents a very high demand for carbohydrate reserves, due to the high amount of active growth points (roots, permanent structures, actively growing canopy, as well as reproductive organs). Moreover, adequate carbohydrate reserves serve to support the energy intensive processes of flowering and fruit set. Insufficient reserves can lead to poor flowering and fruit set, and ultimately lower yield. Vigour and canopy development Carbohydrate reserves make a big contribution to the overall vigour of the vine by influencing the growth and development of leaves and shoots. A healthy, well-developed canopy is essential for optimal photosynthesis, which in turn impacts positively on the vine’s ability to produce and accumulate carbohydrates. Root growth and development Roots grow and develop during specific periods (mainly during flowering and after harvest) when other plant organs also pose a significant demand for photosynthetic product. Sufficient carbohydrate reserves are thus beneficial for good root growth and development. A good, expansive root system increases the productive potential of the vine and ensures the optimal utilisation of soil nutrients. Winter survival In climatic regions where low winter temperatures (< 0°C)

are common, carbohydrate reserves play an important role in protecting plants against cold damage. High concentrations of carbohydrates in the plant cells lower the freezing point of the cell sap and thereby increases the vine’s tolerance against extreme low temperatures. Viticultural practices to ensure carbohydrate reserves Winter pruning Prune to achieve an optimal balance between growth (vegetative) and yield (reproductive). If the bud load is too low, it can lead to dense canopies with suboptimal photosynthesis. If the bud load is too high, the leaf surface and photosynthesis capacity will not be able to meet the demand of the reproductive organs and permanent structures. Both these scenarios will result in lower carbohydrate accumulation. Prune at the correct time. Pruning must be concluded before budding to ensure that carbohydrate reserves are not lost by supporting growth which will be removed later. Pruning should also only take place after all effective leaves have fallen off the vine, to ensure maximum reserve accumulation. Canopy management Canopy management must be applied with the objective of achieving optimal canopy density. A well-aerated canopy with good light penetration is crucial to unlock the photosynthetic potential of the leaf canopy. Carbohydrates that are not used for vegetative or reproductive growth, contribute to the reserve pool. Avoid late topping actions, which remove large amounts of young and active leaves. Regular tipping actions keep growth in check, while also providing for sufficient effective leaves late in the season.

PHOTO 1. An example of the detrimental effects of long term soil moisture/water deficits on initial growth (poor) as result of low reserve levels.

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TECHNICAL YEARBOOK 2024