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.
Irrigation
Sufficient soil moisture/water supply during the critical stages of the seasonal development of the vine, is essential for the build-up and mobilisation of carbohydrate reserves. The irrigation strategy must be especially closely monitored for the budding, flowering, fruit set and post-harvest stages. The regulated deficit-irrigation approach has been proven to control vegetative growth effectively and stimulate root development without sacrificing carbohydrate reserves.
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.
Soil and fertilisation
Apply fertilisation wisely by basing applications on regular soil analyses. Precision applications (spot applications and timing) can decrease variation in growth and variation between blocks, which will allow more value to be added from even the most basic viticultural practices. Indirect practices contributing to the optimal functioning of the vine include the use of cover crops, mulches and organic material. These practices lead to better availability of nutrient contributions to conserve soil moisture, regulate soil temperature and supply organic material. All these actions will contribute to the accumulation of carbohydrate reserves.
PHOTO 2. Competition from weeds for water, nutrients and sunlight also result in lower reserve levels.
Disease and pest control
Follow a good disease- and pest-control programme to keep vineyard canopies healthy. This will lead to an effective canopy and optimal photosynthetic activity.
PHOTO 3. Example of extreme cases of downy (top) and powdery mildew (bottom) infections which resulted in early leaf fall. Given that the crop must be ripened without active leaves, the plant will have to delve deep into the reserve pool, which will then be problematic for the next season.
Harvest
Consistently balance the crop size with canopy capacity. An excessively large crop combined with low canopy capacity will most likely culminate in decreased carbohydrate reserve levels at the end of the season. Leaving grapes on the vine to a higher ripeness degree than the canopy can handle, too will have a negative influence on the reserve status. Together these practices will also shorten the recovery time during the post-harvest period, especially in late-ripening cultivars.
PHOTO 4. Here the effect of harvest time on reserve levels can clearly be observed. The part of the block to the right was harvested early for low-alcohol wine production, while the left side was harvested a month later at higher sugar levels. Note the differences in canopy size and activity.
Post-harvest treatment
Irrigation must be applied to ensure effective photosynthesis. Guard against overirrigation, as the vine’s water requirement is considerably less during this period. Fertilisation must be applied in a timely manner considering soil analyses, as well as vigour and realised yield, which determines nitrogen norms. Protect leaves against untimely fungal infections and prevent regrowth where possible by avoiding stress.
By paying careful attention to all the factors that promote the establishment of carbohydrate reserves, simple management practices can lead to sustainable vineyard blocks. Well-buffered vineyards with sufficient carbohydrate reserves are able to handle climate fluctuations and unforeseen challenges better than those that are already teetering on the edge.
PHOTO 5. Regrowth after harvest will lead to unnecessary utilisation of reserves before budding, resulting in lower reserve levels during budding.
For more information, contact Hennie Visser at henniev@vinpro.co.za.
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