Improve water use of table grapes
By Kobus Burger (Agronomist - Northern Cape Business Unit)
In our country with its low rainfall there are regions further west in the Northern Cape where table grapes are grown with as little as 50 mm to 150 mm of rainfall per annum. Grapes are also irrigated with water from the Orange Rivier. With the different standards used to determine the efficiency of an industry, water use efficiency (WUE) of table grapes also has to be considered. With the WUE of table grapes between 0.92 and 9.05 kg/m3, it is critically important to focus on factors that can improve this relationship to ensure a higher WUE for table grapes.
If drainage water is eliminated, water in the soil is released into the atmosphere in two ways, namely through evaporation and transpiration. There are different practices to determine which part of the water (released into the atmosphere) will be subject to evaporation and which part to transpiration. This information can be used to determine crop factors to calculate the transpiration rate of certain crops of a certain age at a specific time of the year. Many practices have been developed to reduce evaporation losses, for example through the use of mulches in vineyards, improved soil tillage practices, irrigation scheduling and the adaptation of irrigation systems. Transpiration remains a huge challenge, as 97% of the water taken up by the plant is directly released into the atmosphere. Only 2% of the water (taken up) is used for cell enlargement and only 1% for metabolic processes. To increase the WUE of plants, several aspects have to be taken into account. By focusing on the physiology of the plant, it becomes clear which factors are important for WUE.
By looking at the physiology of plants such as table grapes, we can see the direct effect of drought stress on the plant. This gives us an indication where in the plant solutions have to be found to bring about a higher WUE. There are a number of issues that can be focused on during drought stress with regards to the physiology of the plant. During drought stress, the rate of photosynthesis decreases; the concentrations of proline and abscisic acid (ABA) in plant cells increase; the growth tempo of the whole plant decreases; and ultra-structure of organelles in cells change. Optimal plant nutrition is necessary to better maintain plant cells during the stressful circumstances and to increase WUE.
With increasing pressure on the role of plant nutrition to counteract environmental factors, it is extremely important to also focus on the effect of plant nutrition on the improvement of WUE.
Several researchers have indicated that the WUE of crops can be improved by focusing on certain factors. On the physical level of water management much work has been done by looking at soil tillage methods and water scheduling, among others. There are, however, still many challenges on the plant physiology side with regards to the optimal management of plant water. Plant nutrition also plays an important role in how efficiently every plant uses and maintains water in its tissues through membrane stability. WUE can already be increased by:
- SPAD-readings (monitor leaf chlorophyll levels);
- a thorough understanding of the properties of the nitrogen (N) source used in plant nutrition;
- scheduling of fertilisation for optimal uptake of plant nutrients through the season with minimum leaching;
- management of nutrient concentrations (such as the Michaelis Menten system) in root zones can increase uptake of nutrients;
- monitoring of the nutrient balance in plant sap; and
- synergism between elements should be used in nutrient solutions to increase uptake of nutrients.
The above methods should be actively used in the field. Through the correct management, the WUE of table grapes can be drastically increased as indicated. Because the research literature shows that the WUE of vineyards is currently between 0.92 and 9.05 kg/m3, it is critically important to use the above methods to increase WUE and also to look at all the factors that can improve this relationship.