Dr. Carien Coetzee
Basic Wine
27 October 2020
Water stress, due to insufficient water supply, may have direct and/or indirect effects on the physiology of the vine and the composition of the berries. Some of the physiological responses to water stress include1
- Reduced cell division and expansion
- Closing of leaf stomata
- Reduced photosynthesis
- Cell desiccation and death
These physiological effects would normally result in reduced vine growth and increased sunlight exposure in the bunch zone. Further implications would be a reduction in the production and transport of sucrose which reduces the availability of metabolites2.
The leaf water potential estimates the capacity of the cells to retain water by pressurising a leaf with a neutral gas. This is usually done using a pressure chamber. The less free water there is in the plant, the greater the pressure required to cause it to exude. The result is expressed in bar or kPa / MPa, always as a negative value.
A study was conducted to investigate the effect of water stress, measured as the leaf water potential (LWP), on South African Sauvignon blanc vine performance and bunch development. External symptoms at various LWP values were reported2.
Materials and Methods
Twenty-one Sauvignon blanc plots (Stellenbosch) were used in this study. The LWP of the vines was tested using a pressure chamber. Measurements were recorded at pre-dawn (04h00 until sunrise) and midday (11h00 until 14h00)2. External water stress symptoms were also recorded using a vineyard scorecard with adapted evaluation criteria for South African conditions3,4.
Leaf water potential categories
Pre-dawn LWP is generally accepted as the best physiological indication of vine water stress as it is in direct relation to soil water that is accessible by the roots5. During this stage, the stomata of the plant are closed and the grapevine has been able to equilibrate its water potential with the most humid layer of the soil.
The pre-dawn LWP results can be categorised as follows5:
- 0 to -200 kPa = absent/mild water stress
- -200 kPa to -400 kPa = mild/moderate water stress
- -400 to -600 kPa = moderate/severe water stress
- < -600 kPa = severe to drastic water stress
Midday LWP is greatly affected by environmental and weather conditions and the use of this parameter for the scheduling of irrigation has been questioned2.
The midday LWP results can be categorised as follows6:
- -1000 to -1200 kPa = light water stress
- -1200 kPa to -1400 kPa = medium water stress
- -1400 kPa to -1600 kPa = high water stress
- < -1600 kPa = severe water stress
Visual symptoms
Results showed that the measured LWP reflected the external visual stress symptoms well with a good correlation observed between pre-dawn LWP and midday LWP.
- Pre-dawn LWP 0 to -200 kPa = absent/mild water stress
Visual symptoms: Some inactive shoot tips, some yellow-green leaves - Pre-dawn LWP -200 kPa to -400 kPa = mild/moderate water stress
Visual symptoms: Some inactive shoot tips, yellow leaves, tendrils at 90° in relation to shoot apex - Pre-dawn LWP -400 to -600 kPa = moderate/severe water stress
Visual symptoms: Inactive shoot tips, leaves yellow and/or drooping and/or desiccated, tendrils at 90° in relation to shoot apex or wilted - Pre-dawn LWP < -600 kPa = severe to drastic water stress
Visual symptoms: Inactive shoot tips, many chlorotic and necrotic leaves, leaves drooping and abscising, tendrils wilted and abscising
Higher water stress at ripening correlated with shorter shoots, a lower cane mass and reduced berry size and berry volume. According to Prof Alain Deloire5, the pre-dawn LWP should preferably not be lower than -400 to -500 kPa5 as the smaller berry size does not necessarily result in increased quality.
Effect of plant water status on the aromatic development in Sauvignon Blanc berries
The water availability and/or degree of sunlight exposure could affect the concentration of aroma compounds such as the methoxypyrazines (green aromas) and the volatile thiol (tropical fruit aroma) precursors in the berries. Typically, high water availability will result in more vegetative growth and thus increase canopy density and cluster shading. Increased water stress could result in a decreased canopy coverage and increased sunlight exposure in the bunch zone.
A study showed that irrigated vines produced grapes with higher methoxypyrazine content compared to non-irrigated vines7, possibly due to the impact on the canopy density and sunlight exposure (see blog – Managing green aromas in the vineyard: Methoxypyrazines). Another study showed that severe water stress (predawn LWP reaching -1000 kPa) limited the aroma potential in Sauvignon blanc grapes and resulted in a lower volatile thiol precursor content8. To the contrary, a mild water deficit (in combination with a non-limiting nitrogen status) enhanced the thiol precursor development8. Mild water stress can thus be beneficial for the production of the volatile thiol precursors and reduce berry methoxypyrazine concentrations (if desired).
Conclusion
Water stress could directly and/or indirectly affect the development in the berries as canopy microclimate is a result of the inherent water relationship between soil and vine. Studies have shown that water stress symptoms correlate well with field measurements of LWP2 and that the water status can affect the aroma development in Sauvignon blanc berries7,8.
The general recommendation is that water supply during early season should be adequate to quickly fill the trellis system with active leaves but at the same time not to induce too large berries and unbalanced aroma development2. Moderate water stress during berry growth will lead to smaller berries and perhaps a more balanced aroma composition. However, care should be taken to not negatively affect other metabolic processes.
References
(1) Goodwin, I. Managing Water Stress in Grape Vines in Greater Victoria. Department of Primary Industries 2002.
(2) Bruwer, R., Carey, V., Archer, E. The Effect of Plant Water Status on Sauvignon Blanc. Wineland Technical 2004, October.
(3) Smart, R., Robinson, M. Sunlight into Wine. A Handbook for Winegrape Canopy Management; Winetitles: Adelaide, Australia, 1991.
(4) Archer, E. Lighuishouding En Lowerbestuur by Wingerd. Stellenbosch University 2002.
(5) Deloire, A. Grapevine and Water: Influence of Water Deficit on Skin Phenolic Compounds and on Sugar Loading during Berry Growth of Vitis Vinifera L. Consequences for Vineyard Cultural Practices. In Proc. 27th SAWWV Congress; Somerset West, South Africa, 2003.
(6) Bogart, K. Measuring Winegrape Water Status Using a Pressure Chamber. Grape Community of Practice 2019, No. June.
(7) Sala, C., Busto, O., Guasch, J., Zamora, F. Contents of 3-Alkyl-2-Methoxypyrazines in Musts and Wines from Vitis Vinifera Variety Cabernet Sauvignon: Influence of Irrigation and Plantation Density. Journal of the Science of Food and Agriculture 2005, 85 (7), 1131–1136.
(8) Peyrot des Ganchos, C., Van Leewin, C., Tominaga, T., Soyer, J.-P., Gaudillere, J.-P., Dubourdieu, D. Influence of Water and Nitrogen Deficit on Fruit Ripening and Aroma Potential of Vitis Vinifera L. Cv. Sauvignon Blanc in Field Conditions. Journal of Agricultural and Food Chemistry 2005, 85, 73–85.
Photo by Tim Mossholder on Unsplash
