MONDAY, JUNE 23, 2025. BY STAN GRANT, VITICULTURIST.
Part One of this blog post is available HERE.
Heat Related Stresses – How Grapevines Respond
The death of some leaf tissues is a sacrificial, survival response to heat related stresses. It also is a permanent loss of ripening capacity, that may delay harvest if leaf death is extensive (Figure 9, below).
Figure 9. Leaf tissue death is a sacrificial stress response and a permanent loss of ripening capacity.
Photo source: Progressive Viticulture©.
Intact grapevine leaf tissues, on the other hand, have several responses to heat related stresses that lessen their effects. These include the conversion of starch to sugars, the release of volatile isoprenoids, the accumulation of calcium and amino acids, and the production of certain proteins and antioxidant compounds. The net effects of these responses include stimulated antioxidant activities, reduced membrane leakage and rehydrated cells and tissues.
Due to these stress responses, intact leaf tissues can recover with a few days of normal air temperatures. However, stress recovery in leaves redirects sugars away from berries, slowing fruit maturation and possibly delaying harvest.
In roots, carbohydrates are remobilized and translocated to shoots during and following periods of stress. In this way, stored carbohydrate reserves in roots serve as a carbohydrate buffer under stress and impaired photosynthesis. Following stressful periods, certain hormones (cytokinins) produced in roots and translocated to shoots can stimulate lateral buds and promote lateral shoot growth.
The presence of lateral shoots provides a couple of benefits. First, hormones in expanding leaves on lateral shoots delay the senescence of older leaves. Second, the comparatively high photosynthetic capacity of young leaves on lateral shoots compensates, to some degree, for the stress-induced death of tissues in older leaves.
Heat Related Stresses – Vineyard Management Tactics for Lessening Impacts
Before the onset of forecasted triple digit maximum temperatures, temporarily suspend regulated deficit irrigation and liberally irrigate to wet soils, hydrate vine tissues, promote transpirational cooling and limit the impacts of heat on photosynthesis. To do so, daily cycle through irrigation blocks, perhaps running 6 to 8 hours per block.
Next, in portions of vineyards prone to heat stress, prime foliar tissues for antioxidant production, focusing on the afternoon sun sides of canopies. Spray-apply low biuret urea to the vine foliage to increase amino acid synthesis. In these foliar sprays, also include calcium, iron, manganese, zinc and boron as amino acid chelates to ensure adequate tissue levels of these mineral nutrients involved in antioxidant production. Apply these materials no more than 10 days before the onset of high temperatures to achieve the desired stimulatory effect.
Following the heat spell, continue to liberally irrigate for 2 to 5 days before resuming regulated deficit irrigation. Typically, the greater the degree of heat stress, the greater the benefit from liberal irrigation.
During this time, liquid calcium nitrate (CN-9) applied at a low rate (e.g., 3 to 5 gallons per acre) is beneficial. Such applications stimulate translocation of hormones from roots to shoots, where they impede leaf senescence and help activate healthy leaf tissues.
Next, apply potassium to enhance sugar movement towards the fruit. Fertigate with a low rate of a highly soluble potassium fertilizer, such as potassium carbonate (0-0-30) or potassium thiosulfate (KTS). Common application rates following periods of high temperatures are 5 gallons per acre or less. Foliar applications of potassium fertilizers can also be beneficial but are usually less cost effective.
Figure 10. Renewed shoot growth can compensate to some extent for lost ripening capacity due to ripening period stress damage to leaves.
Photo source: Progressive Viticulture©.
Extensive leaf damage and defoliation due to high temperatures require extraordinary actions. In these situations, to replace lost leaves and ripening capacity, apply calcium nitrate at rates sufficient to stimulate renewed shoot growth (Figure 10, above). Appropriate application rates to promote shoot growth are greater than those to activate leaf tissues after a heat spell (e.g., 6 to 12 gallons per acre). Avoid excessively high rates that can lessen stomata sensitivity and inhibit water uptake, thereby predisposing vines to severe water stress.
In spite of diligent management efforts, repeated heat spells can cause leaves to senesce, thereby negatively impacting ripening. This is evident late in the growing season when canopies appear worn and slightly droopy (Figure 11, below). To revitalize senescing leaf tissues and advance ripening, fertigate with a moderate rate of a fertilizer containing a small percentage of ammonium-nitrogen and a large percentage of phosphorus, such as 3-10-10 or 3-12-14-4 (S). Such applications enhance production of antiaging hormones in roots and their movement to shoots.
Figure 11. Senescent grapevine leaves after repeated stressful events during the ripening period.
Photo source: Progressive Viticulture©.
In Summary
During the ripening period, maintaining functional leaves and an undamaged, quality crop are the primary vineyard management goals. Stresses are the greatest threat to these goals. These include crop stress, severe water stress, heat stress, heat with radiation stress, potassium deficiency and some combination of them. Delayed harvest, diminished fruit quality and yields, and decreased return on investment are among the ultimate effects of ripening period stresses.
Given the risks and factors involved, it is imperative for winegrowers to design, develop and manage vineyards to minimize ripening period stresses. Just as important is diligent vineyard monitoring during the ripening period for the onset of stresses and quick responses to them.
The capacity for grapevine recovery depends on ripening period stress intensity and duration. While the symptoms of ripening period stresses occur mainly in canopies, they elicit whole vine responses. As such, below ground, as well as above ground, actions contribute to vine recovery from ripening period stresses. These include supplying water at appropriate volumes and applying specific fertilizer formulations at appropriate rates to elicit stimulatory effects.
This article is based on a presentation made on January 8, 2018 at the Mid Valley Agricultural Services Annual Grape Grower Meeting, and the information was updated for today’s blog post.
Further Reading
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Bernardo, S, Dinis, L-T, Machado, N, Moutinho-Pereira, J. Grapevine abiotic stress assessment and search for sustainable adaption strategies for Mediterranean-like climates. A review. Agron. Sustain. Dev. 38: 66. 2018.
Bertamini, M, Zulini, L, Muthuchelian, K, Nedunchezhian, N. Effect of water deficit on photosynthetic and other physiological responses in grapevine (Vitis vinifera L. cv. Riesling) plants. Photosynthetica. 44, 151-154. 2006.
Bolton and Grant. Ripening period potassium deficiency. Lodi Winegrape Commission Coffee Shop Blog. September 17, 2018.
Conradie, WJ. Seasonal uptake of nutrients by Chenin blanc in sand culture: II. Phosphorus, potassium, calcium, and magnesium. S. Afr. J. Enol. Vitic. 2, 7-13. 1981. Copp, C, Levin, A. Managing grapevines during a heat spike. Oregon State University Extension Service. May 2022.
Edwards, EJ, Smithson, L, Graham, DC, Clingeleffer, PR. Grapevine canopy response to high-temperature event during deficit irrigation. Aust. J. Grape Wine Res. 17, 153-161. 2011.
Gambetta, GA, Herrera, JC, Dayer, S, Feng, Q, Hochberg, U, Castellarin, SD. Grapevine drought stress physiology: towards an integrative definition of drought tolerance. Journal of Experimental Botany. 71, 4658-4676. 2020.
Garde-Cerdan, T, Porto, J, Lopez, R, Santamaria, P. Effect of foliar applications of proline, phenylalanine, urea, and commercial nitrogen fertilizers on Stilbene concentrations in Tempranillo musts and wines. Am. J. Enol. Vitic. 66, 542-547. 2015.
Goldhammer, DA, Snyder, RL. Irrigation scheduling: a guide for efficient on-farm water management. University of California, Division of Agriculture and Natural Resources Publication. 1989.
Grant, S. Five-step irrigation schedule: promoting fruit quality and vine health. Practical Winery and Vineyard. 21 (1): 46-52 and 75. May/June 2000.
Grant, S. Balanced soil fertility management in wine grape vineyards. Practical Winery and Vineyard. 24 (1): 7-24. May/June 2002.
Grant, S. Fertilizer efficiency for wine grape vineyards. Practical Winery and Vineyard. 28 (1): 35-41. March/April 2006.
Grant, S. Regulated deficit irrigation, parts I and II. Lodi Winegrape Commission Coffee Shop Blog. July 18 and August 04, 2014.
Grant, S. Shoot thinning for multiple benefits. Lodi Winegrape Commission Coffee Shop Blog. May 02, 2017.
Grant, S. Managing vineyard mineral nutrient efficiency beyond the 4 R’s. Wines and Vines. pp. 77-78. April 2016.
Grant, S. Post harvest vineyard management. Lodi Winegrape Commission Coffee Shop Blog. October 15, 2019.
Grant, S. Prebloom vineyard management. Lodi Winegrape Commission Coffee Shop Blog. March 02, 2020.
Grant, S. Remote sensing and aerial images in vineyard management. Lodi Winegrape Commission Coffee Shop Blog. March 30, 2020.
Grant, S. Post bloom vineyard management. Lodi Winegrape Commission Coffee Shop Blog. May 11, 2020.
Grant, S. Ripening period vineyard management. Lodi Winegrape Commission Coffee Shop Blog. July 07, 2020.
Grant, S. Systematic vineyard monitoring for effective vineyard management. Lodi Winegrape Commission Coffee Shop Blog. May 3, 2021.
Grant, S. The seasonality of vineyard mineral nutrient management. Lodi Winegrape Commission Coffee Shop Blog. May 24, 2021.
Grant, S. The ultimate goal of vineyard soil management: optimized root zone function. Lodi Winegrape Commission Coffee Shop Blog. December 20, 2021.
Grant, S. There is more to vineyard floors. Lodi Winegrape Commission Coffee Shop Blog. March 28, 2022.
Grant, S. Canopy management revisited – part two. Lodi Winegrape Commission Coffee Shop Blog. January 1, 2024.
Grant, S. Towards an Understanding of Vineyard Potassium. Lodi Winegrape Commission Coffee Shop Blog. August 26, 2024.
Grant, S. In Pursuit of Yield: grapevine capacity, balance, and crop load – Part Two. Lodi Winegrape Commission Coffee Shop Blog. May 28, 2024.
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Keller, M. Deficit irrigation and mineral nutrition. Proceedings of the Soil Environment and Mineral Nutrition Symposium. Christensen, LP, Smart, DR (ed.). Amer. Soc. Enol. Vitic. pp. 267-283. 2005.
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Marschner, H. Mineral nutrition of higher plants. 2nd Ed. Academic Press, London. 1995.
Paulsen, GM. High temperature responses in plants. pp. 365-389. In Boote, KJ, Bennett, JM, Sinclair, TR, Paulsen, GM (ed.). Physiology of determination of crop yield. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America; Madison, WI. 1994.
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