The San Joaquin Valley (SJV) is already considered a hot growing region for winegrapes, and heat stress is expected to become more frequent and severe in this region over the next several decades (Livneh et al. 2015). Heat impacts many aspects of vine physiology, and the goal of this article is to provide consultants and growers with a broad overview of these impacts and the consequences for yield and berry quality.
Vegetative Physiology
Heat strongly impacts grapevine carbon and water fluxes through effects on photosynthesis, respiration and transpiration. Photosynthesis in grapevines is generally optimal from 77 to 95 degrees F and is strongly reduced at temperatures above 105 degrees F (Greer 2018; Greer and Weston 2010). This reflects both direct effects from heat and indirect effects from water stress. Temperatures above 85 to 95 degrees F can directly impair photosynthesis by co-opting the leaf metabolism to generate toxins that damage the membranes where these reactions take place (Carvalho et al. 2015). Heat also increases evapotranspiration and vine water stress. Warmer air molecules spread apart, creating more room to hold water vapor and increasing the driving force for water to evaporate from the soil or vine (measured as a higher vapor pressure deficit, or VPD). Excessive dehydration damages vine tissues, so a higher VPD forces grapevines to restrict transpiration by closing the stomata, which in turn limits the CO2 entering the leaf and available for photosynthesis (Chaves et al. 2016). This process not only reduces the carbon available for growth and ripening but can also increase vine water stress and irrigation demand. Heat also accelerates respiration reactions, causing respiration rates to approximately double with every 18 degrees F increase in temperature (Palliotti et al. 2005). This combination of increased respiration and decreased photosynthesis can limit the carbon available for fruit set and ripening under hot conditions.
Vegetative growth can have complex responses to heat. Up to a point, warmer temperatures can increase vine transpiration and the transport of hormones (i.e., cytokinins) from the roots to the shoots, promoting lateral growth and increasing canopy size (Field et al. 2020). However, vegetative growth is one of the most sensitive physiological processes to water stress, so any positive effects on growth will rapidly reverse if heat is sufficient to produce water stress (i.e., pre-dawn water potentials < -0.3 MPa) (Deloire et al. 2020).
Fruit Physiology
In general, warming has accelerated the rate of fruit development. Over the past 30 years, harvest has shifted 24 days earlier in Germany, mostly due to earlier bud break (10 days earlier) and faster sugar accumulation (i.e., the period from veraison to harvest becoming nine days shorter) (Koch and Oehl 2018). However, extreme heat can interfere with fruit development. The effects depend on temperature, duration and timing. At bloom, temperatures >95 degrees F can interfere with flower fertilization, preventing the pollen from forming the tunnels that allow it to reach the ovary, inducing shatter and berry thinning (Kliewer 1977). Heat generally has less impact during fruit set (bloom to veraison) (Greer and Weedon 2013; Greer and Weston 2010). Extreme heat (>100 degrees F) can limit cell division in the berries, but most impacts from heat during this period are indirect effects of water stress on cell expansion. At this stage, the berries receive most water (~80%) through the water transport tissue (xylem), and the rate and direction of xylem water flow is highly dependent on the water potential gradient between the fruit and canopy (Keller et al. 2015). Vegetative water stress at this stage (i.e., pre-dawn water potentials < -0.5 MPa) can decrease water flow to the berries, berry cell expansion and growth (Deloire et al. 2020).
At veraison, berry water influx switches to the sugar transport tissue (phloem), which is less sensitive to canopy water potentials, and direct effects of temperature become more important. Heat especially impacts quality at this stage, and heat effects can be quite severe, since dark (red) berries can be ~30 degrees F warmer than the air (Venios et al. 2020). Berry temperatures will depend on multiple vineyard design and management factors, including factors affecting radiation exposure from the sky (e.g., trellising, shoot and leaf thinning decisions, shade netting, row orientation) and ground (e.g., cover cropping, fruit zone height) and transpirational cooling (e.g., misting, irrigation) (Keller 2010; Keller and Chang 2023). Heat can have complex effects on sugar accumulation. Warmer temperatures generally increase the rate of sugar accumulation through indirect effects of water stress on the phloem (Salmon et al. 2019). Leaves load sugar into the phloem to create a concentration gradient that pulls in water from the xylem, and this water influx pushes the sugar sap toward the fruit. When the canopy is water-stressed, and water potentials in the xylem are more negative, the phloem needs a higher sugar concentration to pull water away from the xylem, which delivers a more concentrated sap to the berries.
However, severe heat stress can also stall sugar accumulation. In Australia, a four-day heatwave at 105 degrees F downregulated photosynthesis and stopped sugar transport for two weeks, which could reflect persistent damage from heat or water stress (Greer and Weston 2010). Heat also directly impacts berry acidity and pigment (anthocyanin) levels. Heat accelerates berry respiration and the breakdown of malic acid, so that malate accumulation is optimal between 68 to 77 degrees F and significantly degraded above 105 degrees F (Coombe and McCarthy 2000; Venios et al. 2020). Heat also impairs anthocyanin synthesis and increases degradation above 95 degrees F (Cataldo et al. 2023). Heat and light can also interact to produce sunburn, which degrades the waxes in the berry cuticle, leads to severe berry dehydration and alters berry phenolics (Gambetta et al. 2021).
Heat has wide-ranging impacts on vegetative and fruit physiology. Many heat effects are strongly dependent on water stress or light exposure, making it difficult to predict changes in yield or quality metrics as a function of air temperature, though many processes begin to experience problems above 95 degrees F. We also lack important information on the interactions between duration and intensity in determining heat damage.
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