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Soil water tension sensor (left) and soil volumetric water content sensor (right). depleted or the available water content. Soil volumetric water content, on the other hand, offers the percentage of water volume versus the total soil volume and tells you the amount of water stored in the soil and how much you need to irrigate to maintain the desired water content. Growers should irrigate the vineyard when 30% to 50% of allowable water is depleted throughout the root zone (Table 1, see page 35). The soil volumetric water content is also well correlated to plant water stress measured by midday leaf water potential, and growers can potentially use soil volumetric water content to assess grapevine water stress (Figure 1). Overall, growers can use both soil-water tension and soil volumetric water content mentioned above to monitor the soil moisture either indirectly or directly and schedule irrigation. Figure 1: Soil volumetric water content is correlated with midday leaf water potential. Data were collected from various irrigation treatments: 0.2 ETc, 0.6 ETc, 1.0 ETc and 1.4 ETc. Figure is elaborated in Williams and Trout 2005. Continued from Page 35 sensor). Soil-water tension tells you how hard it is for the grapevine roots to pull water from the soil particles, and Sept. 16-17, 2021 the reading is typically negative with units of centibar. The more negative the reading, the harder it is for the grapevine roots to absorb water. Growers can set up the pre-determined value of soil-water tension (e.g., -30 centibar) at a certain soil depth {e.g., two feet). Once the soil-water tension reaches the value, the irrigation should start. The pre-determined soil-water tension is usually between -30 and -40 centibar in SJV, varying across different soil types. However, soil-water tension does not tell you how much water has been Grapevine Canopy Grapevine canopy growth (e.g., budbreak and shoot elongation) depends on the availability of water and nutrients in the soil. Lack of soil moisture during the dormant season increases the risk of freeze damage and hinders the start of budbreak and early season shoot elongation, causing DSG. DSG usually occurs when the grapevine suffers water stress at the beginning of the growing season. If the drought condition persists, shoot elongation might be hampered, and the shoot tip might die off due to lack of water. Under severe drought stress, inflorescences will die off and cause significant yield loss. To prevent early season vine water stress, soil moisture is the key measurement to decide when to irrigate as was previously mentioned. However, canopy appearance and visual assessment can help to confirm the success of an irrigation program. First, upward-growing shoot tips and tendrils are the most obvious signs of a healthy canopy. Second, a pressure chamber to measure midday leaf water potential might offer a powerful tool to validate the irrigation program (Table 2, see page 37). Grapevine water demand is directly SEE PAGE 36 18-19 FOR MORE Progressive INFORMATION Crop Consultant July / August 2021
elated to the amount of sunlight captured by the canopy, and a larger canopy needs more water than a smaller canopy due to receiving more light. Therefore, as the shoot grows and the canopy expands, grapevines generally use more water. Most grapevine irrigation recommendations are based on the canopy size and climatic condition. Canopy management (e.g., leafing, shoot tucking/thinning and hedging) has the potential to influence the canopy size and amount of light received, and ultimately water use. Midday Leaf Water Potential (Bar)* > -10 -10 to -12 -12 to -14 < -14 Grapevine Water Stress No stress Mild stress Medium stress Severe stress *Midday leaf water potential is measured using sun-exposed newly mature leaf during the solar noon from 12:00 p.m. to 3:00 p.m. in SJV. Table 2: Midday leaf water potential readings and related grapevine water stress. Grapevines at different growth stages have different sensitivity and tolerance to water stress. From budbreak to bloom, the grapevine is very sensitive to water stress, and even a mild stress will hinder growth and cause irreversible yield loss. Generally, water stored in the soil profile after winter precipitation is enough to support vine growth. However, in years of drought such as this year, soil moisture may be inadequate to support growth, and irrigation is needed to replenish soil moisture. From bloom to fruit set, the grapevine is also sensitive to water stress, and severe stress causes poor set and yield loss. Fruit set to veraison is a good time to apply some stress if growers are looking to reduce berry size (e.g., smaller berry) and improve berry quality (e.g., color). However, the benefit of improved berry quality might come with the sacrifice of yield. Typically, in SJV, mild stress is recommended at this period to balance quality and yield. From veraison to harvest may be the best time to apply some stress to advance berry ripening and reduce disease pressure (e.g., bunch rot). However, growers need to avoid severe stress which results in excessive defoliation, since a healthy canopy is required for photosynthesis. Postharvest, it is generally recommended to replenish the soil profile, since the photosynthetic active canopy still produces the carbon to refill the reserve of trunk and roots, and the reserve will be used to support the vine growth of the following season. Postharvest irrigation in abundant quantities can also help to leach the salts and alleviate the concern of salinity. Weather Condition As was previously mentioned, grapevine water use depends on two main factors: canopy size and weather. Most growers know crop evapotranspiration (ET) and can calculate grapevine water demand over a given period through the calculation: grapevine evapotranspiration (ETc) = reference evapotranspiration (ETo) × crop coefficient (Kc). Growers can simply use ETc to irrigate the grapevines weekly by using gallons/ vine/week or hours/vine/week after factoring in the application rate. Kc is related to canopy size (Figure 2, see page 38) and ETo is related to weather Continued on Page 38 Upward growing shoot tip and tendril (left), before pressure bomb, bagging leaf before cutting the petiole (top right), during pressure bomb, using magnified glass to observe popping water from the petiole (bottom right). July / August 2021 www.progressivecrop.com 37
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