Yield and Water
This module has been prepared in light of providing growers and industry with immediate information that can be used to aid irrigation management decisions. The complementing powerpoint presentation has been tailored to suit specific winegrape growing regions by providing regional data on yield and irrigation water for Shiraz, Cabernet Sauvignon, Merlot and Chardonnay.
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Components of grapevine yield development
In order to understand how to manage wine grapes under minimal water inputs, it is important to have an understanding of the annual growth cycle of a grapevine. A grapevine’s annual growth cycle includes a vegetative and fruiting (reproductive) cycle (Figure 1.). These main growth events occur across five stages of the grapevine’s development being budburst to flowering, flowering to fruit-set, fruit-set to veraison, veraison to harvest and harvest to leaf-fall. Vines are dormant from May through early August. The cycle of berry/yield development for grapevines tends to extend over two years with bud initiation occurring in Year 1 to harvest in Year 2 (Krstic et al., 2005). The most important stage to determining yield potential is inflorescence initiation and differentiation which begins about 18 months prior to harvest. Floral bud development and budburst are key events that influence yield followed by flowering and fruit-set where a proportion of flowers will successfully set and become berries.
Figure 1. Schemmatic representation of canopy growth and grape berry development during a grapevine’s growing season (adapted from Coombe, 1992; Coombe & McCarthy, 2000).
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Water relations in grapevines
Various studies have used different irrigation regimes to manipulate canopy vigour and have noted that yield and quality at harvest are dependent on when the irrigation is applied in relation to the stage of berry growth. The amount of water required at different stages of grapevine growth will depend on the variety, rootstock:scion interaction, climatic (rainfall and evaporation), soil type/depth and crop load. Figure 2 illustrates the approximate annual percentage of water required by vines at each stage of growth during the season.
Figure 2. Grapevine growth stages and the approximate water requirement at each stage as a proportion of the annual requirement (Adapted from NSW Agriculture (2004).
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Budburst to fruit set
Stages 1 and 2 include budburst, flowering and fruit-set. When combined, these stages utilize approximately 14% of the annual water requirement. They are critical stages for establishing canopy size and potential yield. Irrigations that are applied too soon will be wasted and not utilized by the vines. During this period, rapid vegetative growth ensures the development of an adequate leaf canopy to sustain production. Once the leaf canopy is fully developed, towards early to mid-November, vines flower. Water use at this time is still moderate and water stress should be avoided to ensure optimum flower retention and fruit set. Bud initiation and differentiation for the subsequent season also takes place at around the same time. High water stress at flowering can result in poor fruit set (hen and chicken) or aborted fruit (shot berries) – both of which lead to yield reductions.
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Fruit set to veraison
Stage 3 extends from fruit-set to veraison and uses approximately 35% of the annual water requirements. This is the time that deficit irrigation strategies, such as RDI, can be used to control berry size. Severe water stress can affect bud fruitfulness. During the berry expansion phase, between fruit-set and veraison, water deficits tend to reduce berry size often resulting in a higher sugar concentration per berry. In this period of berry growth the vine is probably least susceptible to moderate water deficits. Several studies on winegrapes have shown that water deficits during this period do not impact on the accumulation of soluble solids and although berry size is reduced (and remains less irrespective of the amount of water applied after veraison) the total sugar content at harvest is not affected.
Veraison to harvest
Stage 4 is from veraison to harvest with vines requiring about 36% of the annual water requirements. This is the period when sugar is accumulating in the berries. It is important to maintain healthy leaf function to maximize berry sugar accumulation. Hence, severe deficits should be avoided to prevent plant defoliation. Once berries have reached maturity in mid to late February water deficits are less harmful to production and after canes have been cut, water deficits may accelerate drying of the fruit.
Post-harvest
Stage 5 is harvest to leaf-fall and uses at least 14% of annual water allocated at this growth stage. It is still important to maintain some leaf function during this period, especially from April-June, to ensure that the vine is able to build up sufficient reserves for the subsequent season before going into dormancy. Water stress during this stage may lead to restricted growth symptoms in the spring, particularly in young vines. In contrast, irrigation to match crop water requirement can lead to re-growth of shoots and this can compete with storage.
Author
Yasmin Chalmers - DPI Victoria, Mildura, VIC
