Grape is believed to originate from the Caspian and Caucasian regions. Most cultivated vines belong to the European type (Vitis vinefera), the American bunch type (V. labrusca and its derivatives) or Muscadine type (V. rotundifolia). The total production of grapes is about 61.95 million tons from 7.3 million ha. (FAOSTAT, 2001)

Grape is grown between about 50°N and S, with suitable areas being small at these limits. The crop needs a long, warm to hot, dry summer and a cool winter. The subtropics with winter rain are most suited. Rain or cold and cloudy weather during flowering may adversely affect fruit setting whereas rain during ripening may lead to fruit rot. Where raisins are produced by sun-drying between the vine rows, at least one warm, sunny month without rain following harvest is essential.

In climates with a cool winter, the grape can survive temperatures down to -18°C, but once new growth begins, minor frost will kill the fruiting shoots. Rapid and succulent growth of shoots starts when mean daily temperatures reach l0°C. During flowering the rate of shoot growth declines and stops when the grapes are mature. The time from flowering to maturity can be expressed as the sum of mean daily temperature above 10°C or S(T - l0°C), which is about 900 degree-days for early varieties and 2000 for late varieties. Under cool to moderate warm weather, fruits ripen slowly and produce dry table wines of good quality. In warmer climates, the heat before and during ripening favours a high sugar content, which makes fruits better suited for port and sherry production. During and after harvest no new shoot growth should take place but leaves should be retained. In the autumn shoots, then also called canes, become woody and lose their leaves.

In the tropics (with less than 1 percent of the world production), the vine is an evergreen and can produce fresh fruits throughout the year. In general, two harvests of relatively poor quality and low yields are obtained annually with harvest dates controlled by adjusting the time of pruning.

Grapes are adapted to a wide range of soils, except when poorly drained or when salt content is high. In general, light soils are preferred. High production can be obtained under rainfed conditions, but without summer rain a deep soil with a high water holding capacity is required. Under irrigation, grape can be grown success-fully on shallow soils of 0.6 m depth or less.

On deep, fertile soil, the largest vines and high yields are produced. On soils of low fertility or limited depth, yields are usually lower, but fruit quality can be better. Fertilizer requirements are 100 to 160 kg/ha N, 40 to 60 kg/ha P and 160 to 230 kg/ha K. The greatest amount of nitrogen is needed during early spring growth and during the flowering period. During ripening, the nitrogen level must be low to prevent continuous vegetative growth.

Grape vines are moderately sensitive to soil salinity and yield decrease is 0% at ECe 1.5 mmhos/cm, 10% at 2.5, 25% at 4.1, 50% at 6.7 and 100% at 12 mmhos/cm.

Most grape varieties are propagated by cuttings, grown in a nursery for one year to produce roots. Where root louse is a problem, grape is grafted on resistant root-stocks. An important (and expensive) operation is the training, pruning and staking of the vines and thinning of clusters. In the tropics the time of pruning determines the time of fruiting. The plant spacing is also influenced by the.pruning and training practices and can vary between 1.5 x 3.5 and 4.5 x 5.5 m.

The graph below depicts the crop stages of grape, and the table summarises the main crop coefficients used for water management.

The crop coefficient (kc) will vary with cultural practices. The kc value, relating the maximum water requirements (ETm) to the reference evapotranspiration (ETo), for clean cultivated conditions, infrequent irrigation and a dry soil surface most of the time is:

Total seasonal requirements vary between 500 and 1200 mm, depending mainly on climate and length of growing period.

Water Supply And Crop Yield

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period are shown in the figure below.

Grape is a perennial crop and can adjust to a certain extent to limited water supply by developing a deep root system. When soil water even over great soil depth becomes limited and overnight recovery from wilting does not occur, growth will diminish and eventually stop. Subsequently leaf and shoot colour change to a dark greyish-green, the shoot tips become dry, the leaves curl, the tendrils abscise and eventually the leaves die and fall.

In the subtropics and temperate climates, flowerbud formation generally occurs during the late summer or autumn and the buds open during the next season. A slight deficiency of water, together with high sunshine and temperatures, is considered to be most favourable to flower bud formation. A dry summer and a relatively low yield appears to be more advantageous to flower bud formation than a wet summer and a heavy yield.

For good fruit production in the same year and the following years, good vegetative growth during the first part of the growing period (vegetative period, 1) is important.Water deficit should not occur during this period of rapid lateral shoot growth. The soil water content should preferably be at field capacity at the end of the winter, by winter rain or by irrigation, to ensure adequate water supply during the first months of the growing period. Especially shoot elongation is very sensitive to water deficits. Adequately irrigated vines have significantly more prunings than those grown under water deficit conditions. If water stress occurs abruptly, the growth is checked and wilting and dieback occur; if water stress develops gradually the vine growth is adjusted by lessened shoot growth, smaller production and earlier ripening. However, vegetative growth should be low during fruit formation and should stop toward harvest to ensure good ripening of the fruit and maturing of the wood.

Prior and during flowering (2), adequate water supply is necessary for flower development. Water deficits at this time retard flower development while severe water deficit reduces fruit set. Also the nutrition requirements of the grapevine are high during this period and the subsequent fruit enlargement period. Leaching of nutrients must be avoided in this period.

Yield formation (fruit enlargement, 3) depends on a steady, continuous water supply, but in this period the crop is less sensitive to water deficits than during the period of shoot growth (1). Water deficits during fruit enlargement reduce fruit size. Later irrigation does not result in undersized fruits becoming normal size. Water deficits prior to or just after veraison (start of ripening, fruits soften and change colour) affect fruit size more than deficits just before harvest.

Severe water deficit causes shrivelling of the fruits at all stages of yield formation (3) and ripening (4) and is first observed in the immature fruits on any cluster. The shrivelling usually disappears after rewatering. Complete desiccation is confined to the smaller fruits (less than about 4 mm in diameter). When the crop is subjected to severe water deficits after v6raison, maturity is delayed while the fruits may not even reach full maturity. A slight water deficit during the ripening period (4) may hasten maturity, while juice concentration is increased.

Water deficits throughout the growing season result in darker wine but may not affect the quality of the wine. Severe water deficit during yield formation (3) and ripening (4) results in the fruit having a dull colour. It also leads to sunburn but reduces the incidence of fruit rot. Severe deficits just after v6raison reduce the content of total soluble solids.

After the fruit is mature and especially after harvest, the vines become adjusted to a limited water supply. Normally no further growth occurs, but leaves are retained and canes ripen even though soil water content is low. In hot, dry regions, water deficit after harvest will cause the leaves to fall; when weather becomes cool in autumn, new leaves can be formed without becoming mature. This will lead to poor production in the next year. Water supply after harvest must therefore be sufficient to maintain the healthy foliage and to prevent premature leaffall. However, too much water after harvest causes new shoot growth, which has the same detrimental effects as new leaf growth. The relation between relative yield decrease and relative evapotranspiration deficit is given for conditions when soil water stress occurs mainly in the second part of the total growing period, and water supply during early vegetative growth (1) and the flowering period (2) is adequate (Fig. 16). To maximize total production if water supply is limited the cultivated area may be extended and crop water requirements partially met, rather than meeting full crop water requirements on a limited area.

When root penetration is not obstructed, mature grapevines are deep rooted up to depths of 2 or 3 m, or more. In deep, coarse sand or gravelly soils, roots may be up to 4 to 8 m deep.The bulk of the roots are usually in the upper soil layer of 0.5 to 1.5 rn. Normally 100 percent of the water is extracted from the first 1 to 2 m soil depth (D = 1-2 m). During vegetative growth (1), flowering (2) and the early part of yield formation (early 3) maximum evapotranspiration will be affected at a soil water depletion (p) of about 0.35 to 0.45, under conditions when ETm is 5 to 6 mm/day. Later in the growing period the soil water can be depleted to a higher level, while toward and after harvest time a high soil water depletion is required.

When winter rainfall is insufficient to fill the full root zone to field capacity, irrigation should be applied before vegetative growth starts. Until the beginning of the veraison water must be applied when 35 to 45 percent of the total available soil water is depleted. Whether irrigation is necessary after veraison depends on the total available water over the root depth in relation to ETm. In shallow and light soils, irrigation will be necessary until harvest, but be applied at higher soil water depletion levels (soil water potential between 1 and 5 bars).

In deep, fine textured soils irrigation should be discontinued in time to achieve the desired soil water depletion level toward harvest time. In warm, dry climates or when the grapes are harvested early, a light irrigation may, however, be required to prevent the soil from becoming too dry (water potential not exceeding 5 to 10 bars).

When sprinkler irrigation is practised, after veraison irrigation should not take place during humid periods in order to assure rapid drying of the leaves (8 to 12 hours), and to reduce foliar burn and the hazard of fruit rot.

Furrow irrigation with 2 or 3 furrows between the rows is mostly used. Sprinkler irrigation becomes more common since it can also be used for spring frost protection where needed. Sprinkler is less advantageous when irrigation is also required during the ripening period because of the likely increase in bunch rot. In new vineyards and especially where irrigation water is scarce, drip irrigation is increasingly being introduced.

Similarly to other perennial crops, yield per vine varies considerably from year to year and from plant to plant. The maximum yield level depends on the variety and growing environment. Good commercial yields in the subtropics are in the range of 15 to 20 kg grapes per vine or 15 to 30 (or more) tons/ha (80 to 85 percent moisture). Yields in the tropics are in the range of 5 to 10 ton/ha. The water utilization efficiency for harvested yield (Ey) for fresh fruits containing about 80 percent moisture is 2 to 4 kg/m3 when grape is grown in the subtropics.