Crop yield response to water - Cra

North Africa and near East ranges from September to December, depending on the beginningof rainfall. Sowing in India can be in summer, winter and kharif period. Extending the sowingtime of sugar beet in winter and autumn has been an important breeding goal in the lastdecades, in order to reduce water requirement by avoiding parts of the summer, whilelengthening the crop cycle and thus increasing productivity (Rinaldi and Vonella, 2005).Minimum soil temperature for germination can be as low as 4 o C. Once established, theseedling enters a period of leaf initiation with virtually no enlargement of the upper portionof tap root for storage. By six weeks the plant has 8-10 leaves but the storage portion of theroot is just enlarging (see Table 1 and Figure 3). From this stage onwards, growth of leavesand storage portion of root occur simultaneously with the root making up an increasingproportion of total plant dry weight.Sugar beets develop an effective root zone of 1.20 m or deeper. In deep soils the crop candevelop a deep tap root system. As is the case for other crops, when the soil profile is relativelywet, normally most of the water is extracted from the upper half of the soil of the root zone,although water extraction down to 3 m has been reported for deficit-irrigated sugar beetin very deep, open soils. Since this crop is cultivated for sugar production, the season lengthinfluences the root sucrose concentration. The length of time when the crop is in the field islimited by the biennial cycle duration and by the water, light and temperature constraints; ingeneral, a prolonged crop cycle during the summer can modify the source-sink relationshipbetween leaves and root and, consequently, reduce the storage root sucrose content that canflow to the leaves.The evolution of canopy cover (CC) is mainly temperature dependent, and often maximum CC(CC x ) is reached after about 8 to 10 weeks. The duration in days is merely indicative becausetemperatures and water availability can influence the length of each phase. How long theleaves (hence the canopy) remain green depends on temperature and water stress. Springplanted crops in western and Eastern Europe and United States are harvested betweenSeptember and November, while autumn planted crops are harvested at the end of springor early summer (May to July in the Northern Hemisphere). Lengths of main crop stages arereported in Table 1 with reference to different cropping regions and sowing dates.When grown at several constant temperatures (range of 7 to 31 o C, spaced 4 to 7 o C apart),fastest leaf growth was obtained at 24 o C, and fastest tap/storage-root growth was observeda considerably lower temperature, at less than 17 o C (Terry, 1968; Milford and Riley, 1980).Leaf growth is a linear function of temperature above 3 o C, and in the field leaf growth isclose to a linear function of growing degree days calculated with a base temperature of 1 o C(Milford et al., 1985).Modern sugar beet cultivars are almost insensitive to day-length, and bolting (appearanceof flowering stalk) and flowering do not take place under normal cropping conditions in thefirst season. Bolting reduces sucrose yield and is induced by a combination of low wintertemperatures (0-10 o C) and long days. Breeders have selected cultivars with very strong flowerinduction requirements that can now be used for early sowing. The plant only flowers in thesecond year when seed production is the goal.204crop yield response to water