Crop yield response to water - Cra

Water use & ProductivitySugar beets consume 500-800 mm of water during the growing season. The seasonal waterrequirements depend on climate and weather conditions, planting date and density, irrigation,and crop management (Allen et al., 1998). Sugar beets are most sensitive to moisture shortagesin the early growing stages but their peak water consumption comes late in the season whenmaximum canopy cover is reached. Soil moisture levels should be maintained above one-halfto one‐third of total available water (TAW) to avoid stomatal closure. Over the peak 30‐daygrowing period, maximum water use ranges from 6 to 9 mm/day, depending on climate.Commonly there is a positive relationship between water use, root production, and sugarcontent. Water stress reduces yield, while overirrigation near harvest reduces root sucroseconcentration and increases processing costs, thus reducing profit (Ehlig and LeMert, 1979).As for water productivity, spring sown beet commonly studied in Northern Europe andAmerica, have reported consumptive water productivity for dry matter production (WP B/et )between 2.1 and 6.8 kg/m 3 (Dunham, 1993). This wide range of values reflect the diversityof climates in terms of evaporative demand and the fact that many WP B/et values were notnormalized for this parameter. It is pertinent to indicate here that the dry matter of concernis mainly composed of storage root dry matter, with shoot dry matter making up only a smallportion (e.g. 20 percent). This is in contrast to all non-root crops, for which dry matter ofconcern is focused on the above–ground portion, neglecting the root portion because it isextremely difficult to extract quantitatively from the soil (Rinaldi et al., 2006).Response to StressesSugar beet is sensitive to water deficits at the time of crop emergence and for a period of aboutone month after emergence. Frequent, light irrigations are preferred during this period, andirrigation may also be needed to reduce crust formation on the soil and to reduce salinity ofthe top soil. Early over-watering may retard leaf development and can encourage floweringduring the first year (bolting). Water deficits in the middle of the growing period (vegetativeand yield formation periods) tend to affect sugar yields more strongly than when occurringduring later periods. Unrestricted water supply near harvest time tends to reduce root sugarconcentration although it increases root fresh weight, with the final effect on sucrose yieldbeing small. Moderate water deficits, together with mild nitrogen deficiency towards the endof the growing period, reduce root growth but increase sucrose concentration. In general, topgrowth toward the end of the growing period tends to be negatively correlated with sugarproduction, apparently the result of competition for assimilates between vegetative growthand root storage. Normally, the irrigation season ends between 2 to 4 weeks before harvest,depending on the water‐holding capacity of the soil, to increase sucrose concentration in thebeets. The soil should, however, not be too dry to hamper lifting the beets at harvest. Thus,except during emergence and early growth, it appears that the crop is not very sensitive tomoderate water deficits. When plants are under water stress, leaves become dark green incolour and if the stress is severe, the leaves fail to recover from midday wilting in the evening(Hanks et al., 1981).Adequate soil nitrogen availability is required to ensure a good vegetative growth in spring.Nitrogen is often given in split applications, a small amount at planting and the rest after206crop yield response to water