Crop yield response to water - Cra

and there is frequent wetting of exposed soil surface by rain or irrigation. The latter increasesthe E part of the ET. WP B/et was found in some studies to be largely independent of nitrogenfertilization and of salinity, but within limits. If nitrogen or salinity is limiting, WP B/et wouldbe reduced (Heuvelink and Dorais, 2005). AquaCrop then requires a local calibration withbiomass produced by a treatment of optimal nitrogen (or a non-saline treatment), in order tosimulate such cases realistically.Response to stressesSimilar to cotton (see Cotton chapter), the vegetative/reproductive ratio of tomato alsodepends on plant-water status, but to a lesser degree. As already mentioned, high waterstatus stimulates vegetative growth and commonly leads to the dropping of flowers andnewly set fruit early in the season. On the other hand, mild to moderate water stress earlyin the season, if lasting for many days, can result in a markedly smaller canopy, and hence,less biomass production resulting from reduced radiation capture. Photosynthesis per unitleaf area is moderately resistant to water stress. Thus, the crop is fairly resistant to moderatedrought once good canopy cover is achieved. Over irrigation causes excessive leaf growth, andplants high in vegetative vigour tend to produce low quality fruit because of reduced contentof soluble solids. Moreover, excess water near harvest can cause nitrate accumulation in thefruit. For some cultivars, wide fluctuations in soil moisture levels during fruit maturation cancause fruit cracking, blotchy ripening, blossom-end rot and varied size and shape.The crop is sensitive to frost. Low temperatures, if persisting for more than a few days, reducesleaf and truss initiation rates, and the plant produces thicker leaves, so they intercept lesslight; fruit set is reduced as a result of poor pollination. Dropping of flowers and young fruitunder cold temperatures has already been noted. Exposure to high temperatures causes areduction in the number of pollen grains and impairs their viability and germinability, markedlyaffecting fruit set. High day and night temperatures cause hastening in flowering and markedreduction in number of trusses, flowers per truss and an increase of blossom drop and fruitabortion. High humidity, combined with temperatures above about 27 °C, also affects pollengermination, resulting in reduced yield. Tomato, as with many other crops, can compensate forday and night temperatures, mitigating the stresses already suffered. Nevertheless, differencesbetween day and night temperatures of less than about 12 °C adversely affect yield of manycultivars (Gent and Ma, 1998). Processing tomato cultivars bred for semi-arid warm climates,however, do not respond negatively to maximum temperatures in the range of 35-40 ºC.As is true for other crops, mineral nutrient requirement is high for high production. Fertilizerapplications may be up to 220-250 kg N, 40-80 kg P 2 O 5 , 300 kg K 2 O, and 20-50 kg MgO perha for crops yielding about 100 tonne/ha. Fresh market tomatoes yielding over 150 tonne/harequires up to 300-600 kg N, 150-300 kg P 2 O 5 , 600-1000 kg K 2 O per ha. Nitrogen is often givenin split applications, at planting and one at the early stage of fruit setting. Nitrogen, either inexcess or when applied late during the growing season, affects fruit ripening by delaying fruitmaturation. Too much nitrogen early in the season induces fruit drop and minimizes fruit set(Adam, 1986; Benton,1999).The crop is moderately sensitive to salinity. The average root zone salinity threshold (EC e ) isabout 2.5 dS/m. Yield decreases as EC e increases, to about 50 percent at EC e of 8 dS/m, and196crop yield response to water