Crop yield response to water - Cra

spring. In double cropping, sowing as soon as possible after harvesting the first crop is alwaysdesirable to minimize risks (cold, rains) at the end of the season. With early plantings, moreof the growing takes place under low evaporative demand, so water productivity is higher,and high summer temperatures during grain filling are avoided. However, the risk of losingthe plant stand increases as planting date is advanced because of cold or frost damage toseedlings, which are quite sensitive up to the sixth-leaf stage (Villalobos and Ritchie, 1992).The cardinal temperatures for crop development are: basal temperature = 4 ºC; optimumtemperature = 28 ºC, and maximum temperature = 40 ºC. The tentative setting of T upper forthe calculation of growing degree day (GDD) in AquaCrop for sunflower is 30 ºC, i.e. GDD doesnot increase further when temperature rises above 30 ºC.Direct sowing is normally practised with a recommended seeding depth of around 5 cm.Plant density depends on rainfall, cultivars, water availability and fertility management.Sunflower is a crop having high plasticity giving similar yields over a wide-range of plantpopulations. Row spacing for water-limited plantings is customarily wider than under amplewater supply (75 -100 cm vs. 50-75 cm). Plant densities between 45 000 and 60 000 plant/ haare common under rainfed conditions, while up to 80 000-100 000 plant/ha can be plantedwith irrigation. In disease-prone areas, planting densities are in the low range (50 000 to70 000 plant/ha) even with ample water and nitrogen availability to reduce the incidenceof disease and lodging risks. Given the plant's high capacity to intercept radiation, yieldsare relatively insensitive to variations in plant density provided near full cover is achieved atmaximum canopy cover, and similar grain yields are obtained over wide variations in plantspacing above 50 000 plant/ha.The duration from sowing to emergence varies between 7 and 30 days. Sunflower requiresaround 4 to 5 days to emerge when shallowly planted in warm soil, while it will take a numberof days more in cooler soils or when planted deeper. Minimum temperature for germination canbe as low as 3 °C, although it may go up to 5 to 10 °C depending on the various field conditions.The maximum germination percentage is maintained from 6 to 23 ºC, declining rapidly above25 ºC (Connor and Sadras, 1992). Seedlings in the cotyledon stage can survive temperaturesdown to -5 °C. Failure of emergence may result in suboptimal plant populations.Sunflower grows rapidly, producing large and rough leaves. Leaf appearance rate is affectedby temperature and photoperiod (Rawson et al., 1984). Temperature affects the growthpattern and final area of individual leaves, along the cardinal temperatures described above.Thermal time per leaf is between 20 and 25 ºC day, calculated with a base temperature of4 ºC (Villalobos and Ritchie, 1992). Maximum canopy cover under standard densities is high,between 90 and 98 percent, and is reached quite quickly. This is because the light extinctioncoefficient of sunflower canopies is quite high and a leaf area index (LAI) of 2 is sufficient tointercept more than 90 percent of the incoming radiation, while a maize canopy with an LAI of2 only intercepts around 70 percent of the radiation. Possibly, as the results of canopy growthcoefficient (CGC) of sunflower, a C 3 species, is considerably higher than that of most other C 3crops, about the same as for maize, a C 4 species. Under near optimal temperatures, sunflowermaximum canopy cover is reached between 40 and 50 days after planting. For early plantingsin cool climates, 60 to 70 days must pass before maximum canopy cover is reached.Floral initiation occurs early in the growth cycle, 25 to 25 days after sowing, and its timingis predominantly controlled by thermal time and photoperiod. The response of sunflower166crop yield response to water