Investigating Performance, Water Productivity, Growth Degree Day Index (GDD) and Evaluating Yield Response Factor of Safflower Plant Under Different Irrigation Treatments

Because safflower contains more than 90% of unsaturated fatty acids, it can play an important role in expanding the area under cultivation of oilseeds and providing oilseeds in the country. Considering the lack of water and the need to investigate the behavior of the safflower plant in low irrigation conditions, it is necessary to have a proper estimate of its performance under water stress conditions. This oilseed plant needs areas with little winter and spring rainfall during the flowering period and is drought tolerant and has long roots with a high ability to absorb water from deeper soil profiles.

In order to estimate the yield production function and yield components of safflower plant variety Sina under water stress conditions, an experiment was carried out in a research farm located in Kermanshah province. This research was conducted using the data collected in two crop years in a research farm in Kermanshah. The yield response factor was implemented at three levels of 70, 60 and 30% of water requirement based on soil moisture balance in three iterations. Based on the data of the first year, the two production functions of Raes and Tafteh, yield response factors of the plant were determined and evaluated using the data of the second year. To calculate the growth degree day (GDD), two methods of direct calculation and modified average were used. The maximum possible temperature for growth is about 30 degrees and the minimum temperature for growth is 0 degrees as the recommended and acceptable low limit (no trend). In this research, the maximum value of 30 degrees and the base temperature value of 4 degrees were taken from the water requirment system database(niwr.ir). Then different periods of phenology were calibrated with this index. Also, the relationship between GDD index and transpiration evaporation coefficients and yield response factors and irrigation requirement were investigated.

The findings showed that the yield response factors of safflower plant to water stress varies between 0.5 and 1.2 in different growth periods and the highest sensitivity is in the flowering period and the middle period of this plant. Also, the results of the investigation of the two production functions showed that the values of the statistical indicators for both functions are the normal error value of 5% and the efficiency value of both functions is about 97%. The results show that the lowest average evapotranspiration in different treatments was related to the 30% water requirement treatment with 189.8 mm and the highest was related to the 100% water requirement treatment with 632.7 mm.

Based on the obtained results, the Sina safflower plant is most sensitive to dehydration in the middle and flowering stages, and the yield response factors reaches its maximum in this stage (about 1.2) and water stress is not recommended at this stage. On the other hand, Rees and Tafteh function with the presented yield response factors can estimate the results with acceptable accuracy in water stress investigation based on the presented coefficients. On the other hand, in the absence of plant data and complete meteorological data, only by using the GDD index can evaluate the values of plant coefficient and plant sensitivity to water stress and plant irrigation requirement  with appropriate accuracy.