Investigation of the effect of drought stress on safflower cultivars and presentation of critical stress limits based on plant canopy temperature

Water scarcity is the most important problem affecting the growth and yield of crops in arid and semi-arid regions such as Iran. Therefore, due to limited water resources, it is necessary to optimize water consumption and use deficit irrigation techniques. In recent decades, agricultural development and declining rainfall have put severe pressure on groundwater resources. In Fars province, which is one of the most important provinces in terms of agricultural production, more than 70% of agricultural water is supplied from groundwater resources. This has caused a sharp drop in the level of groundwater aquifers. The development of new irrigation systems due to the high potential efficiency can contribute to the balanced use of water resources. Of course, according to the FAO, the new irrigation system alone is not enough, and in addition, attention must be paid to controlling water allocation. Previous research has shown that irrigation scheduling, which determines the start and end of irrigation, can be effective in reducing water consumption in farms and orchards. Safflower can be mentioned among the oilseeds compatible with the climatic conditions of Iran. The object of the study was to investigate the effect of different irrigation managements on some agronomic and yield traits of safflower cultivars.

The experiment was conducted in the form of split plots in a randomized complete block design with three replications at Zarghan Agricultural Research Station in 2020. Irrigation levels (cut off irrigation in buttoning stage (I1), supply of 25, 50, 75 and 100% of water requirement until the end of growing season I2, I3, I4 and I5, respectively) as main plots and cultivars of Local Isfahan, Goldasht, Sefeh and Faraman as subplots. Each subplot consisted of four planting rows with a length of 6 m and row spacing and plant spacing per row of 60 and 5 cm, respectively. Some traits such as day to maturity, plant height at harvest time, yield measured. Crop water stress index (CWSI) and economic and physical water productivity were calculated. Economic analyzes were also performed. In order to estimate the economic productivity of water (in Rials per cubic meter), it is necessary to obtain the net profit per hectare. For this purpose, the total planting, maintenance, harvesting and irrigation costs of safflower farm in the year of the research were estimated. The amount of gross income was also obtained by multiplying the selling price of the product by the amount of performance. Benefits to cost ratios and the difference between benefits and costs were also calculated and examined in different cultivars and irrigation treatments, and the best treatment and cultivar were introduced economically. The amount of water required for each treatment was calculated by Penman-Montieth method and measured and delivered using volume meters. Data were analyzed by SAS software and the means were compared using LSD test.

The results of correlation study of different traits showed a significant positive relationship between day to end of flowering, day to physiological maturity, number of bolls per plant, number of seeds per boll, 1000-seed weight and biological yield. The highest grain yield in the treatment of 100% water requirement (I5) was in Faraman cultivar (2688 kg/ ha). The lowest grain yield in water cut treatment in buttoning stage (I1) was related to Sefeh and local cultivars of Isfahan with yields of 758 and 779 kg/ha, respectively, which were in one statistical class and did not differ significantly from each other. The highest irrigation water productivity and total water productivity were related to irrigation cut-off treatment and 75% irrigation treatment with values of 0.383 and 0.274 kg/m3, respectively. Economic water productivity was also higher in (I4). In all cultivars studied, the amount of net profit was higher in (I5) where irrigation was done completely. The difference between the net profit of (I4) and (I5) was about 20 million rials per hectare. The benefit-to-cost ratio also increased with increasing irrigation water, but the difference between (I4) and (I5) was not significant in all cultivars. The highest economic water productivity in all cultivars was obtained in (I4). Among the studied cultivars, the highest and lowest values of the studied economic indicators belonged to Faraman and local cultivars of Isfahan, respectively. Using water stress baselines obtained in this study, by measuring leaf temperature, air temperature and air humidity, the time of occurrence of different water stresses can be obtained and based on that, the start time of irrigation can be determined.