Effect of foliar application of auxin on morphophysiological and biochemical traits of safflower cultivars (spring and autumn) under drought stress

Severe climate changes and rapid population growth have influenced global food security. Oilseeds are the second main source of human food after cereals. Increases in the demand for vegetable oil at global markets and thus increases in the price of these oils, an economic pressure has been imposed on the oil-importing countries. So, concerning population growth and increases in per capita food consumption, it is necessary to increase the area under oilseeds' cultivation and crop yield. Safflower is an oilseed crop of the family Asteraceae with favorable and unique characteristics that make it a valuable oilseed crop. Safflower due to having long roots and the high ability for absorbing water from deep soil layers, is considered as a low water-tolerant plant; but after the rosette stage, it becomes more sensitive to water stress such that since the first signs of inflorescence emergence up to the middle of the grain-filling period, it shows more sensitivity to this kind of stress.

This study was performed during the 2018-2019 cultivation year in a 400-ha farm located in the Seed and Plant Improvement Institute, Karaj, Iran. The geographical location of the study area is 35° 49' N, 51° 6' E, and its elevation is 1321 m AMSL. According to climate data and ambrothermic curve, this area, due to having 150- 180 dry days, is considered as hot and dry Mediterranean climate. Because of having cold and humid winters but hot and dry summers, it has a dry moisture regime. Based on the average of 30-years of climate data, the mean temperature is 13.5 °C, the soil temperature is 14.5 °C, and in terms of soil temperature regime classification, it is considered as thermic areas. Table 1 represents the mean monthly temperature and precipitation during 2018-2019 and 2019-2020 cultivation years in Karaj. The main factor of this experiment was drought stress in two levels, i.e. normal irrigation (control) and irrigation cut-off at flowering stage, and two sub-factors include safflower cultivars (Golmehr and Goldasht) and foliar application of auxin in two levels, i.e. zero-application and applying 4 gL-1 auxin.

Results of the present study illustrated that compared with Golmehr cultivar, Goldasht had higher number of grains per head, grain yield and biological yield. On the other hand, Golmehr cultivar had a higher plant height, percentages of oil, linolenic acid, and oleic acid, compared to Goldasht cultivar. Under drought stress, the activities of antioxidant enzymes and proline were not significantly different between the two cultivars; however, the Goldasht cultivar by accumulating the activities of catalase, peroxidase enzymes, and proline content showed higher resistance to drought condition than the Golmehr cultivar. Irrigation cut-off at flowering stage led to decreases in bush height, number of grains per head, number of plant heads, 1000-grain weight, grain yield, biological yield, harvest index, SPAD index, percentages of oil, palmitic acid, linoleic acid, oleic acid and increases in proline content, activities of antioxidant enzymes, i.e., catalase and peroxidase. In comparison with the no-auxin situation, applying auxin increased SPAD index, proline content and, activities of catalase and peroxidase so, it could alleviate the side effects of drought stress. Therefore, in addition to the reduction of production costs and saving water, especially in arid and semi-arid areas, which results from the foliar application of auxin and cutting of irrigation at flowering stage, also crop yield and oil percentage could be gained same as the ones gained under normal condition. Furthermore, concerning the evaluated characteristics and the reaction of studied cultivars to drought, Goldasht cultivar can be introduced as the preferable cultivar under drought stress conditions.