Introduction: Drought stress is an unavoidable factor that exists in a wide range of environments without boundaries or clear warning, diminishing plant biomass production and quality. It is caused by changes in temperature, light intensity, and precipitation levels. This is despite its cumulative, multidimensional nature, which has a negative impact on plant morphology, physiological, biochemical, and molecular characteristics, as well as the photosynthetic capacity. Plants evolve different adaptation mechanisms to cope with water scarcity, including physiological and biochemical responses, which vary according to species. Plants respond to drought by altering their growth pattern and structural dynamics, reducing transpiration loss through modulation of stomata conductance and distribution, leaf rolling, varying root-to-shoot ratio dynamics, increasing root length, accumulating compatible solutes, and enhancing transpiration efficiency, adjusting hormone levels, and delaying senescence. The purpose of this study was to examine the photosynthetic role of safflower leaves and inflorescence on seed yield and oil content under water stress at Shahid Madani University of Azarbaijan in 2019 using the spring cultivar (Goldasht).
Materials and Methods: This study examined the effects of different irrigation regimes (no stress, water stress from the beginning of flowering stage, and water stress from 50% of flowering stage) and removing different types of photosynthetic organs (control, removal of leaves from the bottom of the plant, removal of leaves from the top of the plant, and inflorescence cover) on the growth and yield characteristics of safflower. SAS software (version 9.2) was used to perform an analysis of variance (SAS Institute Inc. 2002). At a probability level of 0.05, Duncan's Multiple Range Test was used to separate the means. Using a Class A Evaporation Pan, plots without drought and normal stress were irrigated to a depth of 80 mm from the time of planting until the beginning of flowering. Irrigation was performed at 160 mm evaporation time from water evaporation pans on water-stressed plots.
Results and Discussion: The highest 1000-seed weight (42.7 g) was associated with the control treatment, whereas the lowest weight (42.1 g) was associated with 50% flowering stress. During stress (seed filling stage), leaf temperatures were 27.9 °C and 26.6 °C, respectively (control). At the beginning of the flowering stage and at the 50% flowering stage, water stress reduced seed yield by 16.4% and 7.9%, respectively. The oil yield decreased significantly by 4.4%, only when the stress treatment was applied from the beginning of flowering. According to the results, the most important source of assimilates for safflower seed filling and oil yield is the top leaves. Also, in this study, the petals play an important role in filling the seeds, both under conditions of full irrigation and water stress. Under full irrigation, the highest harvest index (39.7%) was associated with non-removal of leaves, while the lowest harvest index (38.6%) was associated with removal of top leaves. Without removing the leaves, the highest seed yield (1305 kg/ha) was obtained without water stress. There was no significant difference in chlorophyll index between removing the top leaves of the plant and the inflorescence cover, but more chlorophyll index was obtained by removing the bottom leaves than when organs were not removed. In both treatments, removing the bottom leaves increased the chlorophyll index by 7.4% compared to not removing the leaves.
Conclusion: Studies confirm that removing the leaves at the plant's base increases the chlorophyll index of safflower plants. Additionally, the inflorescence cover plays a significant role in safflower seed and oil yield. Beginning with the flowering stage, safflower seed and oil yields are negatively impacted by water stress.