Improvement of grain and oil yield-related traits of marigold (Calendula officinalis L.) in response to exogenous application of spermidine under water-deficit and weed infestation conditions

A two-year experiment was performed to evaluate the efficacy of spermidine on the field performance of marigold under water deficit and weed infestation conditions in the 2020 and 2021 growing seasons. The experiment was arranged as the split split-plot design based on a randomized complete block design with three replications. Irrigation intervals (I1, I2, and I3: irrigation after 40, 80, and 120 mm evaporation from the class A pan, respectively) were assigned to main plots, weed infestation (weed free and weed infestation) was assigned to the subplots, and foliar applications of spermidine (1 mM) (4-6th leaf stages and pre-blooming stage) were allocated to sub-subplots. The percentage of ground cover, leaf water content, leaf area index, and oil and essential oil yield decreased and leaf temperature increased in the I2 and I3 and also in weed infestation conditions. This reaction led to a decline in the grain yield of marigold plants. Foliar application of spermidine increased the percentage of ground cover, leaf water content, leaf area index, and oil and essential oil yield. It also decreased the leaf temperature. In conclusion, spermidine positively affected the grain and oil yield of marigold plants under water stress and weed infestation conditions.