Investigating the application of salicylic acid and selenium on agronomic characteristics and photosynthetic pigments of pumpkin (Cucurbita pepo L.) under drought stress conditions

Pumpkin (Cucurbita pepo) is one of the most important plants used in the treatment of some diseases, including benign prostate cancer (Hamissou et al., 2013). The seed of this plant contains 42-54% oil, which contains fatty acids, palmitic acid, stearic acid, oleic acid and linoleic acid, carotenoids, protochlorophyll, and vitamins A and E (Zeynali et al., 2019). Drought is one of the most important environmental factors that reduce the growth and yield of many agricultural and medicinal plants, especially in arid and semi-arid regions of the world. Environmental stresses cause a wide range of reactions in plants, from changes in gene expression and cell metabolism to changes in the growth rate and yield of crops, and limit plant growth more than any other environmental factor (Zafari et al., 2018). Salicylic acid is a plant phenolic compound that is considered as a hormone regulator and also plays a role in defense strategies against biological and environmental stresses. Selenium is a useful element for plants, but its necessity for the growth and development of plants has not been proven (Malik et al., 2011). In a research, it was stated that the application of selenium in the conditions of drought stress had a positive and significant effect on the number of seeds, seed yield, biological yield and oil yield in rapeseed (Zahedi et al., 2009). Due to the lack of sufficient information on the effect of salicylic acid and selenium in modulating drought stress, this study was conducted with the aim of investigating the effect of salicylic acid and selenium in drought stress conditions on the agronomic and biochemical traits of paper pumpkin.
Materials and Methods
In order to investigate the effects of salicylic acid and selenium on some characteristics of vegetative traits of Pumpkin under drought stress conditions, a two-year split factorial study was conducted in the form of a randomized complete block design with three replications. The treatments include drought stress (the main factor), at three levels of irrigation after using 25, 55 and 80 % of Field Capacity (FC) of the soil field, salicylic acid (a minor factor) at three levels of 0, 0, 0.7 and 1.5 mg/l) and selenium (sub-factor) at two levels of 0 and 3 mg/L.

Results & Discussion
The results showed that drought stress decreased plant height,, fruit and seed yield, 1000 seed weight, chlorophyll and carotenoid content of paper gourd leaves. Foliar spraying of salicylic acid and selenium caused a significant increase in all tested traits. The highest amount of seed yield equal to 741 kg/ha was observed in the irrigation treatment after using 25% of the soil field capacity along with foliar spraying with selenium 3 mg/liter and salicylic acid (1.5 mg/liter), which compared to the drought stress treatment. Severe without foliar spraying of salicylic acid and selenium showed an increase of more than 79%. The highest number of seeds in the fruit was 343 in the irrigation treatment after using 55% soil moisture along with foliar spraying with selenium and salicylic acid 1.5 mg/liter, which is an increase compared to the severe drought stress treatment without foliar spraying of salicylic acid and selenium. showed more than 90 percent. Also, the findings of the research showed that in the treatment of severe drought stress, combined foliar spraying of salicylic acid and selenium resulted in 16, 19, and 46 percent improvement in seed yield traits, 1000-seed weight, and number of seeds in the fruit, respectively.
Conclusion
The results obtained from this research showed that drought stress decreased plant height, fruit and seed yield, 1000 seed weight, chlorophyll and carotenoid content of paper gourd leaves. Foliar spraying of salicylic acid and selenium caused a significant increase in all tested traits. Also, the findings of the research showed that in the treatment of severe drought stress, combined foliar spraying of salicylic acid and selenium resulted in 16, 19, and 46% improvement in seed yield traits, 1000 seed weight and number of seeds in the fruit, respectively. Based on the present results, the use of selenium foliar spraying at a concentration of 3 mg/liter and salicylic acid at 1.5 mg/liter can be suggested to improve the performance of paper pumpkin plants in dry conditions.