The effect of foliar spray of naphthalene acetic acid on flowering, fruit production and water use efficiency of cucumber (Cucumis sativus L.) under deficit irrigation stress

Background and objectives: Irrigation is an important limiting factor of crop yield, because it is associated with many factors of plant environment, which influence growth and development. Cucumber (Cucumis sativus L.) demands high soil moisture for satisfactory yield, and under unfavorable climatic conditions, several problems may occur, such as the reduction of female flowers and delay in fruit growth. Availability of adequate amount of moisture at critical stages of plant growth not only optimizes the metabolic process in plant cells but also increases the effectiveness of the mineral nutrients applied to the crop. Auxin plays crucial roles during stress like improving root architecture by increasing lateral root number, expression of stress-related genes and metabolic homeostasis. Naphthalene acetic acid (NAA), has frequently been demonstrated to enhance growth parameters and productivity in vegetable and other crops. This study aimed to investigate the effect of NAA foliar spray on female flower production and fruit yield of cucumber under water deficit stress.

Material and methods: This experiment was conducted at the Research farm of the Agriculture Faculty (University of Zanjan, Iran) in 2023 using a split plot based on a randomized complete block design with three replicates. The experimental treatments consisted of three different irrigation regimes (50, 75 and 100 ETc%) as the main plot and foliar spray of NAA at three levels (0, 50 and 100 mg L-1) as a sub plot. Kish F1 hybrid cucumber seeds were sown with a distance of 50 cm in rows and 120 cm between rows. Different levels of NAA foliar spraying started at 4–5th true leaf stage and were repeated two times with an interval 15 days using a mechanical mist sprayer. Irrigation was calculated based on actual evapotranspiration (ETc%) rates. During the growth period, male and female flowers number was recorded. Fruits number per plant, average fruit weight, total fruit yield, water use efficiency (WUE) and total soluble solid (TSS) and fruit firmness were evaluated.

Results: The results showed that deficit irrigation stress significantly decreased male and female flowers number, but had no significant effect on female to male flower ratio. Also results showed that deficit irrigation significantly reduced fruit number per plant, average fruit weight and yield. Sever water deficit stress (50 ETc%) resulted in a 54.7% reduction in fruit yield compared to irrigation 100 ETc%, leading to a decrease in WUE. Also, water deficit stress significantly reduced fruit firmness but had no significant effect on fruit soluble solids content. Foliar application of NAA improved fruit yield by increasing the number of female flowers, number of fruit per plant, and average fruit weight. Foliar application of naphthalene acetic acid improved fruit yield by increasing the number of female flowers, number of fruits, and average fruit weight. Treatment with NAA 50 mg L-1 under irrigation 100 ETc% increased the number of female flowers, average fruit weight, water use efficiency, and fruit yield by 43.6, 53.2, 24.3, and 24.1 percent, respectively, compared to the control treatment. Application of NAA significantly increased TSS, which depended on the hormone concentration and irrigation level. Also, foliar spray of NAA under irrigation 100 ETc% and deficit irrigation 75 ETc% conditions improved fruit firmness. Maximum fruit firmness (3.7 kg cm-2) was achieved with foliar spray of NAA 100 mg L-1 under irrigation 100 ETc% condition.

Conclusion: According to the results,, it can be stated that the cucumber is a sensitive plant to water deficit stress, which under water deficit fruit yield decreased. This study showed that the application of NAA 50 mg L-1 can be proposed to improve the growth and yield of cucumber under normal and deficit irrigation conditions.