Foliar boric acid application affects yield components, seed boron content, oil quality, and induces vivipary in Styrian oil pumpkin (Cucurbita pepo subsp. pepo var. styriaca)

Styrian oil pumpkin is a medicinal plant whose seeds contain important active substances. In Iran, its yield is lower than the global average, partly due to boron (B) deficiency, an essential micronutrient for reproduction. This study evaluated the effects of foliar boric acid application (0, 50, 100, 150, and 200 ppm) at four growth stages (vegetative, male flowering, female flowering, and early fruit set) on quantity and quality traits of Styrian oil pumpkin in a split-plot experiment based on a randomized complete block design with three replications, conducted at experimental station of the Department of Horticultural Sciences, University of Tehran, Karaj, Iran, during one growing season. Results showed that the highest fruit yield (46,775 kg/ha), seed yield (1,847.7 kg/ha), and oil (46.79%) were obtained at 150 ppm B applied at male or female flowering, where hollow seeds decreased by 53.5%. Conversely, 200 ppm B at early fruit set resulted in the highest seed B content (127.6 ppm) and viviparous seeds (150.7 per fruit). Correlation analysis showed strong positive correlations between seed B content and vivipary ( r = 0.83) and between seed yield and oil ( r = 0.50). Hollow seeds were strongly negatively correlated with the filled-to-hollow ratio ( r = − 0.91) and with yield traits ( r = − 0.60 to − 0.68). The first two rincipal components (PC1 and PC2) explained 62.8% of the total variance, confirming that 150 ppm B at male or female flowering is optimal, while 200 ppm B at early fruit set induces severe vivipary. We recommend foliar application of 150 ppm boric acid at male or female flowering to maximize yield, oil quality, and seed fill while avoiding vivipary in Styrian oil pumpkin.