Using red yeast ( Xanthophyllomyces dendrorhous ) as a biofertilizer to enhance growth, yield, and rhizosphere microbial populations in cucumber ( Cucumis sativus L.)

Microbial biofertilizers can improve nutrient-use efficiency, rhizosphere microbial communities, and crop productivity while reducing dependence on chemical fertilizers. However, the field performance of the carotenoid-producing red yeast Xanthophyllomyces dendrorhous as a plant inoculant has not been evaluated in cucumber. This study assessed the effects of X. dendrorhous applied as a foliar spray or rhizosphere injection on growth, physiological traits, nutrient uptake, yield, and rhizosphere microbial populations of three cucumber (Cucumis sativus L.) hybrids during the 2023 and 2024 growing seasons. Treatments comprised foliar spray, rhizosphere injection, and an uninoculated control. Yeast inoculation significantly improved vegetative growth, including plant height, leaf number, branch number, and leaf area, particularly following rhizosphere injection. ‘Madaen’ generally showed the strongest growth response. Chlorophyll a, chlorophyll b, carotenoids, and total soluble solids were enhanced by yeast application, although foliar spraying occasionally produced greater increases in total soluble solids than injection. Inoculated plants also exhibited higher nitrogen, phosphorus, and potassium concentrations in leaves and fruits, with ‘Bahi’ and ‘Madaen’ displaying the most consistent nutrient improvements. Early, normal, and late fruit number and yield were significantly increased by yeast treatments, especially foliar application. Rhizosphere injection increased culturable fungal, bacterial, and yeast populations, whereas actinomycete populations were generally greater after foliar spraying. Responses varied according to cucumber hybrid, inoculation method, microbial group, and growing season. Overall, X. dendrorhous enhanced cucumber growth, nutrient acquisition, yield, and rhizosphere microbial populations, supporting its potential as a biofertilizer for sustainable cucumber production. Further studies are needed to clarify the mechanisms underlying its plant growth-promoting activity.