Effects of pre-harvest foliar application of plant-regulating substances and their combinations on fruit development and quality formation in blueberry

To explore the effects of different plant-regulating substances (including classical plant growth regulators [PGRs], nutrients, biostimulants, and metabolic precursors) and their combinations on flower-fruit transition, fruit development, ripening progression, and quality formation of blueberry, a field experiment was conducted on blueberry cultivar L25 at a planting base located at 23.4771°N, 103.2510°E with an altitude of 1300 m. All field treatments were arranged in a randomized complete block design with three biological replicates. Four functional categories of exogenous treatments, encompassing nutrition and biostimulation, vegetative growth promotion, ripening modulation, and quality regulation, were established with both single-agent and compound combinations. The study consisted of two experimental phases: screening individual substances at varying concentrations, and evaluating formulated combinations. All reagents were applied via foliar spraying during the key flower and young fruit developmental stages with conventional diluted concentrations suitable for blueberry cultivation. Plant growth, floral and fruit developmental traits, fruit morphological parameters, and internal nutritional attributes were comprehensively evaluated, including chlorophyll content, flower quantity, fruit population dynamics, fruit size, single fruit weight, fruit shape index, soluble solids, titratable acid, anthocyanins, and ascorbic acid (vitamin C) content. The results indicated that different plant-regulating treatments exhibited distinct regulatory performances in modulating blueberry flower-fruit transition, fruit enlargement, color maturation, and quality formation. Naphthalene acetic acid (NAA) and chitooligosaccharides produced consistent effects on regulating fruit longitudinal and transverse growth as well as fruit weight. Forchlorfenuron (CPPU), diethyl aminoethyl hexanoate (CSN-DA-6), and paclobutrazol improved single fruit weight and soluble solid accumulation, and CPPU exerted a relatively stronger promoting effect on fruit morphogenesis. Ethephon, sodium nitrophenolate, and cyanamide mainly promoted fruit ripening and anthocyanin accumulation, while most of these treatments failed to maintain stable ascorbic acid (vitamin C) levels in mature fruits. In comparison, thidiazuron, gibberellic acid, and 24-epibrassinolide comprehensively improved fruit appearance and nutritional performance, and 24-epibrassinolide showed favorable performance in balancing soluble solids and titratable acidity. Each treatment or combination displayed trait-specific regulatory advantages without universal superiority across all measured indices. Combination Treatment 2 exhibited preferable performance in optimizing fruit dimensional traits and single fruit weight. Overall, the selection of plant-regulating substances should be tailored to specific growth stages and cultivation objectives to achieve uniform fruit ripening and high-quality berry production.