Osmotic potential and relative calcium proportion interact to determine fruit structural integrity in blueberry (Vaccinium corymbosum L.)

Calcium (Ca²⁺) is essential for the structural integrity and quality of blueberries; however, its accumulation in developing tissues is constrained by limited phloem mobility. This study evaluated the effects of nutrient-solution osmotic potential (Ψ o ) and relative Ca²⁺ proportion within the total cationic pool of a modified Steiner nutrient solution on Ca²⁺ distribution, vegetative growth, biomass accumulation, and fruit quality of hydroponically grown blueberry cv. Biloxi. A 3 × 3 factorial randomized complete block design combined three relative Ca²⁺ proportions (30, 45, and 60%) with three Ψ o levels (−0.032, −0.052, and −0.072 MPa). Because changes in relative Ca²⁺ proportion involved reciprocal adjustments in K⁺ and Mg²⁺, treatments represent different cationic balances rather than isolated Ca²⁺ concentrations. The relative Ca²⁺ proportion × Ψ o interaction significantly affected shoot length and shoot number, with maximum vegetative performance at 60% relative Ca²⁺ and −0.052 MPa (26.58 cm and 24.58 shoots plant⁻¹, respectively). Biomass accumulation responded significantly to the main effects of both factors, whereas their interaction was significant only for the root-to-shoot ratio. Soluble foliar Ca²⁺ exhibited a pronounced vertical gradient, with concentrations up to 4.3-fold higher in basal than apical leaves. Fruit Ca²⁺ concentration was significantly affected by Ψ o and relative Ca²⁺ proportion, but their interaction was not significant; at 30% relative Ca²⁺, fruit Ca²⁺ decreased by 42.7% from −0.032 to −0.072 MPa. More negative Ψ o was associated with higher soluble solids and lower titratable acidity, while these responses were less pronounced at higher relative Ca²⁺ proportions. Firmness and surface wax load declined under the most restrictive osmotic conditions but were favored by higher relative Ca²⁺ proportions; at −0.072 MPa, surface wax load increased by 147% from 30 to 60% relative Ca²⁺. Maximum firmness and surface wax load occurred at −0.032 MPa and 60% relative Ca²⁺, differing from the condition associated with maximum vegetative growth. Overall, nutrient-solution Ψ o and relative Ca²⁺ proportion within the cationic balance were associated with Ca²⁺ distribution, vegetative development, and fruit physicochemical and structural attributes.