Preharvest Biostimulant–Calcium Application Enhances Blueberry Fruit Quality Through Structural and Cuticular Modifications

The increased demand for higher-quality, longer-lasting blueberries has led to the development of preharvest strategies to improve their structural integrity sustainably. This study analysed the effects of the foliar application of two biostimulant–calcium (Ca) combinations, using Ecklonia maxima extract (EM + Ca) and glycine betaine (GB + Ca), on yield, biometric, mechanical, and histological properties, as well as cuticular wax composition of blueberries. Both biostimulants increased yield per plant and fruit weight and size in ‘Duke’, with superior results for GB + Ca. Fruit yield increased by 80% with GB + Ca and 40% with EM + Ca. Histological analysis showed increases in cuticle thickness, epidermal cell area and thickness, and hypodermal cell area and area/perimeter ratio. This thicker, denser tissue ultimately improved blueberries’ mechanical properties. Specifically, ‘Draper’ berries treated with GB + Ca had 36%, 15%, and 20% higher values for flesh firmness, stiffness, and deformation work, respectively, relative to the control. However, cuticular wax accumulation was more pronounced with EM + Ca for the ‘Duke’ cultivar, increasing by 12%. Overall, GB + Ca had the greatest impact on blueberry structural quality and may represent a promising strategy to improve postharvest quality and commercial production.