Abscisic acid applied to sweet cherry at fruit set increases amounts of cell wall and cuticular wax components at the ripe stage

Cuticle integrity represents an important factor in fruit quality of sweet cherries. Plants hormones, especially abscisic acid (ABA), are involved in fruit cuticle integrity. The effect of pre-harvest application of abscisic acid on cherry fruit cuticles was investigated. Changes of the concentrations of several phytohormones, expression levels of genes related to cell wall-modification, and to wax- and cutin biosynthesis were measured five days after applying ABA to sweet cherry trees (Prunus avium cv. Bing) at the stage of fruit set. At the ripe stage changes in cell wall polymers and fruit cuticle composition were measured. Our results show that pre-harvest application of 0.1 mM ABA at the stage of fruit set resulted in increased levels of zeatin ribose (204-fold), zeatin (126-fold), salicylic acid (30-fold), and ABA (19-fold). Transcription levels of key genes involved in cuticle biosynthesis, the transcription factor PaWINA and the wax biosynthesis-related enzyme PaLACS1 (5.8- and 3.4-fold increase), as well as for the key enzyme PaNCED1, involved in ABA biosynthesis (6.9-fold increase), were increased. At the ripe stage, contents of covalently-bound pectins and hemicelluloses were higher with a concomitant reduction of water-soluble polymers in ABA-treated fruits. Amounts of linear long-chain aliphatic wax compounds, especially C27, C29, and C31 alkanes, were increased (1.2-, 1.3- and 2.3-fold, respectively), whereas no effect of ABA on pentycylcic triterpenoids was observed. We conclude that ABA treatment of cherry trees at the stage of fruit set positively affects sweet cherry fruit quality at ripening by enforcing the cell wall and modifying the cuticle composition.