The role of ethylene and cell wall modifying enzymes in raspberry (Rubus idaeus) fruit ripening

This study focuses on four raspberry (Rubus idaeus) genotypes from two different genetic backgrounds: cvs Glen Prosen and Glen Clova, bred at the Scottish Crop Research Institute (SCRI) and genotypes bred at Horticulture Research International (HRI), East Mailing (EM), EM 4997 and EM 5007. The ripe fruit of each genotype pair were characterised subjectively by raspberry breeders as relatively firm or soft, respectively. Different stages of fruit development from each genotype were used to quantify fruit firmness, rates of ethylene evolution and ripening rate. Penetrometry data confirmed suspected firmness differences. Firmness correlated with rates of ethylene evolution. Rates of ethylene production also correlated with receptacle size. Storage of green fruits in 20 μl 1^-1 ethylene reduced fruit firmness, enhanced respiration rate and colour (anthocyanin) development and stimulated the development of cell wall hydrolase activities. However, during natural ripening in the field, fruit respiration rate declined, which indicates a non-climacteric ripening pattern. In drupelets, the activities of polygalacturonase (PG), pectin methylesterase (PME), Cx-cellulase (Cx) and β-galactosidase (β-gal.) increased substantially as ripening progressed. More detailed studies with ripe fruit of cv. Glen Clova indicated major isoforms of PG at pls 3.3, 8.6 and 10.1; of PME at pls 7.2, 8.5, 8.7, 8.8; of Cx at pl 2.4; and of β-gal. at pls 6.3 and 6.7.