Calcium Reduces Fruit Abscission in Persimmon by Targeting Cell Wall Integrity

In the Mediterranean region, the persimmon cultivar 'Rojo Brillante' may experience up to four waves of fruit drop. The first is a physiological event during fruit set that is common in woody species, while the subsequent waves are induced by rising temperatures and prolonged summer water stress. These summer drops represent the main limiting factor, leading to yield losses of up to 90%. Organ abscission is a complex process regulated by genetic, hormonal, nutritional, and environmental factors. We hypothesise that calcium (Ca) plays a protective role in the abscission zone (AZ) by inhibiting cell wall-degrading enzymes such as polygalacturonase (PG) and pectin methylesterases (PMEs). Calcium applications every 15 days from anthesis onwards significantly reduced fruit drop. Treatments preserved polar auxin transport-through <i>DkPIN1</i> expression-and inhibited stage C of the abscission process, decreasing the relative expression of the <i>DkIDL6</i> gene in the AZ. Moreover, PME and PG activities were significantly lower in Ca-treated fruits, confirming the stabilising effect of calcium on AZ integrity. In summary, pre-anthesis calcium sprays reduced premature fruit drop by about 30% under heat-drought stress by down-regulating key abscission genes (<i>DkIDL6</i>, <i>DkPG20</i>, <i>DkPME41</i>) and preserving cell wall integrity and fruit firmness, supporting the use of Ca treatments as a climate-smart approach to stabilise persimmon yield.