Understanding and Managing Abscission of Apples

Apples are typical climacteric fruit characterized by a marked increase in ethylene production and respiration at ripening. Excessive pre-harvest apple fruit drop, which occurs just before fruit develop optimum red color, maturity and/or size, is one of its faults and usually causes a serious economic loss. Conversely, picking fruit before adequate maturity may lead to poor storability and poor fresh and processed fruit quality. The expression of genes related to ethylene production, ethylene perception, and cell wall degradation increases drastically in both fruit cortex and fruit abscission zones during preharvest fruit drop. However, different members of gene families are responsible for ethylene production, fruit softening, and fruit abscission in fruit cortex and fruit abscission zones. Naphthaleneacetic acid (NAA), a synthetic auxin, aminoethoxyvinylglycine (AVG), an inhibitor of ethylene biosynthesis, and 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action are three compounds that can effectively delay pre-harvest drop of apples. NAA may delay apple fruit drop while increasing fruit ethylene production and fruit softening. This is related to decreased expression of MdPG2 in fruit abscission zones and increased expression of MdACS1, MdACO1, and MdPG1 in fruit cortex after application of NAA. AVG and 1-MCP inhibit fruit ethylene production, reduce preharvest fruit drop, and delay fruit ripening by suppressing expression of MdACS1, MdACO1, and MdPG1in fruit cortex and expression of MdACS5A, MdACO1, and MdPG2 in fruit abscission zones in apples.  The combination of NAA and AVG or 1-MCP is more effective in inhibiting expression of MdPG2 in fruit abscission zones and delaying fruit drop than are NAA, AVG, and 1-MCP alone in apples while maintaining fruit quality.