Citrus tissue culture: regulation of stylar abscission in excised pistils

Lemon pistil explants were obtained by cutting just above the region of the hypogynous disc (A type explant) or at the base of the pistil (B type explant) and cultured on test medium containing Murashige and Skoog salts, 50 g sucrose/L, 100 mg myo-inositol/L, 5 mg thiamine–HCl/L, and 0.5 mg kinetin/L, plus or minus supplements. Under appropriate conditions an abscission zone formed and styles abscised after 6–8 days of culture; in the field stylar abscission occurred 12–15 days postanthesis. Abscission in A type explants was markedly inhibited by 9 μM 2,4-dichlorophenoxyacetic acid but was unaffected by indole-3-acetic, 1-naphthaleneacetic, gibberellic, abscisic, caffeic, or p-coumaric acids. The response to 2,4-dichlorophenoxyacetic acid was reduced in B type explants. In an atmosphere containing 35–200 ppm ethylene, cell division occurred in the zone of stylar abscission producing a proliferating callus, and the content of cellulase increased from 0.6 to 53.7 enzyme units/g fresh weight compared with fresh explants. Stylar abscission was inhibited by 2,4-dichlorophenoxyacetic acid in A type explants of Washington navel orange, Valencia orange, and mandarin pistils, but not of grapefruit pistils. B type explants of Washington navel orange and mandarin pistils were less responsive to 2,4-dichlorophenoxyacetic acid.