RESPONSE OF HYBRID CYMBIDIUM (ORCHIDACEAE) PROTOCORM-LIKE BODIES TO 26 PLANT GROWTH REGULATORS

Plant growth regulators (PGRs) are one of the most effective means of controlling plant organogenesis in vitro. Hybrid orchid production relies on effective protocols to maximize clonal shoot output. This is best achieved when protocormlike bodies (PLBs) are propagated. In a bid to deepen orchidologists’ understanding of basic responses of Cymbidium to PGRs, t his study aimed to establish the organogenic response of hybrid Cymbidium Twilight Moon ‘Day Light’ half-PLBs or PLB thin cell layers (TCLs) to a single application of PGRs (6 auxins; 7 cytokinins; 3 alternative PGRs), 3 herbicides or 7 growth inhibitors/retardants at 4 concentrations (1, 2, 4 or 8 mg·l-1) as well as a control (0 mg·l-1), both in the light and in the dark. The control (PGR-containing TC medium) performed best, but all auxins and growth inhibitors and retardants were toxic to neo-PLB formation, resulting in 100% death. A synthetic auxin (BSAA), a cytokinin (4-CPPU) and two herbicides (dicamba and picloram) were equally toxic. No auxins, TIBA, GA3 or SA induced any organogenic response. 1 or 2 mg·l-1 2,4-D or 1 mg·l-1 TDZ induced embryogenic callus, but 2–8 mg·l-1 2,4-D resulted in abnormal shoots. TDZ induced direct multiple shoots. Only five remaining cytokinins (Ads, BA, Kin, ZR, 2iP) could form neo-PLBs, but always significantly less than the controls, independent of the explant used (half- PLBs or tTCLs) and light conditions (light vs darkness). These five cytokinins could be useful for neo-PLB induction of other Cymbidium hybrids. A new concept, the average cumulative value or ACV, is introduced.