Flowering in snow tussock (Chionochloa spp.) is influenced by temperature and hormonal cues

Snow tussocks (Chionochloa spp.) in New Zealand exhibit extreme mast (episodic) seeding which has important implications for plant ecology and plantinsect interactions. Heavy flowering appears to be triggered by very warm/dry summers in the preceding year. In order to investigate the physiological basis for mast flowering, mature snow tussock plants in the field and younger plants in a glasshouse and shadehouse were subjected to a range of manipulative treatments. Field treatments included combinations of warming, root pruning and applications of two native gibberellins (GAs) GA₃, which is known to be highly floral inductive and GA₄, which is associated with continued floral apex development in another long-day grass. Warming, GA₃ alone and especially warming+GA₃, significantly promoted flowering, as did applications of GA₄ alone and GA ₄+CCC (2-chloroethyltrimethylammonium chloride, which is a known synergist of GA₃-induced flowering in the annual grass, Lolium temulentum L.). Our results provide support for the concept that mast flowering events in tussock species are causally related to high temperature-induced increases in endogenous gibberellin levels. It is likely that GAs (endogenous or applied) promote the continued development of a previously long-day induced floral apex. In addition to the promotion of flowering, applied GA₃ also disturbed the plant's innate resource threshold requirements, as shown by the death, over winter, of many non-flowering tillers. Applied GA₄ did not show this effect, likely due to its rapid catabolic metabolism to an inactive form. High temperature-induced flowering mediated by elevated levels of endogenous floral-promotive GAs could have important implications for regulating the evolutionary interaction between these masting plants and their seed predators.