MARK: FACTORS CONTROLLING IRREGULAR FLOWERING IN Chionochloa

The narrow-leaved snow tussock, Chionochloa rigida (Raoul) Zotov,t which dominates extensively in the low.alpine zone on many mountain ranges east of the Southern Alps, does not flower annuany but at irregular intervals of up to four years, when the majority of plants everywhere are in flower. This phenomenon has been discussed by several people since Barker (1953) first described differential flowering between burnt and unburnt tussocks on the Hunters Hills in South Canterbury. Only recently, however, has the basis for jts control been investigated. The results from a variety of experiments showed that the irregular flowering in C. rigida is largely environmenta11y contro11ed (Mark 1965b. c, d). Physiological induction of flowering occurs only in response to relatively high temperatures received during that part of summer in which day-length exceeds about 14 hours. The intensity of the flowering response, i.e. the number of tillers which are induced and in which inflorescences are subsequently initiated, is controned partly by the duration of high temperatures and partly by the amount of reserves which the plant has available. Further, I have shown (Mark 1965c) that the species is ecotypically differentiated in temperature requirement in a way that synchronizes flowering among populations separated by 1.170m. of altitude and up to 8°C. in mean monthly air temperatures (Mark 1965a). The morphological initiation of inflorescences is usua11y apparent in autumn and a partia11y developed inflorescence overwinters successfully and completes jts development in the fonowing season. Thus, there is a delay of about 12 months between induction and anthesis. Including the 1966-67 season, there have been eight rather irregularly-spaced flowering years during the past 19 years and each has fol1owed a relatively warm summer (Connor 1966).