Diversity in environmental controls of flowering in Australian plants

Abstract

In adapting their flowering to a particular season of the year, plants utilize a number of environmental inputs. Knowledge of these environmental controls of flowering is important for production in commercial horticulture. Such information is also relevant for assessing whether or not a species is threatened by global warming. Here, for five Australian plant species, we document ways in which the environment regulates their flowering. Spring flowering of Crowea exalata ‘Bindelong Compact’ reflects a response to increased daily light integral, these plants showing no hint of a true long day photoperiodic response. Higher temperatures not only cause earlier flowering of this Crowea cultivar but also depress flower production (5% loss per 1 °C increase). By contrast, another Crowea, ‘White Star’, flowers only if exposed to cool temperatures (15 °C) at the time of the increase in daily light integral. Thus, in commercial horticulture, synchronous and rapid flowering of Crowea will be possible by shifting plants from shade to high light conditions. In nature, light intensity will also have a major impact on flowering. By contrast, best flowering of Lechenaultia formosa in spring is a response to short photoperiods at high temperature while L. biloba prefers long days and has potentially spring to summer flowering. Whereas rising summer temperatures could have a deleterious effect on flowering of C. exalata, global warming may have little impact on L. formosa and L. biloba which flower more profusely in warmer conditions. Another spring flowering species, Verticordia chrysantha, responds both to short days and to exposure to cool temperatures so its survival could be threatened by global warming. For Calytrix fraseri its late summer flowering in nature is explained by its requirement for an exposure to long days. When combined with information previously published for Australian plants, it is clear that there are no simple generalizations to explain why a plant species flowers when it does.

Introduction

A number of Australian plant species flower in response to daylength and/or cold winter temperatures (e.g. Evans and Knox, 1969, Shillo, 1985, Sharman et al., 1989, Dawson and King, 1993, Day et al., 1994, Goodwin et al., 1995, Bunker, 1995, Halevy et al., 2001; and references in Johnson and Burchett, 1996). However, novel flowering responses to environment might also be predicted especially because the Australian flora has a unique evolutionary history with a high degree of endemism and genetic isolation (Johnson and Burchett, 1996).

Documentation of environmental regulation of flowering is important where survival of a species may be threatened by agricultural encroachment and, potentially, by global warming. In the latter case, it is clearly important to understand the extent to which temperature affects flower induction and development. In addition, information on environmental regulation of flowering is essential when a species is brought into horticultural cultivation. For many species and especially those originating in the Northern Hemisphere, the best environmental conditions are well known (reviewed in Halevy, 1985). In general, temperatures are close to those experienced in the wild, although greenhouse production will involve some forcing of growth in response to higher temperatures. To date, from a number of Australian species considered as horticulturally promising as flowering pot plants (see Lamont, 1987), extensive information on environmental regulation of flowering is available for Chamelaucium uncinatum and Pimelea ferruginea.

For C. uncinatum and related species, it is now well documented that exposure to short daylengths (SD) is essential for controlling flowering (Shillo, 1985, Dawson and King, 1993; and references in Johnson and Burchett, 1996). Information is also available on the use of plant growth regulators for dwarfing plants and producing an abundance of tightly packed flowers (Shillo, 1985, Lamont, 1986, Dawson and King, 1993).

Studies with the second species, P. ferruginea, have shown that temperatures ranging between 15 and 20 °C are required for its flower induction (Von Hentig and Hass-Tschirschke, 1989, King et al., 1992, Slater et al., 1994, Seaton and Plummer, 2004). There may be limited genetic diversity in this response of P. ferruginea to temperature (King et al., 1996) although it is a wide spread response in Australian species (see summary in King et al., 1992). Additional complexity is found with Hardenbergia violacea where its flowering requires temperatures between 15 and 20 °C and a simultaneous exposure to short daylengths. If the temperature rises too high (>22 °C), already formed flowers abort and no new ones initiate (King, 1998).

In the present study we report in detail on the environmental control of flowering of the third species with horticultural promise, Crowea exalata. It flowers in spring but also throughout much of the year. Three cultivars have been examined for environmental effects on their flowering in response to daylength, irradiance and temperature. We also document some aspects of environmentally regulated flowering in two other spring flowering species, Lechenaultia formosa, and Verticordia chrysantha, in a spring-to-summer flowering species, L. biloba and in Calytrix fraseri which flowers at the end of summer. Some information is also presented on responses to the plant growth retardant, Cultar (Paclobutrazol).