Brunonia australis and Calandrinia sp. are Australian native herbs with potential for commercialization as flowering potted plants or bedding plants. Both species are ideally grown as annuals and, like many traditional short-lived garden plants, are most economically propagated by seed. Nursery managers often schedule plant production for peak market periods outside the natural growing season. Therefore, it is necessary to understand factors that impact propagation as well as growth and development to facilitate production protocols for year-round cultivation. Although Roche et al. (1997) investigated the effects of smoke on seed dormancy of B. australis, this is the first study to investigate the effect of different constant temperatures on seed germination of this species and of Calandrinia sp.
B. australis flowers naturally in spring to early summer from September to December, producing blue inflorescences on long slender stalks (Carolin, 1992; Stanley and Ross, 1986). This species is widespread throughout Australia, including southern Queensland, where average minimum winter temperatures are ≈5 to 7 °C and maximum temperatures are 19 to 21 °C (Bureau of Meteorology, 2010a). In spring and early summer, minimum temperatures are ≈10 to 20 °C with daytime temperatures reaching 25 to 34 °C. Calandrinia sp. is a drought-tolerant succulent (Harrison et al., 2009) with large pink flowers. There are only two known collections of this species, and both occur in the Gascoyne region of Western Australia (F. Obbens, personal communication). There are no published reports on growth and flowering of this species in its natural habitat, but other species within this genus usually flower in spring (September to November). The Gascoyne region has an arid climate where the average minimum temperature during winter is ≈9 °C and the maximum temperature is 21 to 23 °C (Bureau of Meteorology, 2010b). In spring, minimum temperatures are ≈12 to 19 °C, and maximum temperatures range from 27 to 34 °C.
Temperature is an important regulator of seed germination and is thought to be perceived within seed membranes (Mayer and Marbach, 1981). Temperatures that promote seed germination of many species typically correspond to natural environmental conditions, such as adequate soil moisture, that are conducive to seedling survival (Bell, 1999; Bell et al., 1993). Most seeds will germinate at ≈20 to 24 °C (Mayer and Poljakoff-Mayber, 1989). Imbibed seeds may not germinate at low temperatures, but in some species, cold stratification at ≈0 to 10 °C is required to break dormancy (Baskin and Baskin, 2004; Mayer and Poljakoff-Mayber, 1989). High temperatures usually induce or maintain dormancy of imbibed seeds, but seed death may occur as observed in several Australian species (Bell et al., 1993; Bellairs and Bell, 1990).
Germination rate indices are typically used to quantify cardinal temperatures for seed germination and include reciprocal time to median germination and percentage germination per day. Linear regression of a germination rate function versus sub- and supraoptimal temperatures and extrapolation of the same lines to the x-axis (intersect) are usually used to estimate minimum and maximum temperatures (Holt and Orcutt, 1996). The point where the two lines intersect defines the estimated optimum temperature at which germination is typically more rapid and the percentage germination greater. Usually, more than one germination rate index is used, and the adequacy of fit statistically compared to provide the most accurate method for estimating cardinal temperatures.
A robust estimate of the base (minimum) temperature for germination, at which development stops, is an essential requirement for calculating thermal time (Slafer and Savin, 1991). Thermal time, defined as the accumulation of daily mean temperature above a base temperature, is widely used in scheduling ornamental crop production. The approach can be more useful for predicting plant development stages than calendar date when temperatures for germination, or other development stages, are outside the experimental data range (Slafer and Savin, 1991).
The objectives of the study were to investigate seed germination responses of B. australis and Calandrinia sp. at different constant substrate temperatures to establish base, optimum, and maximum temperatures and to determine thermal time requirements. Two germination rate indices, used to provide estimates of cardinal temperatures, were compared. This information will be valuable for scheduling year-round production of these seed-propagated species and for providing insight into germination responses in the plant's natural habitat.
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