Seed-germination responses of Calotropis procera (Asclepiadaceae) to temperature and water stress in northern Australia E. O. Menge A , S. M. Bellairs A and M. J. Lawes A,B A Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT 0909, Australia. B Corresponding author. Email: Michael.Lawes@cdu.edu.au Abstract. Understanding the seed biology of the introduced weed rubber bush (Calotropis procera (Aiton, W.T. Aiton)) is critical to its management in northern Australia. We examined the numbers of seeds produced and the effects of environmental temperature and water stress on germination performance (germinability G; mean germination time MGT) of rubber bush seeds from across northern Australia. Germination trials were conducted using seeds from wild populations monitored for 3 years. Seed numbers per fruit did not vary significantly among the six populations studied (mean  s.e. = 433.2  19.0), but seed mass did (range from 8.32  0.24 to 5.24  0.06 mg), with no negative correlation between the measures. Maximum seed germination (68–100%) occurred at 30  C, associated with a mean germination time of 2.58 days. Under water stress, the proportion of germinated seeds declined significantly with increasing temperature from 92.5  1.1% at 20  C and 0 MPa to 2.8  1.7% at 40  C and –0.4 MPa respectively. Seeds were unable to germinate at ambient temperatures 40  C, but remained quiescent and hence viable. Planting depth influenced seedling emergence, with minimal germination of seeds on the surface (5.8%) but 88.5% germination at 3-cm depth. The effect of water stress was dependent on temperature, with water stress inducing a reduction in optimum germination temperature from 30  C to 20  C. Phenotypic plasticity in G and MGT did not show clear patterns among populations or years. Short MGTs increase seedling survival by rapid transition from endosperm resources to photosynthesis, whereas seed quiescence (cf. dormancy) optimises germination opportunities in a semiarid environment. Thus, the germination traits reported in the present study are likely to promote seedling survival and potential spread of rubber bush in semiarid Australia. Additional keywords: invasive weeds, rubber bush, seed biology, seed quiescence. Received 9 March 2016, accepted 11 July 2016, published online 29 July 2016 Introduction Rubber bush (Calotropis procera (Aiton, W.T.Aiton)) is native to desert regions of North Africa, the Middle East and the Indian subcontinent (Everist 1974; Parsons and Cuthbertson 2001; Grace 2006), but an invasive woody weed of northern Australia (Weber 2003; Grace 2006; Menge et al. 2016). Throughout its invasive range in the semiarid environments of northern Australia, it occurs in regions where mean annual rainfall ranges between 300 and 650 mm (Parsons and Cuthbertson 2001; Bastin et al. 2003; Grace 2006; Menge et al. 2016), and in Caatinga, Cerrado and Restinga of Brazil, where mean annual rainfall ranges between ~240 and 900 mm (Santos et al. 2011; Fabricante et al. 2013; Leal et al. 2013). In Australia, rubber bush forms large and dense monospecific stands that impede pastoral management activities such as the maintenance of native pastures and the mustering of livestock (Parsons and Cuthbertson 2001). Managing rubber bush is a priority for the pastoral industry in northern Australia, but requires an understanding of what promotes its spread and establishment. Successful germination is a prerequisite for the establishment and persistence of weeds and is manifested often as a complex set of responses at the seed, seed germination and seedling stages (Wilbur and Rudolf 2006; Beckmann et al. 2011), and may involve environmentally induced phenotypic plasticity (Relyea 2002; De Jong 2005; Ghalambor et al. 2007; Hulme and Barrett 2013). Seed size is one attribute that can affect germination responses of seeds, which in turn influence seedling survival and final biomass. In general, plants either produce relatively few large seeds or many small seeds. Heavier seeds germinate under a broader range of conditions and their seedlings may have better drought tolerance than those from small seeds (Wulff 1986). Skarpaas et al.(2011) predicted that smaller seeds may disperse farther and have high colonisation potential, but with low germination and seedling growth rates, owing to less endosperm resources to support germination and growth (Wulff 1986). Norden et al.(2008) reported that large seeds have slower germination, and at least one study has concluded that seed mass on its own is a poor predictor of how rapidly germination occurs (Bretagnolle et al. 1995). Rubber bush CSIRO PUBLISHING Australian Journal of Botany, 2016, 64, 441–450 http://dx.doi.org/10.1071/BT16044 Journal compilation Ó CSIRO 2016 www.publish.csiro.au/journals/ajb