Seed-germination responses of Calotropis procera (Asclepiadaceae) to temperature and water stress in northern Australia

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 degrees 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 degrees C and 0MPa to 2.8 +/- 1.7% at 40 degrees C and -0.4MPa respectively. Seeds were unable to germinate at ambient temperatures >= 40 degrees 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 degrees C to 20 degrees 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.