Calotropis procera (giant milkweed) is a hardy xerophytic plant, which is distributed globally in many countries and has important economic and ecological functions. The present study aimed at estimating the effect of temperature, salinity and time of fruit dehiscence on the seed germination and seedling morphology of giant milkweed in urban habitats. Seeds were collected early (in June) and late (in December) from pre-dehiscent (EPRD, LPRD) and post-dehiscent (EPOD, LPOD) fruits. Mature seeds were 100% viable, while premature seeds did not germinate. The highest germination and mean time to germination was attained in full dark followed by dark/light, while no germination occurred at complete light. A significant difference between seeds from pre- and post-dehiscent fruits was assessed, while no significant difference between early and late seeds. Maximum germination was at 25 and 30°C, while germination was inhibited at 35°C, which demonstrates that temperature is one of the critical factors for giant milkweed seed germination. Moreover, salinity more than 2000 mg l-1 NaCl inhibited seed germination. It is likely that high temperature, direct light conditions and high salinity are the limiting factors for the establishment of giant milkweed seeds. The germination of C. procera seeds was significantly affected when germinated with Trigonella foenum-vulgare with the reduction of germination from 100 to 34%, while the later plant was not affected. C. procera is considered as an important medicinal plant; therefore, our results provide useful information for its management under different environmental conditions.
Key words: Giant milkweed, seed morphology, fruit dehiscence, germination time and viability.
EPRD, Early pre-dehiscent; EPOD, early post-dehiscent; LPRD, late pre-dehiscent; LPOD, late post-dehiscent.