Light environment in pre- and post-dehiscent fruits affects seed germination in Calotropis procera

Fruits in Calotropis procera can be distinguished into five discrete but contiguous stages on the basis of diameter and seed color. Seeds from dehisced fruits at stage V germinated >80% on moist substratum in darkness. This was rather unexpected because the seeds developed and matured in an FR-enriched microenvironment (R:FR ratio ∼0.3) of the chlorophyll-containing maternal tissue and displayed low-fluence response (LFR) mode of phytochrome action. In contrast to >80% dark-germinating seeds from dehisced fruits at stage V, about 50% seeds from undehisced fruits at that stage were dark germinating, whereas another 30% seeds required light for germination. The light-requiring fraction of the seed population did not only respond to a very low-fluence R and to a short FR pulse, but also lacked R-FR reversibility thereby indicating to a very low-fluence response (VLFR) mode of phytochrome action. The present study reporting VLFR to non-dormant seed state transition in C. procera suggested that the state of phytochrome and the subsequent seed germination response in dry-seeded species, besides being determined by the light environment immediately before maturation drying, might also be regulated by a post-dehiscence light signal.