Physical dormancy alleviation and soil seed bank establishment in Cassia roxburghii is determined by soil microsite characteristics

Seeds with an impermeable coat, i.e. physical dormancy (PY) persist in the soil for a long-time until suitable environmental cues break dormancy and germination is synchronized with the growing season. However, the dormancy loss of seeds at different microsites has not been well characterized. In this study, the effect of microsite affecting dormancy loss in Cassia roxburghii was studied by artificially burying the seeds in an open site and under canopy and exhuming the seeds at 6 month intervals for two years. The soil temperatures of the two microsites at seed vicinity were determined using a data-logger. The results showed that fresh intact seeds did not absorb water, but mechanically scarifying the seeds resulted in water absorption and germination at a wide range of temperatures both in light and darkness. Thus, the seeds of C. roxburghii have PY. Throughout the study period, impermeable seeds buried in the open site broke dormancy in significantly higher number compared with seeds buried under canopy cover (P < 0.05), presumably due to the temperature variation between microsites. Open sites recorded temperatures above 60 °C during summer with diurnal temperature fluctuations close to 20 °C, whereas under canopy cover temperature remained close to 40 °C during summer and much lower during other seasons. These results suggest that the ability of C. roxburghii seeds to break dormancy and establish soil seed banks is affected by the microsite variation.