‘Safe sites’ for the seed germination of Rhus javanica : A characterization by responses to temperature and light

Germination responses of Rhus javanica L. seeds to temperature and light were investigated with special reference to their gap‐detecting mechanisms in germination, i.e., responses to elevated and/or fluctuating temperatures and sensitivity to leaf‐canopy transmitted light. The seeds, which have water‐impermeable coats to prevent imbibition, were shown to become permeable and germinable after exposure to higher temperatures of 48–74°C for a brief period depending on the temperature. Once the coat impermeability had been removed by such heat treatment, the seeds became readily germinable over a wide range of temperature and light conditions. The lower and higher temperature limits for germination were around 8° and 36°C, respectively, with an optimal temperature of around 25°C. Simple linear relationships were observed between the temperature and germination rates, i.e., the reciprocals of the time taken by the seed subpopulations to show 10–70% germination in the sub‐optimal temperature range, where the required ‘thermal time’ for germination was 2300–3600 Kh. The presence or absence of light or a simulated ‘canopy light’ had little effect on the germination of this species. It was concluded that the seeds of R. javanica are furnished with a gap‐detecting mechanism in the form of a heat requirement for the breakage of water‐impermeable seed dormancy, which may be fulfilled by either daytime elevation of the surface temperature of exposed soil, or more effectively by fire.