Germination Responses ofPinus densifloraSeeds to Temperature, Light and Interrupted Imbibition

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Abstract
Germination responses of Pinus densiflora Sieb. et Zucc. seeds to temperature and light were examined with a special interest in the gap-detecting mechanisms in germination, i.e., responses to fluctuating temperature and sensitivity to leaf-canopy transmitted light. Simple linear relationships were observed between the temperature in the suboptimal range and the germination rates, i.e., the reciprocals of the time taken to germinate by the seed subpopulations with 10–90% germination. The ‘thermal time’ concept could thus be used effectively to analyse the responses to varying temperatures and light. Neither shifts nor alternations of ambient temperature affected the thermal time required for germination; the profile of the change in percentage germination plotted against thermal time for the seed population was almost identical among various constant and varying temperature regimes. Seed germination was completely inhibited by simulated leaf-canopy light, thus indicating that P. densiflora seeds have a gap-detecting mechanism in the form of canopy-light sensitivity. Moreover, 1–7 d interruption of imbibition with dehydration had little effect on the final percentage and thermal time required for germination, suggesting that the germinating seeds of this species have a great capacity to withstand the fluctuating moisture conditions of the exposed surface of soil.