Effect Of Thermal Time On Seed Germination

Temperature is an important environmental factor influencing seed germination. Germination ability and seed germination responses to environmental temperature conditions affected the distribution of vegetation and dynamics in plant community. Our objective was using the thermal time model to research the germination response to different temperatures. It was a case study on 33 plants of Compositae and Poaceae from the eastern Qinghai-Tibet plateau. We combined the Logistic function and the thermal time equation, and used non-linear regression to estimate the base temperature and thermal time for germination of each species. The results showed that:(1) For most of the studied species, the average germination percentage is greater than 80% at the five temperature gradient, their germination percentage was relatively higher. The result of one-way analysis of variance showed that the five temperature gradient had no significant effect on the germination percentage of the studied species (P<0.05).(2) Germination rate of most of the studied species accelerated with increasing temperature. 5-25℃is in the suboptimal temperature range for germination of these species roughly. The optimum temperature for germination of the studied species is around 20-25℃roughly.(3) The average of base temperatures and thermal time of the studied species were 1.55℃and 80.28℃·d respectively. The base temperature was lower and the thermal time was higher compared with previous researchers'findings. Species with lower base temperature had higher thermal time would avoid the risk of seeds germinating earlier in an environment with changeable temperature. These are the result of their long-term adaptation to temperature environment of the Qinghai-Tibet plateau.(4) There was a significant negative correlation between base temperature and thermal time (P<0.001). This mechanism of trade-off between base temperature and thermal time of seed germination makes most of seeds germinated at appropriate season, so that increase the surviving opportunity of seedling.(5) There was a significant negative correlation between altitude and base temperature of germination (P<0.001), it reflect that the base temperatures of these seeds" germination are adapted to their temperature environment. Also a significant positive correlation between altitude and thermal time of germination (P<0.001). The thermal time require for seed germination increased with increasing altitude. This spreaded the risk of germination early in the complicated and volatile temperature environment of high altitude areas through extending the duration of germination.(6) There existed significant difference on thermal time between different life forms of species (P<0.001), but their difference on base temperature was not significant (P>0.05). Perennial species had higher thermal time than annual species. So the annuals germinated earlier made them occupied the priority of space resources and benefited from the competition in the early community succession than the perennial species.(7) A significant positive correlation was existed between seed mass and thermal time for germination of 29 perennial plants (P<0.05). Small-seeded species had lower thermal time than large-seeded species in perennial plants. Under the case of little differences between their base temperature, the small-seeded species germinated faster than the large ones, that made them occupied the priority in the early community succession.(8) Habitat had no significant effects on the base temperature and thermal time for seed germination.(9) There existed significant difference on base temperature for germination between Compositae and Poaceae plants as well as between different genera, but their difference on thermal time was not significant.