Abstract:

Background and Aims Ammopiptanthus monglicus, endemic to the semi-arid desert region of central Asia, is an endangered evergreen leguminous shrub. Having difficulties to recruit into its original habitat, A. mongolicus population is experiencing difficulties in regeneration, and the area it ever possessed is shrinking. It is included in the China Red Data Book of Endangered Plant as the third-class protected plant. Despite the importance of seed germination characteristics in developing an adequate understanding of both primary and secondary colonization processes, there still not a few previous studies on the germination of A. mongolicus. To get a comprehensive understanding about the germination characteristics and seedling performance of A. mongolicus, experiments were, therefore, conducted to define the optimum environmental condition for germination. Further, the seed size effects on the seedling performance is also investigated here.
Methods In order to find out optimal temperature for soaking, Seeds were soaked in water of 21.5 (room temperature), 40, 60, 80, 90 ℃ for 5 minutes respectively. Treated seeds were placed on moistened filter papers in 11cm diameter Petri dishes, then, transferred into a laboratory incubator at 28 ℃. To test the germination response of the seed to the environmental temperature, seeds presoaked in 60 ℃ hot water were put into incubator under 15, 20, 25, 30, 35, 40 ℃ for germination, respectively. In the case of container seedling culture, presoaked seed were sowed with different cultural media, sandy soil, clay loam, mixed soil (sand soil mixed with clay loam at the ratio of 1∶1). Attempt to answer the question that whether the seeds with different size will differ in seedling performance, seeds were categorized into three groups according to the their diameter, those less than 5 mm, those between 5 and 6 mm, and those that of more than 6 mm. Pretreated seeds of its respective size were sowed into the raised seedbed separately. Observations on the seedlings survival rate, leaf width, leaf number, and plant height were recorded almost twice a month.
Key Results Seeds presoaked in water at 60 ℃, gave the highest germination vigor that 56.4% seeds were germinated within one day's incubation at 28 ℃. The optimal temperature for hypocotyl and radicle elongation of seedling is 25 ℃ to 30 ℃, lower than that for seed germination initiation (30 ℃). Temperature above 40 ℃ would likely have a detrimental effect on seedling growth in the germination test, it cause the hypocotyl and radical tissue damage and eventually decay of the seedlings. Interestingly, the percentage of abnormal seedlings with tightly twisted radicle and split tip, which are less likely to survive in the seedbed, were just 5.2%-8.6% under the temperature of 30 ℃ to 40 ℃, greatly less than that of 28.1% under the temperature of 15 ℃. In the case of container seedling culture, sixty three percent seedlings emerged out of the sand soil within 7 days, while in clay loam only 11%. Compared with seedlings emerged from the smaller one, seedlings emerged from seeds with size more than 5 mm tend to be stronger and growing more vigorous than the smaller one during the early stages of growth. This trend is further evidenced by the weak and high susceptibilities to pathogen and continuous death of seedlings from the smaller seeds within the 70 days growth. Seedlings from larger seeds took only about 57 days to attain a constant survival rate of more than 56.3%, while seedlings from the smaller ones took more than 70 days to attain 35.4% and still on a decreasing trend.
Conclusions Extremely high or low temperature have a detrimental effect on germination and seedlings performance of A. mongolicus shortly after the germination initiation, despite the fact that their adult counterpart have a extremely high tolerance to the hardness of its environment. This is accordance with our filed observation that very few seedlings or saplings were seen in the natural habitat. Soaking in hot water between 40 ℃ to 60 ℃ did promote the germination rate of A. mongolicus, and decrease the percentage of remaining hard-coated seeds. Seeds less than 5 mm in diameter showed reductions in seedling quality and seedling performance suggests that seeds with different size may have significant impacts on future community composition in the prospect habitat and should be treated discriminatingly in nursery and re-introduction activity.

Key words: Ammopiptanthus mongolicus, Soaking, Temperature, Germination percentage, Seed size, Seedling emergence