| Summary: | 博士 === 國立臺灣大學 === 農藝學研究所 === 96 === The thesis aims to study the seed eco-phsiology of Ageratum houstonianum Mill. and Ageratum conyzoides L., including how the germination was affected by various environment factors. The second propose of the thesis is to explore the possibility and method establishing these two weeds as cover plants for fallow land field.
Both the seeds of A. houstonianum and A. conyzoides were low in germination percentage under constant temperature; the percentage was higher under alternate temperatures such as 30/25, 35/30 and 23/130C. The base temperature (Tb) for the germination of A. houstonianum and A. conyzoides seeds were 6.40C and 6.60C respectively. The thermal time for seed germination of A. houstonianum was 700Cd; for A. conyzoides it was 720Cd. High water potential affected seed germination. Part of seed of A. houstonianum still germinated under -0.8MPa and 25/200C regimes. Seeds of A. conyzoides were more tolerate to low water potential; some seeds germinated at as low as -1.0MPa. The seeds of A. houstonianum required more light photons to germinate than those of A. conyzoides. Under light and anoxia condition the germination percentage of A. houstonianum was higher than that of A. conyzoides. Soil pH showed the same effect on seed germination of both two species, the germination percentage was highest at pH7. The seeds of both species were not able to germinate at burial condition deeper than 2 cm. The lower the temperature, the less depth the seeds might tolerate in terms of germination.
The base temperature, base water potential (Ψb(50)), hydrothermal time (θHT), and base water potential variance (σΨb) for seed germination of A. houstonianum were 7.0°C, -0.436MPa, 250CMpa-day, and 0.316Mpa respectively. The seed germination model for this species was: Probit(G) = [Ψ-25/(T-7.0)Tg-(-0.436)]/0.316.
Under dark germination condition, the exhumed seed of both species showed conditional dormancy at the beginning of burial experiment; the dormancy was higher in seeds of A. houstonianum. Dormancy profile were different between burial experiments initiated at different season. Burial experiment initiated in winter upland showed a loss of about 35% in seeds after 6 months burial, and 70% after two years. More than 70% of the buried seeds remained germinable after 4 months burial; however, there were less than 10% at the end of the two-year experiment. Burial experiment initiated in summer upland showed a loss of 10 and 15% in seeds of A. houstonianum and A. conyzoides respectively after 3 months burial. Less than 20% seeds remained germinable after 6 and 3 months burial for A. houstonianum and A. conyzoides respectively. Burial experiment initiated in summer paddy field showed a loss of 5 and 8% in seeds of A. houstonianum and A. conyzoides respectively after 3 months burial. Less than 30% seeds remained germinable after 8 months burial for both Ageratum species. The disappearances of seeds were slower and the germinability higher in those seeds buried in paddy field than those in upland field. Both species showed a peak emergence in October each year.
Monthly plowing treatment showed that the seeds of both species could emerge all year round. In the field the emergence percentage of the sowed seeds of both species reached 80% after 3 months. The plants of both species grew well when sowed in March, June, and September. Field covering rate reached 77% at 60th days after sowing. The growth appeared slower when sowed in December. Seed number produced by a single plant varied from 3000 to 7000 and from 2000 to 22000 for A. houstonianum and A. conyzoides respectively, dependent on the sowing month. In a flooding rice field, a sowing rate of approximately 1300-1600 seeds (0.2g/m2) of Ageratum could grow 80-100 plants after 4 months at rice harvest. Volunteer Ageratum appeared after the harvest of the 2nd rice crop had a flowering period of 45 days, and 80 days for those appeared after the harvest of the 1st rice crop.
In autumn crops of corn, peanut, sesbania fields or let fallow, sowing approximately 300 seeds of Ageratum (0.05g/m2) resulted in 89-124 plants and could reach full covering with a flowering period of 60 days. While in spring crops, the same treatment resulted in 57-93 plants and could reach full covering with a flowering period of 80 days. About 58-116 volunteer Ageratum per meter square appeared after the harvest and plowing of the autumn crops, with a full flowering period of 45 days; while for those of the spring season, 66-90 plants appeared with 72 days of full flowering period.
After rice harvest, more Ageratum appeared in the fields that were managed with shallow plowing or straw-mulching than that managed with straw-burning. In spring crop of corn and fall crop of peanut, burning the straw after harvesting also reduced the appearance of Ageratum. In a stand mixed sowing with sesbania and Ageratum, Ageratum could be able to grow quickly to cover the farmland if the sesbania be damaged by pests, according to the result of sesbania cutting experiment.
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