|其他摘要||Tolerance to desiccation is acquired continuously during seed maturation. Three categories of seed storage behavior are generally recognized among plant species: orthodox, intermediate and recalcitrant, depending on response to desiccation and low temperatures. Some indices, such as 1000-seed weight, seed coat ratio (SCR), water content at shedding and habitat, can be used to predict the desiccation sensitivity in seeds. Also, seed dormancy, which has developed in the course of long-term evolution, is a trait of major adaptive importance to their survival in a certain habitat, reflecting the intimate connection between dormancy and seed development and germination. Special dormant mechanisms and germination pattern ensure seed germination and resultant establishment in a favorable temporal and spatial condition. Comparative analyses of different storage behavior, dormant mechanisms and germination pattern in seeds of plant species growing in different habitats are highly valuable for ex situ preservation and sustainable use of plant diversity in the future. Based on the working hypothesis of habitat-correlated variation in seed nature, we investigated distinct seed characteristics of eight plants inhabiting in heterogenous environments. The major results and correlative conclusions were shown as following:
1. Lasia spinosa (Araceae) seeds were recalcitrant and not dormant at maturity. The favorable germination temperature was between 25℃ and 30℃, and the appropriate temperature for wet storage of freshly-mature seeds appears to be 10℃. On desiccation from 66.9% to 25.9% moisture content (mc), viability of seeds was reduced by 50%. After seeds with 37% mc were hermetically stored for 6 months, their viability was reduced to 18%. Mimusops elengi (Sapotaceae) seeds were intermediate. The favorable temperatures for germination were 25, 30 and 20/30℃. Seed tolerated desiccation to 10.7%, but viability was reduced to 23% on desiccation to 5.4% moisture content. After M. elengi seeds with 9.8% mc were hermetically stored at 4℃ for 1, 3 and 6 months, respectively, they germinated at 25℃ to 84, 83 and 79%, respectively. Based on the relationship between the habitat and seed characteristics of two seeds described above, we inferred that desiccation-sensitive seeds are shed to coincide with high water availability, albeit in this case related to spatial rather than temporal patterns.
2. Cellular membranes of M. elengi seeds were seriously damaged during desiccation, resulting in loss of viability of seeds. During desiccation, free radical, which includes reactive oxygen species (ROS), was strong oxidants and caused peroxidation of membrane lipids leading to impairment of membrane structure and function. At the same time, anti-oxidative enzyme activities also changed significantly. When desiccated to low moisture content, the activities of superoxide dismutase (SOD) in seeds firstly increased (6.1% mc) and then decreased. There was a similar pattern in catalase (CAT) and peroxidase (POD）activities, that is, firstly enhanced (10.7% mc) and then declined. Unlike the changes of SOD, CAT and POD activities, the enzyme activity of ascorbate peroxide (APX) decreased in whole on desiccation to 5.4% mc. The activities of dehydroascorbate reductase (DHAR) and glutathione reductase (GR), however, initially increased (10.7% mc) and then sharply decreased. We conclude that a decrease in enzymic protection agaist oxidative attack in M. elengi seeds was directly associated with lipid peroxidation and cyto-toxic free radical formation and that these events taken together might contribute to the loss of viability in the intermediate seeds.
3. M. lasiocarpa (Musaceae) seeds represented a new kind of combinational dormancy (PY+MD+PD) which consisted of physical dormancy (PY), morphological dormancy (MD) and physiological dormancy (PD). Water uptake was passive and germination poor after an experimental period of 5 months in mature, potentially viable seeds. Removal of the operculum of the Musella seeds permitted water to reach the embryo, but did not result in germination. This indicated that water entering the seed is unable to hydrate the embryo, which meant Musella must have an embryo imposed dormancy. During stratification at room temperature, 4, 15 and 20℃, the embryos differentiated gradually, and the rate of embryonic differentiation was faster at room temperature, 15 and 20℃ than that at 4℃. A rudimentary embryo is small and inverse T-shaped. Musella seeds which had stratified for 5 months and differentiated organs did not germinate. However, the final germination of the seeds stratified for 9 months at room temperature was only 3%, and the germination increased to 17, 32, 29, 32 and 37%, respectively, at 4, room temperature, 4/15, 15 and 20℃ after 12-month stratification. With the extension of stratification duration, with the exception of 4℃, the germination of the rest increased slightly during extended stratification. These results confirmed that embryonic differentiation and growth in seeds were temperature-depended and a matured embryo in morphology and physiology was prerequisite of loss of dormancy. Also, the particular deep dormancy of M. lasaicarpa seeds was the major block of sexual regeneration. We inferred that it was difficult for population extension of this species in the field. During warm stratification (15℃), the contents of endogenous GA3 and ABA in Musella seeds significantly fluctuated. Meanwhile, enzyme activities of both α-amylase and β-amylase enhanced. Judging by the relationship between seed dormancy and amylase activity, β-amylase might be considered to be more significant in germination.
4. Lepidium perfoliatum (Cruciferae), Olimarabidopsis pumila (Cruciferae), Scorzonera pusilla (Compositae), Ixiolirion tataricum (Amaryllidaceae) and Tulipa iliensis (Liliaceae) are desert spring ephemeral plants inhabiting the same habitat. Their seeds were small and light, and absorbed water well and exhibited orthodox storage behavior. Seed water contents of the five species were ＜ 9% at shedding. Seeds of L. perfoliatum and O. pumila regenerating only by seeds were dormant at maturity, and undergone after-ripening under heat (30℃, which occurs in summer) or chilling (4℃) conditions. As additional afterripening occurred, L. perfoliatum and O. pumila germinated well in light between 15 and 25℃. In addition, light is required for seed germination of L. perfoliatum and O. pumila. However, the seeds of S. pusilla, I. tataricum and T. iliensis were not light-requiring and germinated very well at low temperatures, such as 4, 5 and 4/10℃. Soil moisture was frequently the overriding factor controlling the timing of germination in deserts. The results indicated that there is more than one germination pattern in a given habitat.|