关键词: 羊草种子, 发育动态, 种子休眠, 盐碱胁迫, 温度, 稃, 萌发, 幼苗生长

Abstract:

Leymus chinensis (Trin.) Tzvel. is a perennial rhizomatous high quality species of the Poaceae family. It not only has high forage values and good palatability, but also has great tolerant to drought, cold and salt-alkaline conditions. This plant is a dominant grass species in Songnen Plain of Northern China, and also considered an ideal grass for rangeland use in the arid and semiarid regions. In recent years, with the development of animal husbandry and the great efforts on the ecological environment, artificial Leymus chinensis grasslands were constantly built. Therefore, the request on the quantity and quality of the seeds was also higher and higher. In this paper, we investigated seed development, dormancy characters and salt-alkaline tolerance of Leymus chinensis. On the one hand, we clearly determined the optimum seed harvest time of this species, providinga scientific basis for agricultural production and high quality seed harvest. On the other hand, we excavated the seed germinating potential, deeply analysed the stress tolerance mechanism, and provided a theoretical basis for the utilization rate and breeding new varieties of Leymus chinensis. The main results and conclusions in our experiments were listed as follows:
(1) Results of seed development of Leymus chinensis showed that, seed color changed from green to light green, yellow and heavy brown into the final along with the seed maturity. 1000-seed weight increased constantly, and reached highest at 33 d after peak anthesis. However, water content and electric conductivity of the seeds showed a declining trend, water content reached lowest at 36 d after peak anthesis, and the values of electric conductivity of the two experimental years were different, which reached minimum at 27 and 30 d after peak anthesis, respectively. The results of standard germination test showed that germination percentage was highest at 39 d after peak anthesis. At this time, germination starting days, 50% germination days and germination energy were all reached the optimal value. Although a slight difference was found between the standard germination test and accelerated aging test, seeds at 39 d after peak anthesis also had the strongest anti-aging capability. Above results shows that seeds at 39 d after peak anthesis have the highest vigor and best quality, and can be considered the optimum seed harvest time of Leymus chinensis.
(2) Response to burial depth and salt-alkaline stress of Leymus chinensis seeds at different maturation time were also greatly different. The ability of seedling emergence and growth was decreased with the increasing burial depth, and 1 cm was the most suitable planting depth. At this time, the seedling emergence rate was highest, time to seedling emergence was shortest, and the length and biomass of the shoot and root were also highest. In addition, the ability of seedling growth of the seeds at different maturation time was also different. Seeds at 39 d after peak anthesis had the highest vigor, the above index of seedling growth were optimum. Germination percentage and energy were significantly affected by seed maturation time, salt-alkaline stress and their interactions. Highest germination percentage and germination energy were occurred at 39 d after peak anthesis, especially at the highest salinity stress (400 mM). The recovery test showed that recovery percentage was also the highest at 39 d after peak anthesis. Above results show that although seeds at different maturation time have germination ability, the ability of seedling emergence and resistance to salt-alkaline stress is highest at 39 d after peak anthesis, which further support the view that 39 d after peak anthesis is the optimum seed harvest time of Leymus chinensis. In addition, shallow sowing is suitable for this species, and 1 cm is the most optimum planting depth.
(3) Artificial treatments can obviously break seed dormancy of Leymus chinensis. The results showed that many methods such as H₂SO₄, cold stratification, PEG, GA₃, KNO₃ and soaking in the water were all enhanced the germination percentage, germination rate, starting germination time, and 50% germination time. However, in production practice, both efficiency and economic cost should be considered, and combined with our results, we recommend the way of soaking seeds in water under lower temperature for 20 d in order to break seed dormancy and increase the germination percentage of Leymus chinensis.
(4) Lemmas are important factors inhibiting seed germination of Leymus chinensis, but it can also improve salt resistance of the seeds. We investigated the effects of lemmas on seed imbibition and dehydration, germination responses to various temperature regimes and the impact of different duration salt stress on seed germination. The results showed that lemmas could significantly enhance water absorption, and slowed down the dehydration rate under drought conditions. Lemmas, temperature regimes and their interactions significantly decreased germination percentage and germination rate, indicating that lemmas could inhibit germination process of Leymus chinensis. Under different duration of salt stress, recovery percentages of non-germinated seeds with lemmas were higher than that without lemmas, especially under long duration of salt stress, indicating that lemmas could enhance the salt resistance of the seed. Once precipitation and melting snow decreased salinity concentration in the soil, seeds with lemmas can geminate again.
(5) Germination percentages and rates were inhibited by either an increase or decrease temperature from the optimal temperature 20-30℃, and this temperature can alleviate the inhibitory effects of salt-alkaline stress on seed germination. With the increasing salinity and alkalinity, seed germination and germination rate were both decreased, and the reductions were much greater under alkaline stress. Under salt stress, when salinity < 200 mM, lower temperature was the main factor inhibiting seed germination. With the increasing salinity, higher temperature aggravated the adverse effects. While under alkali stress, germination percentage and germination rate were both decreased markedly at 25-35℃ even though the alkalinity was very low. Recovery percentage of non-germinated seeds increased with the increasing salinity, but increased at first and then declined under alkaline stress, and recovery percentages were lowest in both stresses at 25-35℃, especially under alkaline stress. Seedling growth had similar response to the interactions of temperature and salt-alkaline stress, 20-30℃ was also the optimum temperature. In addition, both of the two stresses inhibited root growth much stronger. Due to the above results, early July sowings in the field would be recommended, when temperature is appropriate and salinity-alkalinity concentrations are always reduced by the high rainfall.
(6) Under mixed salt-alkaline stresses, germination percentage and germination rate were decreased with the increasing salinity under all the stress treatments, and the reductions were greater under treatment which the proportion of alkaline salts was greater. At 250 mM salinity, germination percentage of treatment A that without alkalinity was 6.5%, but the other five treatments were 0. Seedling growth was also affected by salinity, pH and their interactions. However, radicle length decreased more markedly with the increasing salinity and pH. Stepwise regression analysis results showed that salinity was the dominant factor for seed germination under mixed salt-alkaline stress conditions. However, once radicl break through the seed coat, and pH changed into the dominant factor for seedling establishment. Above results indicates that mixed salt-alkaline stress has different impacts on germination and early seedling stages of Leymus chinensis. The interactions of high salinity and high pH have the strongest inhibition on seed germination and seedling growth.
(7) Both saline stress and alkaline stress were significantly decreased seedling length, fresh weight and water content, and the reductions were much greater under alkaline stress. The Na⁺ concentration, Na⁺/K⁺ ratio increased but K⁺ concentration decreased under both stresses, and the changes were greater under alkali stress. Under salt stress, shoots mainly accumulated Cl^- and little change was found in organic acids. While under alkali stress, the concentrations of Cl^-, NO₃^- and H₂PO₄^-♂ were all decreased, and organic acids were accumulated, especially malic acid and citric acid. In addition, soluble sugar was the same osmoregulation under the two stresses. Above results indicate that alkali stress inhibited early seedlings of Leymus chinensis much greater because of the high pH, different accumulation characters of Cl^- and organic acids indicate that different adaptive mechanism to saline and alkaline stresses are existing of Leymus chinensis during early seedling stage.

Key words: L. chinensis seeds, Seed dormancy, Salt-alkaline stress, Lemmas, Germination