作 者 ：朱雅娟, 董鸣, 黄振英
期 刊 ：植物生态学报 2005年 29卷 5期 页码：730-739
关键词：沙鞭；沙埋深度；种子大小；种子萌发；幼苗出土；种子生态学；
Keywords:Psammochloa villosa, Sand burial depth, Seed size, Seed germination, Seedling emergence, Seed ecology,
摘 要 ：该文研究了野外条件下不同深度的沙埋对沙鞭（Psammochloa villosa）种子萌发和幼苗出土的影响，以及温室条件下种子大小对不同深度沙埋后的种子萌发和幼苗出土的影响。结果表明，沙埋深度显著影响沙鞭的种子萌发率、幼苗出土率和种子休眠率。沙子表面的种子不能萌发。2 cm的浅层沙埋时的种子萌发率和幼苗出土率最高，1 cm 沙埋的种子萌发率和幼苗出土率次之。沙埋深度超过2 cm之后，沙鞭的种子萌发率和幼苗出土率与沙埋深度呈负相关。2 cm的种子休眠率最低。从2 ～12 cm，种子休眠率随着沙埋深度的增加而增加 。在幼苗能够出土的深度（1～6 cm），幼苗首次出土所需的时间随着沙埋深度的增加而延长。种子大小对沙鞭的种子萌发率没有显著影响。但是在深层沙埋（6 cm）时，与小种子相比，大种子产生的幼苗的出土率较高。从2～6 cm，大种子形成的幼苗的茎长度都较长。
Abstract:Sand burial is a common phenomenon in sand dune ecosystems all over the world. Although many studies concerning the effects of sand burial on seed germination and seedling emergence have been conducted in many parts of the world, such studies are rare in China. In the summer of 2003, we studied the effects of sand burial depth (0, 1, 2, 4, 6, 8, 10 and 12 cm) and seed size on seed germination and seedling emergence of Psammochloa villosa, an important psammophyte that establishes on mobile sand dunes in deserts and sandy lands of North China. Seeds were collected in their natural habitat in Mu_Us Sandland, Ordos Plateau, in 2002 and stored at -18 ℃. The average seed mass of P. villosawas (5.51±0.05) mg. Seeds were sorted into 3 groups based on their mass: small (4 - 4.9 mg, mean = (4.489 ± 0.012) mg), medium (5 - 5.9 mg, mean = (5.457 ± 0.012) mg), and large (6 - 6.9 mg, mean = (6.415 ± 0.011) mg). We conducted two experiments , one in the field and the other in a non_heated greenhouse. In the second exper iment, burial depth was 1, 2, 4, 6 and 8 cm. Before burial experiments, all seeds were treated with cold stratification in 5 ℃ for 4 weeks in order to break dormancy.We found that sand burial depth had significant effects on seed germination and seedling emergence of P. villosa. Experiments revealed that the highest percentage of seeds that germinated and seedlings that emerged occurred at 2 cm burial depth. Few seeds lying on the surface of the sand germinated, and, those that did germinate, could not anchor their roots into the soil and seedlings did not establish successfully. The next greatest germination and emergence percentage occurred at the 1 cm burial depth. The timing of emergence was delayed with deeper burial depth. As burial depth increased from 2 to 12 cm, the percentage of seeds that germinated and emerged declined, a higher number of seeds remained dormant, and the number of days before first emergence increased. As sand burial depth increased, some seeds were able to germinate but the seedlings were not able to emerge possibly because the seeds did not have enough energy. Seedlings that could not emerge, etiolated, died and decomposed in the sand. At all depths, the seeds that could not germinate remained dormant. Possible reasons for sustained dormancy include poor aeration, high sand moisture, low soil temperature, poor O2 content, higher CO2 levels in soil and low light intensity. The dormant seeds, which form soil seed banks, are very important for the long_term survival of this species. The dormant seeds can germinate when sand erosion decreases the burial depth and the seeds are exposed to optimal environmental conditions, such as sand depth. However, our experiment revealed that seeds buried at deeper depths had higher mortality rates (15 - 35%). The high mortality might be due to fungal infection, water_soluble germination inhibitors in the seed coat, and an unfavorable microenvironment and low O2 concentrations. Seed size did not affect the percentage of seeds that germinated or remained dormant, but had a remarkable effect on the percentage of seedlings that emerged at different sand burial depths. At greater depths (>6 cm), large seeds had higher rates of seedling emergence than medium and small seeds. From 1 to 6 cm depth, the seedling shoots that originated from large seeds were longer than those from small seeds; also, at 2 to 6 cm depth, seedling shoots that originated from medium sized seeds were longer than those from small seeds. Therefore, we suggest larger seeds have ecological advantages in sand dune ecosystems. Larger seeds have a larger endosperm, contain more energy, and thus can emerge from deeper burial depths. This will benefit seedling growth in early stages and help them to suc cessfully establish under extreme desert conditions. Sand burial might be a sele ctive pressure on seed size; therefore, seed polymorphism as an evolutionary str ategy can benefit the renewal of P. villosa populations. In sand dune habitats, P. villosa produced different sizes of seeds that could emerge from different depths of sand burial. The diversity of seed size could enhance opportunities for seedling establishment in this heterogeneous environment allowing plants to respond to different sand burial depths, enhance their ability to adapt to the chang ing environment, and increase survival.