沙鞭(禾本科)种子在沙丘上的萌发策略及幼苗的耐干燥特性

Strategies of seed germination on sand dune and seedling desiccation tolerance, of Psammochloa villosa (Poaceae)

沙鞭是鄂尔多斯高原流动沙丘上分布的多年生禾草。对位于沙层不同深度的种子萌发和幼苗出土进行了测定。结果表明 ,种子的萌发和幼苗的出土随着沙埋深度的增加而减少 ,最高的幼苗出土率来自 0 .5～ 2 .0 cm浅层沙埋的种子 ,并且沙埋的深度影响种子萌发的时间。沙埋越深 ,越多的种子不能萌发并且处于强迫休眠的状态。但是 ,当被埋较深种子上层的沙层被移走使种子位于较浅的沙层时 ,种子能够克服强迫休眠。种子的强迫休眠还能够通过改善沙层中的空气含量来克服。在自然生境中 ,沙丘的流动性能够使萌发后的幼苗整个暴露在空气中遭受干旱胁迫 ,或者萌发后的幼苗在因无后续的降雨而变干。但是 ,沙鞭在幼苗的早期具有忍受干燥 ,在下次水合后能够恢复生长的能力。

Psammochloa villosa is a sand dune perennial grass mainly distributed in the moving sand dunes of Ordos Plateau, China. This species is one of the pioneer plants of sandland vegetation succession, and is usually found in sand dune habitats. It occurs in large sand covered areas where desertification is becoming quite serious. A general survey indicated that seed germination is quite low and slow, and only a few new seedlings each growing season can be found in their natural habitats. Hardly any research has been done on the strategies and adaptation of L. racemosus seed germination and seedling emergence in sand, which are important for the survival of this plant under extreme sandy desert conditions with unpredictable, small amounts and distribution of rain. To this end, we studied seed germination and seedling emergence from different depths of sand and seedling desiccation tolerance after a period of drought. Sand depth influences both seed germination and seedling emergence. The deeper the seed in the sand, the lower was the germination and seedling emergence. Higher percentages of seedling emergence were obtained from the shallowly buried seeds, ranging from 0.5～2.0cm, because it increases soil contact and relative humidity around the seeds, thus enhancing the probability of germination. Seedlings of some germinated seeds cannot emerge to the sand surface. This may be because the seeds do not have enough energy to grow to the surface. Later they decompose in the soil. The deeper the seeds were buried, the more remained ungerminated and in the condition of enforced dormancy. Many factors may cause enforced dormancy. Poor aeration at the greater sand depth has been shown to be one of the causal factors of enforced dormancy of P. villosa seeds. However, the seeds were able to overcome the enforced dormancy when the upper sand layer was removed and the buried seeds were at the suitable sand depth for germination, or meliorate the sand aeration content at greater depths. This enforced dormancy may have ecological advantages for P. villosa by maintaining a long-term seed bank in the sandy soil, in that seedlings can be produced when erosion reduces the depth of sand and the seeds become exposed to a suitable depth for germination. In natural habitats, the germinated seedlings may be wholly exposed to the air by sand erosion and undergo drought stress. However, the young seedlings have the ability to resist drought and start to recover after rehydration. The root length at dehydration and drying periods from 7 to 60 days influenced seedling revival ability. The later the stage of seedling development, the longer is the root and the smaller is the endosperm that remains. Further field observations need to be done in order to study the importance of the present findings on the ecological adaptations of this species to its natural habitats.