Texas, USA 2010 - International Herbage Seed Group

increases germination to its potential percentage (Figure.1). Also the pericarp seems to not affectgas exchange because removal of the pericarp near cotyledon (RCP) did not increase seedgermination significantly, but interesting, removal of the pericarp near radical (RRP) largelyimproves seed germination, this implies that a mechanical resistance on radical protrusion isimposed by the pericarp. Further, the pericarp mechanical resistance just partly explains pericarpimposed seed dormancy because RRP germinated to 44% only which indicate other factors areinvolved in the dormancy of the remaining seeds. In the present study, water flushingsignificantly increased germination percentage of seeds with pericarp or with a part of pericarp,and this result is consistent with previous research (Clrak, et al., 2007) and implies a germinationinhibitor existed or generated by the pericarp. Based on this, we suggest that pericarp imposeddormancy in Z. xanthoxylum seed is a combination of mechanical resistance and chemicalinhibition generated by pericarp.Seed dormancy has an important role in determining the time and place of seed germination inthe field to spread the risk of germination failure, and thus may help ensure long-term seedsurvival, especially for wild species growing in harsh environments (Baskin & Baskin, 1998). Itis clear from the current field experiment that pericarp imposed dormancy is responsible for thelack of germination from December 2007 to June 2008. During this period seeds with pericarpdo not germinate even when returned to optimum conditions for germination (Figure. 2), and thisis consistent with previous research in other species (Hu et al., 2009b). Seeds without a pericarpcan germinate readily at any time of the season depending on environmental conditions. Hu et al.(2009a) also reported that seeds with a pericarp exhibited a lower germination but higherseedling establishment compared to seeds without a pericarp because most seedlings from nakedseeds had a greater spread of germination time and many died from subsequent droughtconditions.Seed decay is another factor affecting seed persistence in addition to changes in dormancy.Present experiment clearly showed that the presence of a pericarp significantly reduced thenumber of decayed seeds regardless of burial depth and burial time. Low levels of seed decayearly in the season may be a consequence of low soil moisture at the experimental site (similarlyslowing the process of seed aging). When soil moisture rapidly increases late in the season,environmental conditions become optimum for seed germination and most seeds germinaterather than lose viability and decay. In addition, more than 90% of the variance in terms of seeddecay could be explained by the pericarp, suggesting that the presence of the pericarp plays adeterment role in limiting seed decay across a range of conditions.AcknowledgementsThe study was funded by the National Key Basic Research Special Foundation Project of China(2007CB108904).References102