Effect Of The Pericarp And Germination Time On Soil Seed Bank And Life History Of Isatis Violascens(Brassicaceae)

Isatis violascens Bge.(Brassicaceae) is an annual ephemeral. In China, this species only grows on fixed and semi-fixed sand dunes in the Garbantunggut Desert of Xinjiang. When seeds matured, the dispersal unit of this species is dry indehiscent silicle with a wing. In this study, the effect of the pericarp on dispersal, dormancy/germination and seed soil seed bank, and the effect of germination time on life history of this species were studied. The ecological adaptive strategies of this species in the desert were explored. The key results are as follows:(1)In the natural habitats, all dry indehiscent silicles were dispersed by wind, in c. 20 d and within the 0-560 cm zone. Thus, the silicles of this species dispersed nearly surrounding the parent plant. In still air, the winged silicles took longer to fall than those without wings, the fall rate of the winged silicles was significantly slower than those without wings, and the wing significantly increased dispersal distance. This suggested that a very obvious effect of the wing is that it facilitates dispersal.(2)Isatis violascens has two levels of PD. Twenty to twenty-five percent of seeds had an afterripening(come out of dormancy) during summer and germinate in autumn if the soil is moist; otherwise, germination is delayed until spring. It showed that this part of the seeds have nondeep PD(C1b). Seventy-five to eighty percent of seeds germinated in the next spring. The seeds required ≥ 12 weeks cold stratification for dormancy break; however, an afterripening period in summer significantly decreases the length of the cold stratification period required to break dormancy. Thus, this part of the seeds have intermediate PD(C2a).(3)This species forms a transient soil seed bank. Both under laboratory conditions and on soil in the experimental garden, seeds inside silicles germinated to significantly lower percentages than those removed from silicles, indicating an inhibitory effect of the pericarp on germination. Thus, the pericarp can prolong the duration of the soil seed banks.(4)This species behaves as winter annuals and spring ephemerals. For seeds germinated in autumn and in spring, they formed autumn-germinating plants(AP) and spring-germinating plants(SP), respectively. Germination time significantly affected the interval from sowing to emergence, seedling period, interval from 4-leaf rosette to flowering, flowering period, fruiting period, post-germination life span and other life history traits of AP and SP. AP had a longer post-germination life span, larger plants and higher silicle production than SP. The proportion of the reproductive period for the whole post-germination life span of SP was significantly longer than the proportions for AP, and SP allocated a larger proportion of total biomass to reproduction than AP.In conclusion, the wing of the dry indehiscent silicle promotes dispersal by wind and prolongs the duration of the transient soil seed banks. The presence of both nondeep and intermediate PD in the seed cohort ensures that the seeds germinate in autumn and in spring, respectively. Thus, this species formed autumn- and spring-germinating plants, which the post-germination life span, plant size, silicle production, the reproductive biomass to reproduction and other life history traits were highly significantly different. It may be viewed as adaptive bet-hedging in an unpredictable environment.