Aerial Seed Bank In A Cold Desert Annual-ephemeral Species Euclidium Syriacum (L.) R.Br.:role Of Anatomical Structure,Dispersal Dynamics And Germination Behaviors

Canopy seed bank is a unique phenomenon of desert annual plants formed in the course of evolution to adapt to extreme environment.Euclidium syriacum(L.)R.Br.is an annual-ephemeral plant of the Brassicaceae,widely distributed in Central Asia,Iran and the Caucasus.In China,this species is main to distributed in the northern Xinjiang,and is an important component of plant groups in this region.According to the field investigation,the fruit ripening period of the species has the characteristics of stem lodging and silicles delayed dispersal.In this study,the effect of canopy seed bank role of anatomy,dispersal dynamics and germination behaviors.The main results of are as follows:(1)Role of anatomical structure:?Compared with the Goldbachia laevigata(M.Beib.)DC.(control species),the cross section of the stem,the vascular bundles of the E.syriacum are more closely arranged,xylem and phloem are thicker;more phloem fibers are arranged in a continuous loop outside the vascular bundle;there are more small-caliber vessels,which make the stem stronger mechanical support;the appearance of thicker perimedullary zone,lower pith/stem ratio,and do not have pith cavity.The anatomical features of these stems explain the reason why the stem of the E.syriacum are erect and not lodging during ripening.?At the same time,from the longitudinal section of the base of the fruit stalk at the ripening stage of the E.syriacum,there was no cell dedifferentiation and cell wall disintegration in the cortex,so the base of the stalk did not form the detached layer.This is also the reason that fruit lodging in fruit maturity of the species does not fall off the plant.(2)Dispersal dynamics:?Under natural conditions,most of the silicles(85.1%,79.4%and 96.3%)and the seeds(84.5%,79.2%and 95.8%)in the three parts of the canopy(upper,middle and lower)remained on the plants before snowfall in winter(end of October).however,the percentage of silicles and seeds remaining on the sequence of the lower part of the canopy of plant was higher(P<0.05)than that on the silicles and its seeds in the upper and middle part of the canopy.During the snow melting period of the following spring(April 2019),most of the plants fell to the ground due to the pressure of snow cover,and the vast majority(about 80%)of the silicles(seeds)remained on the lodging fruit sequence,most of the seeds had germinated in the silicles,but about 14%of the fruits remained on the unflodded plants,and the seeds in the silicles did not germination.All the silicles of the three sites and their seeds fell off on the plants until late summer(early July 2019).During this period,there was a significant positive correlation between wind speed and the diffusion of fruits in the upper(r=0.74,P<0.05),middle(r=0.70,P<0.05)and lower(r=0.69,P<0.05)part of the seed bank of the plant canopy.It can be seen that the silicles can last at least one year remaining on the plant,so it forms a canopy seed bank.?Compared with natural rainfull,in increasing precipitation treatment significantly increased the percentage(P<0.05)of the silicles and seeds on the sequence of the three positions of plant in the critical period before snow cover and during the snow melting period in the spring of the following year.(3)Germination behaviors:Compared with soil?soil treatment(The silicles still buried in the original habitat soil;control),the germination rate of silicles in the canopy?soil treatment(To simulate the treatment of seed bank in natural habitat,the silicles is suspended in the crown first,and then buried in the soil at the appropriate depth when the fruit falls off in natural habitat)was significantly lower in each period(<20%),At the end of the experiment 15 months after treatment),the canopy?soil treatment of the viability of the seeds in the fruit under soil?soil treatment(90%)was significantly higher than the control treatment(34.2%)(P<0.05).Increasing precipitation treatment had no significant effect on seed germination and viliabily in silicles placed under canopy?soil treatments during most treatment periods(P>0.05).It can be seen that this species prolongs the germination time and viability of seeds in silicles by planting canopy seed bank.In conclusion,the anatomical characteristics such as stem and base of fruit stalk are a main reason for the formation of canopy seed bank in this species,which can prolong the time of seed dispersal and germination,not only reduce the risk of seed in adverse environment,but also guaranteed seed germination and establishing of seedling by choosing the right time to spread and germinate.The results can not only accumulate theoretical data for exploring the seed bank types and dynamic changes of desert annual plants,but also provide theoretical basis for analyzing the ecological maintenance mechanism of desert plants.