Histological Anatomy Of Infection Process Of Arceuthobium Sichuanense

A.sichuanense was a hemiparasite angiosperm that only grew in China and infects Picea mainly. It could obtain so much nutriment and water from its host via its enormous and complicated endophytic system that the tree vigor became weak and dead finally. Branches of P. crassifolia which were being infected and had been infected were collected in Xianmi forest area in Qinghai Province. Those branches were used to observe infection progress of A.sichuanense and build and development of its endophytic system with paraffin sectioning and light microscope from the anatomical level. Buds of A.sichuanense fell off by ethephon. Branches of P.crassifolia had been infected were sprayed with different concentrations of ethephon and collected. Those branches were used to observe effects of ethephon on buds and endophytic system of A.sichuanense and host tissues with paraffin sectioning and light microscope from the anatomical level. The main results are as follows:(1) A portion of nutriment needed by germination of embryo and infection of radical were provided by endosperm of seed of A.sichuanense wrapped in viscin and no seed coat. Holdfast was generated when the apex of radical contacted host, then sinkers could be generated on the edge or in the center of bottom of hold fast. The periderm of host became thickened when the apex of radical contacted host, penetration site was transferred when penetration peg could penetrate into host in its first penetration sites, epidermal layer of host was squeezed by penetration peg and tissues of host became sunken and broken. Those phenomena showed resistance of host to infection of A.sichuanense. The number of host cortex cells increased where localized infection of A.sichuanense occurred.The endophytic system consisted of cortical strands within the bark and sinkers embedded in the xylem. The cortical strands extended by squeezing the cortex cells of host. When penetrating the periderm of host, the cortical strands developed aerial shoots; the cells of bark strands could divide and generate many new bark strands. When contacting with and near to the secondary phloem of host, the cells of outermost layer of cortical strands divided and developed sinkers. Then, the sinkers penetrated the phloem and cambium of host, extended to the xylem and grew along with the xylem rays of host. The periderm of host became thickened when the cortical strands penetrated it.(2) Exfliation of buds of A.sichuanense in different degrees was caused by ethephon with different concentrations. Mortality of buds was caused by ethephon in 1:200 was higher than ethephon with other concentrations. Ethephon started to kill the cells of bud and host near the bud from touchpoints between bud and host, and then ethephon permeated through host tissues gradually, and finally bud fell off. Subsequently, ethephon continued to permeate along bark strand which generated bud and kill the cells of bark strand and host near the bark strand till pesticide effect of ethephon disappeared completely. The results indicated that ethephon only killed the bark strands which could buds, this phenomenon showed that ethephon could not prevent and control endophytic system of A.sichuanense. Ethephon in high concentration 1:200) could cause larger damage to needles、buds and branch of host than ethephon in light concentration(1:500).