Morphological and cytological studies on Arceuthobium (Viscaceae) in relationship to host phloem with studies on the healthy phloem in Pinus sabiniana (Pinaceae)

Abstract

Anatomical and developmental tissue relationships between Arce~thobium spp. endophytic tissues and host vascular tissues were examined by light and transmission electron microscopes. The host-parasite pairs studied were Psudotsuga menziesii/A. douglasii, Tsuga heterophylla/A. tsugense, Pinus sabiniana/A. occidentale, and P. lambertiana/A. californicum.The morphological form and growth characteristics of A. douglasii in different aged host tissues was found to be coordinated with growth and maturation of the host. It is proposed that morphological forms of Arceuthobiwn endophytic tissue be categorized as 1) primary, 2) diffuse-secondary, or 3) localized-secondary in order to semantically clarify the relationship of endophytic morphology with primary or secondary growth stages of host tissue.In localized-secondary endophytic forms, the parasite integrates with host rays to form multiseriate infected rays. At both the light and electron microscope level, parasite cells can usually be identified by their distinctive chromocentric nuclei and abundant lipid bodies or lipid ghosts. Sinker cells of Arce~thobium have unusual plastids which resemble etioplasts and which do not store starch. Sinker cells have distinctive mitochondria with unusually large nucleoid areas. They have abundant endoplasmic reticulum. Wall/plasmalemma specializations increase the membrane surface area in relation to cell volume in sinker cells.The walls of host and parasite are fused at the middle lamella common to both organisms and the organisms share a common apoplast. Pit-like regions in the fused walls of the host/parasite interface were commonly seen in light microscope studies, although such interspecific pitting is seen less than intraspecific pitting. It was determined that, whereas intraspecific pits are traversed by complete plasmodesmata, interspecific pits had no plasmatic channels, or, only half plasmodesmata on the host side of the pit. In one case a half plasmodesmata was seen on the Arceuthobiwn side of the host/parasite interface.On the basis of the electron microscope studies of the host/parasite interfaces it appears that plasmatic connections between host and parasite do not normally occur. Because of this plasmatic isolation it can be concluded that nutrient acquisition does not involve direct flow of nutrients via interspecific symplastic bridges. Therefore, photosynthate, normally housed within phloem cells, must be leaked into the common apoplast of both host and parasite before becoming available for absorption 3 into the parasite symplast. Since host and parasite lack symplastic continuity but share a common apoplast, apoplastically-mobile herbicides should be tested for their ability to accumulate in parasite tissues. 4 Cytopathological effects on the host cells were relatively mild although a significant increase in the ratio of radial to axillary vasculature was noted in infected tissue. other modifications included a tendency for increased numbers of specialized phloem parenchyma (Strasburger cells) in infected ra...