Some Ultrastructural Aspects of Metaphloem Sieve Elements in the Aerial Stem of the Holoparasitic Angiosperm Epifagus Virginiana (Orobanchaceae)

Abstract

A light and electron microscope investigation was conducted on phloem in the aerial stem of Epifagus virginiana (L.) Bart. Tissue was processed at field collection sites in an effort to overcome problems resulting from manipulation. At variance with earlier accounts, Epifagus phloem consists of sieve elements, companion cells, phloem parenchyma cells, and primary phloem fibers. The sieve elements possess simple sieve plates and the phloem is arranged in a collateral type of vascular bundle. In addition, this constitutes the first study on phloem ultrastructure in the aerial stems of a holoparasitic dicotyledon, an entire plant which could be viewed as an “ideal sink.” Epifagus phloem possesses unoccluded sieve plate pores in mature sieve elements and a total lack of P‐protein in sieve elements at all stages of development. Mature sieve elements lack nuclei. Plastids were rarely observed in mature sieve elements. Vacuoles with intact tonoplasts were encountered in some mature sieve elements. Otherwise, the ultrastructural features of sieve elements appear to differ little from those described by investigators of non‐parasitic species.

Structure of the Host-Parasite Interface of Boschniakia Hookeri Walpers (Orobanchaceae)

“…In two other genera of Orobanchaceae, Orobanche and Epifagus, sieve elements have been conclusively demonstrated (DOERR & KOLLMANN 1975;WALSH & POPOVICH 1977), and phloem may therefore be expected in at least the stem also of Boschniakia. In the tissues adjoining the interface, however, we have failed to find any phloem-like cells.…”

Shoot Structure of Boschniakia hookeri Walpers (Orobanchaceae)

“…Howeuer, no "sieue tubes" with sieue plates were obserued by Perciual ( 1931), leading him to the conclude that they had been eliminated in this aduanced parasite. Perciual's ( 1931) obseruation stands in conflict with Boeshore ( 1920), who concluded that from the hemiparasitic figworts through the holoparasitic broomrapes Of interest, the ultrastructural study of Walsh and Popouich { 1977) found that the sieue tube members in the aerial stem of the broomrape Epifagus uirginiana lacked P-protein, a condition nouel to a holoparasite. Further study, particularly at the ultrastructural leuel, is needed to confirm the presence and significance of P-protein in the sieue elements of holoparasitic angiosperms.…”

“…Further, Boeshore ( 1920) concluded that a perennial parasite fauored selection for host-specificity. The aduanced holoparasitic broomrapes are eHemplified by the monotypic genus Epif agus which parasitizes only members of the genus Fagus (Cooke and Schiuely 1904;Walsh and Popouich 1977), C:onopholis which parasitizes members of the genus Ouercus (Boeshore 1920, Perciual 1931, and Boschniakia hookeri which parasitizes only perennial genera of the Ericaceae. This study confirms that B.…”

“…Due to the problems associated with the study of phloem, few workers haue conuincingly demonstrated the presence of sieue tube members in parasitic members of Scrophulariaceae and Orobanchaceae (Dorr and Kollman 1975;Dorr, Kollman and Uisser 1979;Kuijt and Dobbins 1971;Walsh and Popouich 1977; Worsdell 1895). The accepted euidence of sieue tube members is sieue plates containing pores.…”

“…eHamination of the perennial parasitic body. Sieue tube members haue been identified in phloem at the haustorial interface in Orobanche(Dorr and Kollman 1975).I nuestigations of aerial shoot structure in broomrapes haue included B. rossica (6ariliuk 1965), Christisonia(Worsdell 1895), Eoiphegus(Cooke and Schiuely 1904) or Epifagus(Walsh and Popouich 1977), and Orobanche(Beck 1930;Tate 1925) species. The shoots of these broomrapes haue been generallycharacterized as hauing a single ring of uascular bundles, abundant phloem, sclereids in the ground tissues, reduced numbers of stomata, scale leaues with mesophyll not differentiated into spongy and palisade parenchyma, and greater deuelopment of phloem than Hylem (Boeshore 1 920; Metcalfe and Chalk 1950; Solereder 1908).…”