Sieve Elements of Graft Unions

Abstract

In grafted plants the various relations between scion and stock are based on exchange of heterogeneous substances between the partners; many of them being organic compounds which are translocated in the symplast (see reviews by Brabec 1965 and Carr 1976). Although symplastic continuity between cells in the graft interface has often been postulated, reliable experimental proof was difficult to obtain because of the uncertainty in localizing the exact boundary between the partners at the cellular level. This problem has been overcome by investigating heterografts with species-specific cell markers (Kollmann and Glockmann 1985; Kollmann et al. 1985). Secondarily formed plasmodesmata have been recognized in the fusion walls interconnecting the partner cells in these heterogeneous systems. However, of main importance for the success, i. e. for a long-lasting survival of the graft, a connection of the vascular bundles of scion and stock has to be achieved (Vochting 1892; Simon 1908, 1930; Kuster 1925; Funk 1929; Hayward and Went 1939; see also the reviews by Krenke 1933 and Brabec 1965). Vascular connections are formed within a region of mixed tissue derived from both partners, which we define as the graft union in the strict sense. While xylem contact has been recognized with certainty and convincingly described in the early literature (Vochting 1892; Simon 1908; Crafts 1934; Silberschmidt 1935/36); continuous phloem connections have always been difficult to demonstrate unambiguously (Herse 1908; Crafts 1934) on account of the small size and irregular shape of the sieve elements (see also the review by Krenke 1933).