STUDIES OF VEGETATIVE COMPATIBILITY-INCOMPATIBILITY IN HIGHER PLANTS. IV. THE DEVELOPMENT OF TENSILE STRENGTH IN A COMPATIBLE AND AN INCOMPATIBLE GRAFT

Three phases of adhesion between the stock and scion are observable during the formation of a compatible autograft in Sedum telephoides and an incompatible heterograft between Sedum telephoides and Solanum pennellii. The first phase of adhesion is similar in both systems in that it 1) lasts 2 to 3 days, and 2) is characterized by an average increase in tensile strength of 1 g breaking weight (BW)/mm2 graft area (GA)/day. In the compatible Sedum autograft, the second phase of adhesion lasts from Days 3 to 11 after grafting and is correlated with a 28-fold increase in the tensile strength of the graft union to approximately 56 g BW/mm2 GA by 11 days after grafting. The third phase of adhesion in the compatible autograft is characterized by a leveling off of the tensile strength of the graft union at approximately 56 g BW/mm2 GA, roughly equal to that of an ungrafted internode. Graft formnation is now complete. These results suggest that the ratio of the tensile strength of the graft union: tensile strength of a comparable ungrafted internode provides an estimate of the percent development of compatible autografts. In the incompatible heterograft between Sedum and Solanum, Phase II adhesion 1) lasts from Days 2 to 5 after grafting, and 2) peaks at 12 g BW/mm2 GA at 5 days after grafting. Phase III adhesion in the incompatible heterograft occurs subsequent to Day 5 after grafting and is characterized by an average decrease in the tensile strength of the graft union of 0.3 g BW/mm2 GA/day. The results of this study are discussed relative to the quantitative contributions of various structural events to the tensile strength of a graft union. ALTHOUGH STEM GRAFTS provide an excellent system for studying cellular and tissue interactions in plants, most aspects of graft development have been little investigated and remain poorly understood. This is not to say, however, that advances are not being made toward understanding some of the processes associated with graft development. Recent research on plant grafting has taken several different approaches, including studies of the structural (Stoddard and McCully, 1979; Moore, in press; Moore and Walker, 198 la, b), biochemical (Gur, Samish, and Lifschitz, 1968; Copes, 1978; Yeoman et al., 1 978; Moore and Walker, 1981 c), and physiological (Stoddard and McCully, 1980) aspects of graft formation. Indeed, the most thorough documentation of a proposed mechanism for graft incompatibility has resulted from some of this work (Gur, Samish and Lifschitz, 1968). The I Received for publication 10 December 1981; revision accepted 14 April 1982. This research was supported by grants from the University Research Committee of Baylor University, Sigma Xi, and the American Philosophical Society. Special thanks are extended to Ms. Barbara Wimpee for her excellent technical assistance. salient features of many of the studies cited above have been reviewed recently (Moore, 1981a). This paper is the forth in a series aimed at elucidating the mechanisms of compatibility and incompatibility that occur in plant grafts. We have previously reported that lethal cellular senescence characterizes the incompatibility response between Sedum telephoides (Crassulaceae) and Solanum pennelii (Solanaceae) (Moore and Walker, 198 ib). Furthermore, this senescense is positively correlated with the release of the hydrolytic enzyme acid phosphatase (E.C. 3.1.3.2) into the cytosol of the Sedum cells located along the graft interface (Moore and Walker, 1981 c). In this study, we have experimentally monitored graft formation in the 1) compatible autograft in Sedum, and 2) the incompatible heterograft between Sedum and Solanum by measuring changes in the tensile strength of the graft union over time. MATERIALS AND METHODS-Plant growth conditions-Clonal populations of Sedum telephoides and Solanum pennellii were grown in a porous soil mixture in a controlled-environment growth chamber. The plants were placed under a daily regime of 15 hr of light and 9 hr