Selection of Rootstocks for Grafting
Grafting is a process that can be traced back in Plato’s writing (Barnes, 2005a). Since then, progress has been made in the selection and development of improved under stocks. This report would focus on the under stocks used to graft woody plants. The progress in the selection of rootstocks for woody plants involves interspecific and intraspecific grafts, improvement of precocity, production of quality rootstocks through tissue culture technique, and development of pest resistance.
The process of grafting generally involves the union of two different plants through a union of the cut surfaces (Matthews). It is composed of two parts, the rootstock or under stock and the scion. The rootstock is the lower part where the root system is established, on the other hand, the scion is the upper portion of the graft where the new shoot system take place (Matthews). Matthews elaborated the importance of rootstocks in the overall health and quality of plants. Primarily, it has influence on ‘stress tolerance, pests’ resistance, root anchorage ability, and graft compatibility’ (Matthews).
Plants are grafted either interspecifically or intraspecifically, depending on the degree of kinship exhibited by the plants. There is no specific pattern or trend available due to the complex nature of plant taxonomy which can prevent grafting with the same genus or even in the same species, as can be illustrated by Acer Rubrum and genus Quercus (Barnes, 2005 a). The process of looking for compatible combination involves rigorous research starting from the examination of seeds, flower formation and proximity of genus. In a study conducted by H.W. Barnes (2005a), there has been a favorable result implying the possibility of using Picea abis as rootstocks of Cedrus libani. Cedrus libani is native to Asian region and slow growing with a maturation of about 40 years. Thus, the use of grafting can improve its propagation.
J.H. Alexander (1998) highlighted that current taxonomic studies based on DNA is now being used to define the systemic relationships that can be beneficial for plant propagators. Rootstock selection, as mentioned above, is highly dependent on plant taxonomy. This explains the failure involved in grafting some plants that are thought to be of the same species. H. Barnes identified a major revision or reclassification on Chamaecyparis nootkatensis due to its incompatibility to Juniperus species which is highly unusual for common Chamaecyparis. The reclassification ensue after phytochemical and DNA studies proved that indeed Chamaecyparis nootkatensis is not related to Chamaecyparis genus. It is now changed to Callitropsis nootkatensis. The reclassification implies a change in rootstock selection from genus Chamaecyparis and Juniperus to genus Callitropis and Cupressus (Barnes, 2005b).
Precocity or early maturity is important in crop production. Recently, acorns are considered as edible crops. A research was conducted by Mark Coggeshall and colleagues (1998), from the University of Missouri, regarding the effects of rootstock and scion to the growth and early acorn production of Swamp White Oaks (Quercus bicolor). In reference to the report conducted by Dew and colleagues, that 3.5 % of White Oaks saplings produces acorn in as early as three years, Coggeshall and co-researchers (1998) found out that early-fruiting trait is under a strong genetic control. This conclusion is based on the data and observation that shows that irrigation and fertilization does not affect the precocity of the seedlings. Therefore, it is recommended to use grafting rather than seed propagation.
The increased demand for rootstocks generates an increase in production. In the Forest Home Nursery, a tissue culture facility was established in 1991 to meet the market needs for rootstocks. Basically, a first class rootstock is ‘disease free, robust and has high productivity (Hazell, 1999).’ Tissue culture has the potential to achieve high yield in shorter time (Hazell, 1999). It creates clones of superior quality rootstocks thus; it is inevitable that a mass production of high quality rootstocks can be produced. An increase in the production of quality rootstocks increases the productivity of crops that is essential to meet global standard and demands.
Disease is another concern for rootstock propagation. Jelle Heimstra and Bart van der Sluis (2005) studied the Development of Verticillium-Resistant Acer Rootstocks. Verticillium dahliae is a fungi infection affecting most of the shade trees (Heimstra and van der Sluis, 2005). The resistant seedlings are cloned and will be made available for growers in the future. The development of pest resistant or disease resistant rootstock is essential in grafting, since rootstocks can be used in combination to other species. The development of Verticullum-Resistant rootstock can be beneficial to other shade trees compatible with Acer rootstocks.
Grafting is usually used for propagation of high-quality woody plants. In the last 50 years, progress in the selection and development of rootstock includes the consideration of scion-rootstock compatibility, productivity, pests or disease resistance, and application of new technology. The changes in genus classification also have implication of the selection of rootstocks since it determines compatibility. The selection process in rootstock is complicated due to the complexity of plant taxonomy which permits or limits the possibility of grafting.
Alexander, J.H. “A Summary of Graft Compatibility from the Records of the Arnold Arboretum”. International Plant Propagators’ Society. Volume 48. (1998): 371.
Barnes, H.W. (A)“The grafting of Cedrus libani ‘Pendula’ onto Picea abies”. International Plant Propagators’ Society. Volume 55. (2005): 441-42.
Banes, H.W. (B) “A Taxanomic Revision of Chamaecyparis nootkatensis f. pendula and Implications for Rootstock Selection for Grafting”. International Plant Propagators’ Society. Volume 55. (2005): 443-45.
Coggeshall, M.V., Van Sambeek, J.W. and Garrett, H.E. “Scion and Rootstock Effect on Growth and Early Acorn Production of Grafted Swamp White Oaks”. International Plant Propagators’ Society. Volume 48. (1998): 418-22.
Hazell, B.S. “Propagation of Apple Rootstocks by Tissue Culture”. International Plant Propagators’ Society. Volume 59. (1999): 180-83.
Heimstra, J. and van der Sluis, B. “The Development of Verticullum-Resistant Acer Rootstocks”. International Plant Propagators’ Society. Volume 55. (2005): 421-22.
Matthew, H. “Discussion Group: Rootstocks of Choice”. International Plant Propagators’ Society. Volume 49. (1999): 626-28.