Exploring bridge-grafting as technique te restore growth in girdled Ocotea bullata and Curtisia dentata in the Southern Cape forest area

Abstract

In South Africa, there is a growing concern regarding the sustainability of bark harvesting due to the reduced availability of medicinal trees in natural areas and the slow growing and slow-reproducing nature of South Africa’s indigenous trees, of which some have specific habitat requirements and a limited distribution. With an estimated 80% of all Africans in South Africa still relying on plant material for their basic healthcare needs, approximately 200 000 traditional healers and an estimated 63 000 commercial harvesters, medicinal plants are being exploited to extinction. The aim of the study was to determine whether bridge-grafting could be used to restore growth in girdled Ocotea bullata and Curtisia dentata trees as these two medicinal tree species are listed on the South African Red List as ‘endangered’ and ‘near threatened’ due to overexploitation. These trees were also historically used in furniture production and general carpentry, which increased these trees’ vulnerability to extinction. Bridge-grafting is a technique widely described in horticultural literature but has not been used to restore growth in medicinal tree species with extensive harvesting damage. Metabolomics as analytical method is a relatively new science, but it is very useful, accurate and repeatable in obtaining knowledge on the metabolites present in a plant, and for determining the concentrations of metabolites. NMR metabolomics is, however, not sensitive enough to quantify metabolites with very low concentrations such as plant hormones. The results achieved showed that O. bullata and C. dentata responded differently to girdling and bridge-grafting regarding location of callus initiation, callus development, rate of callus development, basal shoot initiation and development and their sucrose source-sink relationships. NMR metabolomics showed that seasonality was an important factor affecting metabolite responses in both species. NMR metabolomics also showed that after one year, there were no differences in responses above and below the girdle wounds, nor were there differences between the grafted trees and the normal control trees. iv Although the method of bridge-grafting was successful in restoring the growth of O. bullata and C. dentata with this study, much more research needs to be conducted to prevent endangered girdled medicinal trees from becoming extinct, to maintain tree species diversity, genetic diversity and biodiversity.