The relationship between graft incompatibility and phenols in Uapaca kirkiana Mu ¨ ell Arg. Simon A. Mng’omba a,b, *, Elsa S. du Toit a , Festus K. Akinnifesi b a Department of Plant Production and Soil Science, Faculty of Natural and Agricultural Sciences, University of Pretoria, 0002 Pretoria, South Africa b World Agroforestry Centre, SADC-ICRAF, Agroforestry Programme, Chitedze Agricultural Research Station, P.O. Box 30798, Lilongwe 3, Malawi 1. Introduction Uapaca kirkiana Mu ¨ ell Arg. (wild loquat), locally known in Malawi as ‘masuku’, is a highly valued indigenous fruit tree of the Miombo woodlands. The fruit is the most preferred by commu- nities in southern Africa (Maghembe et al., 1998) and can be eaten fresh or processed into juice, jam and a variety of wines. However, all the harvest comes from wild populations and there is little effort to renew these vital forest resources. The propagation of U. kirkiana has relied on conventional techniques which include the use of seeds, grafting and air- layering (Akinnifesi et al., 2004). Grafted U. kirkiana trees have fruited after 2–3 years compared to those that are raised from juvenile materials (seedlings) which have fruited after 10–12 years (Akinnifesi et al., 2006). Furthermore, grafted trees are dwarf, and hence easier and cheaper to manage than large trees (Usenik and S ˇ tampar, 2001; Webster, 2001). About 80% graft-take has been achieved with skilled grafters at Makoka Agricultural Research Station in Malawi (Akinnifesi et al., 2004). However, growth irregularities for the grafted U. kirkiana trees have been observed in the field and these growth irregularities could be signs of graft incompatibility (Mng’omba et al., 2007). The recent assessment of grafted U. kirkiana trees has shown declining survival trends from 98% at 12 months after field establishment to 67% at 33 months after field establishment (Akinnifesi et al., 2008). This low field survival could be attributed to graft incompatibility which is characterised by visibly poor callus formation, phenol accumula- tion and necrotic layer at the union (Mng’omba et al., 2007). These signs are prevalent above the union, especially for less compatible scion/stock combinations. U. kirkiana plants release a lot of exudates (phenols) in response to wounding. According to Hamisy (2004), DNA extraction from U. kirkiana leaves was adversely affected due to high production of phenols. Also, fruit darkening after harvest U. kirkiana fruits has been associated with high production of phenols. Histological study on grafted U. kirkiana trees revealed accumulation of deposits (phenols) at the union leading to cell or tissue death and lacuna layers above the union (Mng’omba et al., 2007). Phenols have been implicated in graft incompatibility (Pina and Errea, 2005) and differences in quantity or specific phenols above and below the union area play a role in reducing graft compatibility (Facteau et al., 1996; Usenik et al., 2006). Graft incompatibility is complex and involves many physiolo- gical and biochemical processes (Usenik and S ˇ tampar, 2000), but the main cause largely depends on the plant species (Andrews and Marquez, 1993). Although U. kirkiana plants produce a lot of Scientia Horticulturae 117 (2008) 212–218 ARTICLE INFO Article history: Received 1 August 2007 Received in revised form 20 March 2008 Accepted 27 March 2008 Keywords: Callus Euphorbiaceae Exudates Field survival Secondary metabolites ABSTRACT The objective of the study was to determine the relationship between phenols and graft incompatibility in Uapaca kirkiana. Phenol quantification and identification were carried out using Folin-Ciocalteau reagent procedure, fluorescence microscopy and reverse phase high performance liquid chromatograph (RP-HPLC) above, below and at the graft union. Results showed no vascular cambium continuity above the scion/stock unions. Significant differences in total soluble phenols and cell wall bound phenols were obtained. Fluorescence microscope indicated the presence of flavonoids and other polymers above the union. The RP-HPLC identified ferulic acid as a major phenol component found in U. kirkiana plant cells and responsible for wood discolouration. High phenol concentrations were obtained in less compatible combinations than in compatible combinations. High peaks of r-coumaric acid were obtained above the union. It is concluded that phenols, especially r-coumaric acids and flavonoids caused poor callus formation at the union, and hence implicated in graft incompatibility. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author at: World Agroforestry Centre, SADC-ICRAF, Agroforestry Programme, Chitedze Agricultural Research Station, P.O. Box 30798, Lilongwe 3, Malawi. Tel.: +265 1 707 332; fax: +265 1 707 323. E-mail address: SMngomba@cgiarmw.org (S.A. Mng’omba). Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti 0304-4238/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2008.03.031