S01 - Session P6 - Progress towards early identification of factors important for scion-rootstock compatibility in Litchi sinensis in South Africa

Authors: Karin Hannweg *, Oscar Maphanga, Mark Penter, Ziyanda Shezi, Elliosha Hajari

South Africa's litchi industry relies on only a few cultivars, thus reducing exposure on markets. Because of the short marketing window, several cultivars were sourced from Australia and Israel to determine not only their adaptability to South African climatic conditions, but also their ability to extend South African litchi penetration into the market. Unfortunately, many of the newly-sourced cultivars appeared to be incompatible with 'Mauritius' rootstock, which is traditionally used in South Africa. Scion-rootstock compatibility is a prerequisite for successful grafting and subsequent plant growth, and a fully functional vascular system must develop between the cultivar and rootstock, thus ensuring optimal uptake and transport of water, nutrients, assimilates and hormones. Graft incompatibility results in unhealthy trees, which may survive but do not perform optimally in the field. The success of any graft is dependent on graft healing at the graft union. Healing results in collapse of the ruptured cells caused by cutting, followed by adherence of the cells of the rootstock and scion at the graft junction and subsequent callus formation. The final critical step is callus differentiation and formation of vascular tissue. The actual underlying reasons for graft incompatibility are unknown, but are thought to be dependent on several variables including anatomical, biochemical, physiological and genetic factors. Several rootstock cultivars were assessed for compatibility to selected scion cultivars by measuring factors, such as graft success, shoot growth and graft union development, and physiological factors, such as chlorophyll content and assimilation rate. Some differences between the various combinations were already observed six months after grafting. Although compatibility is difficult to predict, the success of graft compatibility generally only occurs between cultivars and rootstocks that are taxonomically 'closely' related, and therefore incompatible grafts do not form proper vascular continuity due to insufficiently close relationships. With this in mind, the results obtained will be used towards developing a model for predicting compatible combinations. Compatibility is essential for ensuring that the best quality young plants are produced for optimum and sustainable field performance.