The 'ideotype concept' and its application to the selection of cultivars of trees providing agroforestry tree products

Abstract

The ideotype concept has been developed and modified for a number of different crops, including forest trees, over the last 35 years. In recent years it has been used in the domestication of agroforestry trees producing Agroforestry Tree Products (AFTPs), as an aid to the multiple-trait selection of superior trees for cultivar development. For example in west and central Africa, 'fruit' and 'kernel' ideotypes have been identified in Irvingia gabonensis (Bush Mango), based on quantitative characterization of a number of fruit, nut and kernel traits. Subsequently, recognizing the opportunity to develop different markets for the commercially important kernels, the 'kernel ideotype' has been subdivided into 'oil' and 'food-thickening' ideotypes, with options for further subdivision of the latter into ideotypes with either high viscosity or high drawability characteristics. Similar opportunities for the development of single-purpose cultivars from multi-purpose species have been identified in Sclerocarya birrea (Marula), indicating that with increasing information about the variability of AFTPs it is possible to develop a hierarchy of different ideotypes to meet different market opportunities. In S. birrea and Dacryodes edulis, future options exist to extend ideotypes with recognition of the variability in both nutritional and organoleptic qualities. In Australia, a very broad-based ideotype has been developed for the indigenous shrub Kunzea pomifera (Muntries) established on both morphological and physiological traits. Similar approaches are identified for timber trees (e.g. Triplochiton scleroxylon) and those like Santalum austrocaledonicum producing essential oils. It is concluded that ideotypes are a useful tool for visualizing and conceptualizing how to combine specific rare combinations of visible and invisible traits, aimed at the maximization of harvest index, even when the traits are only weakly related.