Light quality regulates shoot cluster growth and development of MM106 apple genotype in in vitro culture

Abstract

The interaction between shoot proliferation and regulative action of light quality was investigated in terms of cluster growth and development. In the commercial laboratory, the shoot multiplication procedure is normally conducted by growing clusters of shoots during several sub-cultures. Thus in this work the regulative action of light quality was evaluated by analysing the shoot cluster proliferation rate, the evolution of axillary bud differentiation, the axillary shoot development on both initial explant stem (order 1 axis) and three representative lateral branches (order 2 axes), growing from the lateral buds located at the bottom of the initial explant. Experiments showed that light qualities regulate bud differentiation and interact with apical dominance, leading to strong control of axillary bud outgrowth, and, finally, resulting in a differentiated architectural formation of clusters and type of branching. Two independent actions were observed, one dependent on blue light and the other dependent on red light, evoking the role of specific photoreceptor systems in plant response. Blue and UV-A light increased the number of buds differentiated from the apical meristem, without affecting apical dominance. An opposite trend was induced by red, yellow and green light. The involvement of phytochrome became clear under red light, while possible specific receptors and/or specific interaction among the known photoreceptors can be postulated under the other two light qualities. Results confirm that in apple, proliferation is the product of the interaction between two biological responses, lateral bud differentiation, which can be defined as potential proliferation, and new shoot development, which is the effective proliferation, and both responses are regulated by light quality.