How can plants choose the most promising organs?

Abstract

Branch death is an important component of the processes that generate tree form. Many branches are formed and the plant appears to choose the most promising alternatives for survival and further development. The purpose of this chapter is to consider how this choice could occur in the absence of a central decision center. Experiments were carried out on a model system of pea seedlings with two shoots. In most plants only one of these shoots continued to grow. We studied the conditions that influence the choice of the winning branch. These included removing entire branches or leaves at different ages and modifying the environment of the entire plant or of individual shoots. The evidence from experiments and from comparative observations of many trees supports the following hypothesis. All components of a shoot are sources of auxin and possibly of other signals. The level of auxin synthesis depends on the immediate environment and the developmental stage of individual leaves. The responses to auxin include the orientation of vascular differentiation towards organs that are its strongest source. This oriented auxin response results in competition between alternative organs. This is a prominent example of developmental selection, an alternative to developmental programs and prepatterns, which also has various other roles in the generation of biological form.