Abstract
It is commonly assumed that the modular structure of plants results from two basic processes: the apical growth process by which the plant builds the monopodial shoot, and the branching process initiated by the development of the axillary meristem. The architectural analysis proposed some 40 years ago by Hallé and colleagues (Hallé et al., 1978) considers the actual morphology of a plant as resulting from a balance between the deterministic genetic blueprint specific to the taxon and the environmental cues. A finite number of criteria have been defined to analyze tree architecture which all refer to the behaviour of meristems: fate (i.e., vegetative or reproductive), growth orientation (i.e., vertical, orthotropic or any other directions, plagiotropic), growth rhythmicity (i.e., continuous or rhythmic), distribution along the parent shoot, development with (delayed or proleptic) or without (immediate or sylleptic) a resting period, etc. These criteria refer to growing meristems. However non-growing meristems play an important role in plant architecture. These meristems belong to two categories. The first one includes meristems remaining latent (dormant) as buds. They usually serve as a reserve of meristems within the individual plant and are typically involved in the proleptic reiteration process by which the plant regenerates the whole or a part of its architecture. Another non-growing meristem category includes meristems which permanently lose their growth potential through abortion ultimately involving cladoptosis, i.e., the shedding of a single shoot or of a whole-branch complex. These phenomena are usually described as an adaptation to environmental stresses, e.g., shade and drought, possibly mediated by a decrease in the hydraulic conductance. However, it is known that the permanent loss of meristematic activity may be highly predictable revealing a genetic or an epigenetic basis. This is typically the case of meristem extinction which has been coined in apple to describe meristem abortion with the following characteristics: independent of external (light, temperature) conditions and likely involving a genetic or epigenetic determinism, precocious (as soon as in the first weeks following bud burst), and in most of the cases involving reproductive laterals. A relation between frequency of extinction and development of the remaining laterals, especially an increase of the frequency of flowering, has been shown. When and where extinction occurs is thus as important in defining shoot and plant architecture as the analysis of the distribution of growth characteristics of shoots. This interweaving of growth and death processes in the dynamics of plant form follows a known Atlan’s principle, since it appears to be a specific case of the statement established by Henri Atlan (1979) at cellular and tissular levels, mainly in animal science, “Sans processus de mort contrôlée, pas de processus de vie”.