Un cas de ramification sympodiale à déterminisme endogène chez un système racinaire:Platanus hybridaBrot.

Abstract

Summary Morphological study of the Plane tree shows that the root system of this species has a completely sympodial structure. The layout of root sympodial units leads to a pseudomonopodial organization, never been observed in roots. The endogenous origin of this sympodial development is discussed. Abridged version Different works show us the occurence of sympodial branching in roots (Gill and Tomlinson, 1977; Oldeman, 1990). Usually, this type of branching is considered as a resuH of injury which accidentally destroy apical meristem (Wilson, 1970). Thus, analysis intended to understand structure and development of plane tree root system (Platanus hybrida Brat. Platanaceae) lead us to revise this interpretation. We made our observations in natural habHat near Montpellier in the South of France. We have completely freed root systems of thirty individuals from seedlings to aduHs developing Hs first main orthotropic branches. For each individual, we have described the morphology of each root taking in account its direction, its cambial growth, its life expectancy, and its branching patterns. By comparing different root systems at different stages of the plant development, we have deduced morphogenetic processes occuring during ontogenesis. At first, we made a morphological analysis of branching patterns. We first observed very frequent death of apical meristem in each freed root. The death of root apices is often accompanied by the formation of one or several subterminal axes which replace and extend the root (Fig. 1). This typical sympodial branching gives rise to units or modules, all showing the same organization: a basal part bearing non woody laterals with determinate growth and an apical part near the dead tip, bearing woody laterals Including one or several replacement axes (Fig. 1). We have studied the extent of this process in Plane tree root systems. We have observed that all woody roots of this system, whatever their age and stage of development, are built up by these modules (Fig. 2, 3, 4). Sympodial branching occurs regularly in the whole root system, except in non—woody roots (Fig. 5), but may occur in two different ways: In the first case (A4 on Fig. 4a), each module bears only one replacement unit. The root (A4, Fig. 4a) is composed of a linear series of modules, some of which bearing differentiated woody laterals in their sub—terminal part. The whole resembles a monopodium. In the second case, each module bears two or three replacement members (arrow on Fig. 4a) in such a way that the root forms a folK composed of two or three branches. The frequency of syrrpodial branching, the regularity of its occurence during ontogenesis and the similarity of the modules it gives rise to can't be explained only by a simple traumatic interpretation whose necessarily random character doesn't agree with our observations on Plane tree root system. On the contrary this species shows us that root sympodial branching may have an endogenous origin. We have now to analyse the extent of this process in woody species, the results of this pattern of branching on the way of building root systems and the morphogenetic process on which it is based.