A mathematical model for storage and recall functions in plants

In plantlets of Bidens pilosa L., under severely limiting environmental con ditions the growth of the buds at the axil of the cotyledons (cotyledonary buds) is asymmetric (i.e. one of the buds starts growing before the other o ne), this asymmetry being oriented by the pricking of one of the cotyledons (i.e. pricking one cotyledon increases the probability that the bud at the axil of the other cotyledon be the first to start to grow). As long as the plant apex (i.e. the terminal bud) is present, the growth of the cotyledon ary buds is inhibited (apical dominance), bur the souvenir of the asymmetri c message caused by sub-optimal environmental conditions and the orientatio n given by the cotyledon pricking is always present in the plant and can be revealed by removing the apex. Depending on the conditions for removing th e plant apex and/or on the application of a variety of symmetrical treatmen ts (e.g. thermal treatment, symmetrical pricking treatments, etc.) the stor ed asymmetry will either take effect (the bud at the axil of the non-pricke d cotyledon will be the first to start to grow more often than the other on e) or not (both buds will have equal chance to be the first to start to gro w). This has been termed 'recalling' the stored asymmetry. By combining sev eral successive symmetrical treatments, it is possible to reversibly switch on and off the recall function several times. This recall of the stored pl ant-asymmetry is analogous to the evocation function of a memory system. In this paper, we will present first a discrete logical version of the observ ed interaction structure between the main components of the bud growth syst em, then a continuous differential version, taking into account the main fe atures of the observed experimental reality and trying to explain this phen omenology. The interaction structure of both the discrete and the continuou s models presents similar positive and negative feedback circuits, necessar y condition for observing multistationarity and stability. (C) 2000 Academi e des sciences/Editions scientifiques et medicales Elsevier SAS.