Following the initiation and development of individual leaf primordia at the level of the shoot apical meristem: the case of distichous phyllotaxis in Begonia

Background and Aims: By using the technique of replicas of a developing apex it is possible to obtain a direct measure of phyllotactic parameters (plastochrone and platochronic ratio) involved in the initiation of two successive primordia at the level of the SAM. The goal of this study is to compare, in a real time setting, the value of phyllotactic parameters in distichous sytems using Begonia as a case study, with the value of the same parameters in spiral phyllotactic systems. Methods: To determine the real-time sequence of events at the level of the SAM, replicas were made of the developing apex at different intervals using previously described techniques. Impression moulds were made at 24-h intervals. The following phyllotactic parameters were measured: plastochrone, angle of divergence, plastochrone ratio and ratio between the diameter of the leaf and the apex. Results: The time between the appearance of two successive leaves is 15-20 d. The average value of the plastochrone ratio (R) is 1.3, and the ratio of the leaf to the diameter of the apex ( Gamma ) is 2.5. The angle of divergence varies from 165 degrees to 180 degrees . The speed of advection of the primordium from the apex, varies from 0.28 to 0.37 micro m d-1. Conclusions: The speed of advection of primordia in Begonia is lower than that of Anagalis. This is not in accordance with theoretical simulations that predict the opposite. In Begonia, the plastochrone ratio does not reflect the real time of appearance of two successive primordia. The time separating the appearance of two primordia is not directly related to the distance of these two primordia from the centre of the apex but is related instead to the enlargement of leaves..