Formation of pseudowhorls in Peperomia verticillata (L.) A. Dietr. shoots exhibiting various phyllotactic patterns

Abstract

Pseudowhorls are composed of leaves attached at almost equal levels and separated by single fully elongated internodes. In Peperomia verticillata, pseudowhorls form regularly in shoots exhibiting both spiral and truly whorled patterns of phyllotaxis. In spiral systems, they are composed of successive leaves positioned on the ontogenetic helix. In whorled phyllotaxis, leaves of two adjacent whorls occur at almost the same level and this way form a pseudowhorl. The number of leaves per pseudowhorl depends on the type of phyllotactic pattern and also the system of primordia packing. In all the shoots, regardless of the type of phyllotaxis, the number of leaves per pseudowhorl equals the number of leaf primordia in physical contact with the apical dome. It is the same as the higher number in contact parastichy pairs in spiral patterns or the number of orthostichies in whorled phyllotaxis. The pseudowhorled pattern is already manifested in the arrangement of leaf primordia. In spiral and whorled phyllotaxis the plastochron ratio calculated for primordia or whorls belonging to adjacent pseudowhorls is always higher than that calculated for members of one pseudowhorl. Moreover, angular distances between primordia of one pseudowhorl in spiral patterns are more uniform than expected in Fibonacci phyllotaxis. These observations were made on plants both growing in pots and cultured in vitro. 6-Benzylaminopurine, a synthetic cytokinin, added to the medium increases the mean number of leaves per pseudowhorl. It seems that this effect is indirect: phyllotaxis changes first rather than the destiny of a particular internode in a process of selective elongation.