Relations between cytokinin level, bud development and apical control in Norway spruce, Picea abies

Abstract

In conifers such as Norway spruce, the extent of shoot growth is predetermined by the size and number of embryonal organs of the buds laid down the previous year. As it is known that cytokinins have a key role in bud development a possible hypothesis is that the level of cytokinin in the buds during their formation determines their size and complexity. As a first step to test this hypothesis we compared cytokinin levels in buds of different size of annual shoots from 15‐ to 20‐year‐old trees of Picea abies (L.) Karst. Apical buds from the leaders, and from branches in lower parts of the trees, were collected in April, July and August. The difference in size of the buds and the shoots growing from them was considerable in these three positions. Extracts were purified by immunoaffinity columns, and the retained compounds were separated by high‐performance liquid chromatography (HPLC). Quantification was made by enzyme‐linked immunosorbent assay (ELISA), and the accuracy of this method was checked by measurements with liquid chromatography‐mass spectrometry (LC‐MS) and UV absorption. Zeatin riboside (ZR) was the most abundant cytokinin, but isopentenyladenosine (iPA) was also present in all samples. The large apical bud of the leader contained much higher cytokinin concentrations than the considerably smaller buds from lower positions, and during the period of secondary growth in July, similar relationships were found for annual stem tissue from different positions. The possible role of ZR as a controlling factor in bud development and apical control is discussed. Our conclusion is that the level of zeatin‐type cytokinins appears to play an important role in the establishment of differences in bud size and, thereby, the architecture of the tree crown.