The influence of shoot growth on the pattern of axillary development on the long shoots of young apple trees (Malus domestica Borkh.)

The influence of shoot growth on the pattern of axillary development was investigated in apple (Mains domestica Borkh. cv. Braeburn) long shoots, either monopodial (all the metamers are produced by the same meristem) or sympodial (composed by a proximal inflorescence, the bourse, and a relay-axis, the bourse-shoot). All of the studied shoots were characterized by the formation of two growth flushes during the same year. The study first involved measuring primary growth components of individual metamers (leaf size and length of the internode below) in the year of shoot growth. Then, for each metamer, the type of related axillary structure was noted, either during the same year (anticipated axillary shoots) or during the next one after budbreak. In the latter case, five types of axillary structures were distinguished: vegetative axis, inflorescences from which fruit was set, inflorescences from which fruit was not set, aborted bud, and blind node. The first analysis compared the two growth flushes. It showed that the distribution of types of axillary structure in the first flush of growth differed considerably between monopodial and sympodial shoots in that monopodial shoots tended to have more inflorescence buds and anticipated laterals than sympodial shoots, which had more blind nodes. The distribution of types of axillary structure was rather similar in the second growth flushes of both monopodial and sympodial shoots. The second analysis substantiated the significant role of the parent metamer in determining the absence, if under a minimal size, or the presence of vegetative or generative branching. The third analysis showed a high dependence between the types of axillary structure of adjacent metamers along a given growth flush. This spatial analysis revealed the following order between types of axillary structure: blind node, vegetative bud, inflorescence bud that did not set fruit later in the season, and finally, inflorescence bud that set fruit. When proceeding from the proximal to the distal part of a growth flush, axillary structures were ranked as described above and then in the reverse sense. Since these axillary structures are known to correspond to various numbers of preformed appendages, these results suggested that the location of a metamer on a shoot strongly conditioned the axillary organogenetic activity. The shoot growth characteristics, notably the presence of successive growth flushes in the same annual increment, not only conditioned the location of vegetative side branches but also influenced the distribution of blind nodes and inflorescences.