Báo cáo khoa học: Effect of a feedback control by early secondary xylem formation on the outcome of competition between leafy shoots in apple (Malus x domestica Borkh.)

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Báo cáo khoa học: "Effect of a feedback control by early secondary xylem formation on the outcome of competition between leafy shoots in apple (Malus x domestica Borkh.)"Effect of a feedback control by early secondary xylemformation on the outcome of competition between leafyshoots in apple (Malus x domestica Borkh.)A. Jebari J. CrabbéMorphogen6se V6g6tate Appliqu6e, Facult6 des Sciences Agronomiques, Gembloux, Belgique by the ratio of their upper shoots is evaluatedIntroduction lengths (L Furthermore, anatomical inves- ) 2 /. 1 tigation was carried out in order to evaluate initiation and expansion of cambial activity, newFrom bud break onwards, a developing secondary xylem characteristics and their varia-shoot induces, within its bearing stem, a tions.strand of secondary xylem, which progres- experiment was limited to the spring Thesively expands basipetally along the stem growth flush, i.e., from bud break to late June.and spreads tangentially around it. Therate of this process and the differentiatedwood characteristics, e.g., frequency andwidth of vessels, depend upon the rate of Resultsshoot elongation (Jebari and Crabbe, inpreparation). The varying conducting ca- On a pruned woody axis, the outcome ofpacity resulting from these early events competition between the 2 uppermostaffects the outcome of shoot competition shoots is very uncertain. When a largeand plays a part in the complex control of population of plants is pruned as statedthe apical dominance syndrome, in woody above, a continuum is generally observedplants. between 2 extreme habits: strong domi- nance of shoot 1 on one side and forking due to subequal elongation of both shoots on the other.Materials and Methods In fact, in most plants, shoot 1 has, owing to its distal position, a precedencyOn 1 yr old apple trees, cv. MM 106, the unique (acrotony) which makes it start growingaxis was pruned at ca 60 cm high before budburst and debudded except for the 2 uppermost first - or, at least, make up some initialbuds (designated as buds/shoots 1 and 2, 1 retardation and overtake shoot 2 - andbeing the most distal) and, when stated, bud 4 then amplify its advance during the re-or 5, located on orthostichies next to bud 1 or 2, maining season. However, when shoot 1respectively. is very delayed, a fork arises because All shoots present were periodically mea- shoot 2 is only able to maintain its smallsured. The degree of dominance between the 2 upon cultural conditions andinitial advance and cannot amplify it by depending treatments (see below). Fig. 1 showsgrowing faster than shoot 1. The latter it may these extremes in shoot elongation is far from uncommon:case behavior in selected subpopulations.amount to 40% of the total population, dominance in selected subpopulations Among the conditions favoring either where deviation infringes upon the smallextreme behavior, we could mention the angle between 1 and 2 (top) or on thepresence of a third shoot. For example, larger one (bottom) and where bud 1 is fa-bud 4 which is next to bud 1’s orthostichyincreases the probability of strong domi- vored by the presence of bud 4 (left) ornance of this latter; whereas bud 5 near bud 2 by that of bud 5 (right).bud 2 favors the occurrence of a fork Anatomical investigation gives a pos-(Jebari and Crabbe, 1984; Crabbe, 1985). interpretation of these rather sur- sible prising results. In a large population of This is, however, further complicated by plants differently treated, 5 groups werethe frequent helical deviation of the basi- selected according to the time at whichpetal progression of the secondary xylem shoot 1 overtakes shoot 2 or to mean finalstrands, w ...