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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Nội dung Text: 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 xylem formation on the outcome of competition between leafy shoots in apple (Malus x domestica Borkh.) A. Jebari J. Crabbé Morphogen6se V6g6tate Appliqu6e, Facult6 des Sciences Agronomiques, Gembloux, Belgique by the ratio of their upper shoots is evaluated Introduction lengths (L Furthermore, anatomical inves- ) 2 /. 1 tigation was carried out in order to evaluate initiation and expansion of cambial activity, new From 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 The sively expands basipetally along the stem growth flush, i.e., from bud break to late June. and spreads tangentially around it. The rate of this process and the differentiated wood characteristics, e.g., frequency and width of vessels, depend upon the rate of Results shoot elongation (Jebari and Crabbe, in preparation). The varying conducting ca- On a pruned woody axis, the outcome of pacity resulting from these early events competition between the 2 uppermost affects the outcome of shoot competition shoots is very uncertain. When a large and plays a part in the complex control of population of plants is pruned as stated the apical dominance syndrome, in woody above, a continuum is generally observed plants. 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 precedency On 1 yr old apple trees, cv. MM 106, the unique (acrotony) which makes it start growing axis was pruned at ca 60 cm high before bud burst and debudded except for the 2 uppermost first - or, at least, make up some initial buds (designated as buds/shoots 1 and 2, 1 retardation and overtake shoot 2 - and being 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 1 respectively. is very delayed, a fork arises because All shoots present were periodically mea- shoot 2 is only able to maintain its small sured. The degree of dominance between the 2
upon cultural conditions and initial advance and cannot amplify it by depending treatments (see below). Fig. 1 shows growing 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 small extreme behavior, we could mention the angle between 1 and 2 (top) or on the presence of a third shoot. For example, larger one (bottom) and where bud 1 is fa- bud 4 which is next to bud 1’s orthostichy increases the probability of strong domi- vored by the presence of bud 4 (left) or nance 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 were the frequent helical deviation of the basi- selected according to the time at which petal progression of the secondary xylem shoot 1 overtakes shoot 2 or to mean final strands, which proceed seemingly at ran- degree of dominance. The last of these dom clock- or counterclockwise. The out- groups comprises the forked plants. come of competition between the 2 upper- most shoots consequently results alto- Considering the relative elongations of gether from which one sets growth on first, both shoots together with the relative from the presence of a third bud and from width of new xylem formed below the the direction of this deviation. Fig. 2 dis- shoots in the bearing axis, it clearly plays the distribution of the degrees of appears (Fig. 3) that, in the last group,
shoot 2 invests relatively more matter in sion cannot be discarded as part of the wood formation per unit shoot length than explanation. However, cambial activity shoot 1. The resulting increase in con- also interferes by long-term effects ducting capacity explains why shoot 2 through variation in the conducting capaci- successfully opposes and stays abreast of ty of the main stem. its competitor. References Discussion and Conclusion Crabbe J. (1985) Aspects of the apical control branching in one-year-old caulinary axes of on The control of secondary xylem differentia- woody plants. Acta Univ. Agric. Brno 33, 555- tion by shoot growth in the bearing stem 560 below is not easy to explain. It differs Crabbe J. (1986) Hi6rarchie et opportunisme markedly according to the complex domi- dans le développement de I’appareil caulinaire nance relations of the shoots: extreme des arbres. Nat. Monspel. Ser. Bot. suppl. 339- 350 habits, like strong dominance of the distal Jebari A. & Crabbe J. (1984) Aspects of the shoot or forking, proceed from different interaction between the two upper shoots left on modes of regulation. a pruned one-year-old axis in apple. 4th Congr. In the competition among shoots, the Eur. Fed. Soc. Plant Physiol., book of Abstr. 244-245 ‘classical’ hormone-directed nutrient diver-