Comportement de drageons de pommier (Malus pumila Mill.) issus des cultivars Golden Delicious et Stark Golden Spur

Après enracinement sous brouillard de boutures des cv. Golden Delicious et Stark Golden Spur, il a été possible d’obtenir des drageons sur les racines de ces deux variétés. En général, les arbres multipliés à partir des drageons de Stark Golden Spur ressemblent aux arbres issus des drageons de Golden Delicious traduisant ainsi l’existence d’un génotype non muté dans les couches profondes de Stark Golden Spur. Un drageon fait exception, vraisemblablement néoformé à partir d’une racine issue de la couche LII et, par conséquent, totalement constitué de cellules de type spur. Ce nouveau clone devrait constituer un type spur stable en culture. Par ailleurs, tous les arbres issus de drageons diffèrent de leur variété témoin pour des caractéristiques bien marquées, telles qu’un accroissement des ramifications en nombre et en longueur, une réduction des surfaces foliaires, un retard dans la mise à fleurs. Ces différences s’estompent avec le temps et ont pratiquement disparu après 5 ans. La signification de ces modifications physiologiques est discutée par rapport aux phénomènes de juvénilité.

Rooted cuttings of apple cvs Golden Delicious Standard and Stark Golden Spur were grown for several years according to figure 1. The trees were then cut off at the roots. Neoformed buds developed on the roots and gave new shoots. Five buds of these shoots were then propagated on a clonal root stock (MM 106). The maiden trees which originated from neoformations on Golden Delicious roots (R in the tables) were all different from the mother variety by common features : increased branching, smaller leaves (at significant level) and taller and thinner habit (table 1). Under cultivation for 5 years, growth, measured by trunk girth and total shoot length, was close to the control (table 3 and fig. 2). Number of flowers per tree remained lower than the control during the first two years (table 4), gradually reaching the control level (fig. 3). Similarly, leaf areas increased progressively to the control size (fig. 3). According to Zimmerman’s scheme on juvenility (1976), this phenomenon can be interpreted as a shifting back, more or less complete, to the juvenile stage of a seedling. The maiden trees which originated from neoformations on Golden Stark Spur roots fell into two patterns. On the one hand, those which originated from r1, r3 and r4 roots differed strongly from the Stark Golden Spur control by increased growth (trunk girth, number of branches and total shoot length). Due to various diseases (chlorotic leaf spot, stem pitting and rubbery wood), these trees were significantly smaller (10 %) than those of Golden Delicious, but their general habit was of a standard type (fig. 2). These data suggest strongly that the deeper layer LIII of Stark Golden Spur, from which the roots originated, still carries the genetic standard character of growth. As the spur character has been shown to be transmitted through the gametes (DECOURTYE 1969), which originated from layer II, the chimeral structure of Stark Golden Spur is thus established. This structure can account for the instability of the clone which occurs in commercial orchards. On the other hand, trees issued from r2 root remained closely similar to the Stark Golden Spur control for most characters, as well for nursery features (table 2), as for orchard ones (table 3). This r2 clone has now been made virus- free, which should afford a stable clone of Stark Golden Spur for growers.