© 2005 Heron Publishing—Victoria, Canada
Variations in sucrose and ABA concentrations are concomitant with heteroblastic leaf shape changes in a rhythmically growing
species (Quercus robur)
R. Le Hir (1), N. Leduc (1), E. Jeannette (2), J.-D. Viemont (1) and S. Pelleschi-Travier (1, 3)
1. Unité Mixte de Recherche SAGAH (Sciences Agronomiques Appliquées à l’Horticulture) A-462 (Université d’Angers/Institut National
de la Recherche Agronomique/Institut National d’Horticulture), Laboratoire de Morphogenèse des Ligneux, UFR Sciences, 2, Boulevard
Lavoisier, 49045 Angers Cedex 01, France / 2. Laboratoire de Physiologie Cellulaire et Moléculaire des Plantes, Unité Mixte de Recherche Centre National de la Recherche
Scientifique 7632, Université Pierre et Marie Curie, Casier 156, 4, Place Jussieu, 75252 Paris Cedex 05, France / 3. Corresponding author (sandrine.travier@univ-angers.fr) / Received January 27, 2005; accepted May 7, 2005; published online November 8, 2005
Summary
In rhythmically growing woody species such as common oak (Quercus robur L.), stem growth is discontinuous and a bud forms at regular intervals at the shoot apex. These buds are composed of different
types of leaves: laminate, aborted lamina and scale. The change in heteroblastic leaf shape from laminate to aborted lamina
leaves is regarded as one of the events marking shoot growth arrest. To better understand the determinism of heteroblastic
leaf shape change and thus, of rhythmicity, we studied morphogenetic events during the early days of the second flush of growth
in oak, as well as changes in sucrose metabolism and abscisic acid (ABA) concentrations in control plants expressing the heteroblastic
leaf shape change and in defoliated plants showing no heteroblastic leaf shape change and producing only laminate leaves.
In control plants, the leaf shape change was underway on Day 5 of the second flush with the differentiation of the first two
aborted lamina leaves. Sucrose concentration in the apices of control plants decreased between Days 3 and 5 during differentiation
of the aborted lamina leaves. An inverse pattern was observed in defoliated plants, suggesting that sucrose acts as a signal
triggering heteroblastic leaf shape changes. During the same period, acid cell wall invertase activity was high in young stem
and laminate leaves of control plants, whereas the activity remained constant and low in the apices. If the laminate leaves
were removed, the increase in apical sucrose concentration was proportionally higher than the decrease in apical acid vacuolar
invertase activity, suggesting that, in the absence of young leaves, sucrose is imported to the apex. The sucrose concentration
in the apex is therefore likely to be affected by trophic competition with the expanding laminate leaves. The decrease in
apex sucrose concentration may be one of the mechanisms driving heteroblastic leaf shape change. Differentiation of aborted
lamina leaves was followed by a decrease in the organogenic activity of the shoot apical meristem (SAM) between Days 7 and
9. High concentrations of ABA are associated with differentiation of aborted lamina and scale leaves and with low SAM organogenic
activity. Shoot apical meristem organogenic activity remained high and ABA concentration in the apex remained low in defoliated
plants producing only photosynthetic leaves. These results suggest that (1) ABA is involved in the gradual conversion of embryonic
leaves to abnormal leaves, thereby regulating heteroblastic leaf shape changes and (2) changes in ABA concentration influence
the intensity of SAM organogenic activity. Heteroblastic development and therefore rhythmic growth could be the result of
competition between apices and laminate leaves, with competition first involving sucrose and thereafter ABA.
Keywords:
abscisic acid, bud, common oak, invertases, leaf morphology, rhythmic growth, shoot apical meristem.
