Identification of biomechanical factors involved in stem shape variability between apricot tree varieties.
OBJECTIVE
Stem shape in angiosperms depends on several growth traits such as elongation direction, amount and position of axillary loads, stem dimensions, wood elasticity, radial growth dynamics and active re-orientation due to tension wood. This paper analyses the relationship between these biomechanical factors and stem shape variability.


METHODS
Three apricot tree varieties with contrasting stem shape were studied. Growth and bending dynamics, mechanical properties and amount of tension wood were measured on 40 1-year-old stems of each variety during one growth season. Formulae derived from simple biomechanical models are proposed to quantify the relationship between biomechanical factors and re-orientation of the stems. The effect of biomechanical factors is quantified combining their mechanical sensitivity and their actual variability.


RESULTS
Re-orientations happened in three main periods, involving distinct biomechanical phenomena: (a) passive bending due to the increase of shoot and fruit load at the start of the season; (b) passive uprighting at the fall of fruits; (c) active uprighting due tension wood production at the end of the season. Differences between varieties mainly happened during periods (a) and (b).


CONCLUSIONS
The main factors causing differences between varieties are the length/diameter and the load/cross-sectional area ratios during period (a). Wood elasticity does not play an important role because of its low inter-variety variability. Differences during period (b) are related to the dynamics of radial growth: varieties with early radial growth bend weakly upward because the new wood layers tend to set them in a bent position. The action of tension wood during period (c) is low when compared with passive phenomena involved in periods (a) and (b).
UMR 1098 Biologie du développement des Espèces Pérennes Cultivées, INRA-ENSAM-IRD-CIRAD, Equipe 'Architecture et Fonctionnement des Espèces Fruitières', 2 Place Viala, 34060 Montpellier Cedex 1, France. t_almeras@hotmail.com
Tancrede
T
Almeras
Evelyne
E
Costes
Jean-Claude
JC
Salles
D001696:Biomechanical Phenomena; D008954:Models, Biological; D018547:Plant Stems / Q000254:growth & development*; D027861:Prunus / Q000145:classification / Q000254:growth & development*; D013314:Stress, Mechanical; D013718:Tensile Strength; D013997:Time Factors