Identification of biomechanical factors involved in stem shape variability between apricot tree varieties

Abstract

Background and aims: 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).

Fig. 1. Schematic shape of the three varieties studied: (A) ‘Lambertin no. 1’ (upright shape); (B) ‘Modesto’ (spreading shape); (C) ‘Palsteyn’ (weeping shape).

Fig. 2. Descriptors of stem shape: angle at the base (α) and angular deviation (γ).

Fig. 3. Bending of a straight beam of length, L, and diameter, D, initially leaning at an angle, ϕ, from the horizontal, subjected to a mass, M, at position, pL, from the base, resulting in variation of angular deviation, Δγ.

Fig. 4. Uprighting of a beam of diameter, D, subjected to a diameter increase, ΔD, and then unloaded of ΔM, resulting in a variation of angular deviation, Δγ′.

Fig. 5. Schematic representation of a stem cross‐section of initial diameter, D, subjected to a diameter increase, ΔD, with a sector of tension wood in its upper part, characterized by its angular extension, β.

Fig. 6. Mean dynamics of fruit load, shoot load and diameter increment for the three varieties studied, highlighting key dates in the year: T0 (bud‐break), T1 (physiological drops), T2 (before harvest), T3 (after harvest), T4 (end of growth) and T5 (fall of leaves).

Fig. 7. Mean shape of the stems for each variety at each key date (T0–T5, starting with T0 at left with each successive stage indicated by the arrows), demonstrating the angle at the base and the angular deviation.

Fig. 8. Comparison between observed and predicted stem re‐orientations (in radians). (A) Period T0–T1 (whole sample); (B) period T2–T3 (sub‐sample with fruits); (C) period T3–T4 (sub‐sample for which anatomy was studied).

Fig. 9. Position of the varieties relative to each other for each re‐orientation factor. Each bar represents the difference between the mean log‐value for a given variety and the overall mean log‐value. (A) Period of passive bending (T0–T1); (B) period of passive uprighting (T2–T3); (C) period of active uprighting (T3–T4). A positive value means that the factor promotes reorientation for that variety more than for the other two varieties.