Mechanical stimuli regulate the allocation of biomass in trees: demonstration with young Prunus avium trees

Abstract

Background and aims: Plastic tree-shelters are increasingly used to protect tree seedlings against browsing animals and herbicide drifts. The biomass allocation in young seedlings of deciduous trees is highly disturbed by common plastic tree-shelters, resulting in poor root systems and reduced diameter growth of the trunk. The shelters have been improved by creating chimney-effect ventilation with holes drilled at the bottom, resulting in stimulated trunk diameter growth, but the root deficit has remained unchanged. An experiment was set up to elucidate the mechanisms behind the poor root growth of sheltered Prunus avium trees.

Methods: Tree seedlings were grown either in natural windy conditions or in tree-shelters. Mechanical wind stimuli were suppressed in ten unsheltered trees by staking. Mechanical stimuli (bending) of the stem were applied in ten sheltered trees using an original mechanical device.

Key results: Sheltered trees suffered from poor root growth, but sheltered bent trees largely recovered, showing that mechano-sensing is an important mechanism governing C allocation and the shoot-root balance. The use of a few artificial mechanical stimuli increased the biomass allocation towards the roots, as did natural wind sway. It was demonstrated that there was an acclimation of plants to the imposed strain.

Conclusions: This study suggests that if mechanical stimuli are used to control plant growth, they should be applied at low frequency in order to be most effective. The impact on the functional equilibrium hypothesis that is used in many tree growth models is discussed. The consequence of the lack of mechanical stimuli should be incorporated in tree growth models when applied to environments protected from the wind (e.g. greenhouses, dense forests).

Fig. 1.

The experimental device for bending sheltered trees. (A) View of the SB treatment. (B) The stem inside the shelter is attached to two fixed points and a moving arm piloted by an air-pressurised piston imposes a lateral displacement, which results in the bending of the stem.

Fig. 2.

Thigmomorphogenetical signals generated by the applied bending in the basal part of the stems and their action on stem growth. The bending applied on the basal part of the stem triggers a signal that will affect the radial growth in the basal part, but the signal will also reach the elongating part. Due to the small diameter of the shelter, the elongating part is counter-bent so that a signal is generated within the elongating zone. This signal may act on-site and lead to a modification of the annual shoot elongation. The signal might also act on the radial growth of the basal part. With the available data, it is only possible to estimate the mechanical state of the bent basal part of the stem.

Fig. 3.

Kinetics of elongation of the annual shoot and cambial growth at the stem base within treatments. Treatments: NSW, no shelter and wind; S, shelter; SB, shelter + bending; NSSt, no shelter + stake. (A) Kinetics of elongation of the annual shoot in each treatment. The installation of the bending system resulted in a transient reduction of stem elongation that disappeared after 2 August. Two weeks after the beginning of the application of artificial bending the elongation of the SB trees is almost zero. The elongation of NSW trees was the lowest throughout the experiment; the staked trees exhibited an elongation smaller than the sheltered trees but higher than the NSW trees. (B) Kinetics of cambial growth at the stem base in each treatment. The installation of the bending device did not lead to a visible modification of stem cambial growth. The cambial growth of the NSW trees was the highest throughout the experiment. Before the beginning of the application of artificial bending the cambial growth of S, SB and NSSt trees was the same. Two weeks after the application of artificial bending the cambial growth of SB trees increased and finally reached the cambial growth of the NSW trees. The cambial growth of the S and NSSt trees was similar throughout the experiment.

Fig. 4.

Cambium growth response to treatments: (A) Average cumulative response in terms of cambial growth at 15 cm from the stem base; (B) average cumulative response in terms of cambial growth at 65 cm from the stem base. On both graphs, the NSW treatment is considered as the control. Before the beginning of the application of bending, the differential is negative for the three treatments S, SB and NSSt. For the SB treatment, the differential takes positive values during the application of bending, indicating that the cambial growth is more important for SB trees than for NSW trees, demonstrating the thigmomorphogenetical effects of artificial bending.

Fig. 5.

Kinetics of elongation rate of the annual shoot for each treatment. Treatments: NSW, no shelter and wind; S, shelter; SB, shelter + bending; NSSt, no shelter + stake.

Fig. 6.

Biomass partitioning between shoots and roots before and after treatments, expressed as a percentage of the total biomass of the tree. (A) Biomass partitioning at the date of planting; there were no significant differences in the biomass partitioning between the four sets of trees. On average, the shoot biomass represented 53 % of the total biomass. (B) Biomass partitioning after treatments at the end of August, by which time there were differences between the three treatments SB, NSW and NSSt. Artificial bending in the shelter induced an allocation of the biomass towards the roots. There was no difference between the NSW and NSSt treatments.

Fig. 7.

Evolution of the applied strain state with time. Through time and radial growth, the stems were subject to increasing strain: for an average plant, the applied strain increased from 1.5 to 2 % at 65 cm from the stem base, and 3.8 to 5 % at the stem base.

Fig. 8.

Relationship between artificial bending and differential cambial growth at 65 cm from the stem base. Trees from the NSW (no shelter and wind) treatment were used as controls to determine the differential response.

Fig. 9.

Differential growth response vs. time for trees subject to artificial bending. The growth response is plotted using NSW (no shelter and wind) trees as controls, for growth (A) in length, (B) in diameter at 65 cm from the collar and (C) 15 cm from the collar. General trends are indicated by the arrows.