Journal of Experimental Botany, Vol. 51, No. 352, pp. 1813-1824,
November 1, 2000
© 2000 Oxford University Press
Original Papers |
Biomechanical study of the effect of a controlled bending on tomato stem elongation: global mechanical analysis
1 INRA, Unité associée Bioclimatologie-PIAF, 234 avenue du Brézet, 63039 Clermont-Ferrand cedex 2, France
2 INRA, station d'Ecophysiologie des Plantes Fourragères, Les Verrines, 86600 Lusignan, France
3 Unité associée Bioclimatologie-PIAF, Université Blaise Pascal, Les Cézeaux, 63177 Aubière, France
4 Ecole Nationale Supérieure d'Horticulture, 8 rue Le Nôtre, 49000 Angers, France
5 Laboratoire d'énergétique et phénomènes de transfert, Ecole Nationale Supérieure des Arts et Métiers, Esplanade des Arts et Métiers, 33405 Talence cedex, France
Received 26 April 2000; Accepted 9 June 2000
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Abstract |
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An experiment was designed to apply a controlled bending to a tomato stem and simultaneously to measure its effect on stem elongation. Stem elongation was measured over 2 d until steady and equal rates were obtained for the control and the treated plants. Thereafter, the basal part of the stem was submitted to a transient controlled bending at constant displacement rate using a motorized dynamometer. After load removal, stem elongation was again measured for 2 d. The tested plants were mature (height visible internodes) and only the basal part of the stem, which had already finished elongation, was loaded (hypocotyl and the first three internodes). A few minutes after the application of bending, elongation stopped completely for 60 min. Thereafter it took 120–1000 min to recover a rate of elongation similar to the control. The growth response was exclusively due to the bending of the basal part of the stem. It was shown that the side mechanical perturbations on the roots and on the stem tissues interacting directly with the clamp were not significantly involved on the elongation response. These results give evidence for mechanical perception and plant signalling from the basal stem to the upper elongating zone. However, none of the variables characterizing the global mechanical state of the bent part of the stem (i.e. the maximal force, bending moment, inclination, mean curvature of the stem, stored mechanical energy) could quantitatively explain the variability of the growth response. A more local mechanical analysis is therefore needed to elucidate how the mechanical stimulus is perceived.
Key words: Bending, biomechanics, growth, thigmomorphogenesis, tomato.
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Introduction |
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Thigmomorphogenesis is defined as the influence of mechanical perturbations on plant growth and development (Boyer, 1967
; Jaffe, 1973). Physiological and molecular studies have shown that it involves a cascade of biological events starting with a transduction of the mechanical stimulus into a biological signal and ending into such a global response as modification of growth (Trewavas and Knight, 1994).
A thigmomorphogenetic influence on stem growth has been reported in many herbaceous (Biddington, 1986) and woody species (Stokes et al., 1995, 1997; Telewski and Pruyn, 1998). It usually results in a reduction in elongation, but in some species also leads to a stimulation of radial growth. Experimental evidence has been obtained in natural conditions, by tutoring plants in such a way as to prevent wind sways (Jacobs, 1954; Larson, 1965; Goodman and Ennos, 1997). To get more insight on the underlying mechanisms, many experimental studies in more controlled conditions have been conducted, using different kinds of mechanical loading such as bending (Patterson, 1992; Gartner, 1994; Osler et al., 1996), rubbing (Depège et al., 1997) and brushing (Latimer and Thomas, 1991). Altogether, a rather large body of knowledge is available on thigmomorphogenesis. However, the authors' view is that, from a biomechanical point of view, three major problems are now limiting the progress in the field: (i) the characterization of the mechanical perturbation, (ii) the differences in time scales of the perturbation and of the studied response and (iii) the identification of the perceiving tissues within the plant and the related problem of plant signalling. The situation for each problem is rather different, however, and it is necessary to detail them briefly to specify an experimental strategy.
Characterization of the mechanical perturbation
In general, the perturbations have been very poorly characterized and controlled from a mechanical point of view (e.g. ‘manual flexion during 1 s' or ‘gentle mechanical distortion’). The quantification of the perturbation has seldom been carried out, excepting in a first attempt (Jaffe et al., 1980). Moreover, qualification of the rheological behaviour of the tissue during the loading has not been studied. However, the effect of a mechanical perturbation on the stem is likely to depend on the rheological behaviour of the tissues during loading. If non-elastic behaviour occurs, then part of the work transmitted to the stem can be dissipated as heat (i.e. probably lost for transduction?), and irreversible strains can be produced. For the same external load the mechano-perception could differ from what occurs if the tissue behaviour is elastic.
This lack of quantification and qualification of the stimulus raises obvious problems of standardization and repeatability, but more importantly prevents a quantitative assessment of the relation between the stimulus and the response (Jaffe et al., 1980).
Time-scales of the perturbation and of the response
In many cases, there has been a difference in time scale between the perturbation (usually applied for a few seconds, although often repeated) and the measurement of the growth responses (often the length increment on a daily basis or on difference in final length with the control). This is particularly problematic if a time integration of the thigmomorphogenetic effect in a variable environment is targeted (e.g. wind). Only a few studies involving continuous growth measurement are available in the literature (Peacock and Berg, 1994; Garner and Björkmann, 1996).
Identification of the perceiving tissues and plant signalling
In general, the mechanical perturbations are applied directly on the elongating zones or on the whole plant (i.e. including the elongating zones). It is thus not clear if the non-elongating tissues are able to perceive mechanical perturbations or not, and if the perception occurs at the same place as the response or if it can be transmitted along the plant. A clear demonstration of intra-plant signalling controlling growth response has been given for wounding perturbations, mainly in seedlings (Desbiez et al., 1981). Moreover the occurrence of rapid, long distance transmission along the stem of possible signal supports have been documented for plants in a large range of developmental stages, including tomato (Boari and Malone, 1993). But the evidence in the case of non-wounding stimuli remains sparse and arguments for internodal compartimentation in adult plants do exist in some species (Desbiez et al., 1982).
This paper is thus part of a research effort toward (i) the control, localization, quantification, and qualification of the mechanical stimulus, and (ii) the assessment of quantitative phenomenological relations between the stimulus and the growth response. The first aim of the work presented here was to design an experiment allowing controlled and quantified loading at various locations in the plant, and a simultaneous precise quantification of stem elongation. Bending (rather than punctual compression as in Jaffe et al., 1980) was chosen in reference to natural windy conditions where plants stems are submitted mainly to bending (and twisting). Additionally, it can be noted that the ‘brushing technique’, which is already used in horticulture to control stem elongation, also induces stem and petiole bending (Mitchell et al., 1977; Garner and Björkmann, 1997).
It was decided to work with mature vegetative plants and to start by applying the bending stimulus to the basal part of the stem (i.e. away from the primary growth zones). This choice was dictated by three reasons. First, mechanical loading of elongating tissues has a direct physical consequence on elongation through creep (Lockhart, 1965; Cramer and Bowman, 1991; Coutand et al., 1997), and it can be difficult to distinguish this from real thigmomorphogenetical effects (i.e. involving a transduction). Additionally, non-elongating tissue can more easily be loaded in their elastic domain (Moulia et al., 1994), thereby restricting the stimulus to a non-damaging one. Secondly, basal stem bending enables the application of bending without inclining the elongating zones therefore preventing side uncontrolled loading and gravitropic reactions in the elongating zone (Pickard, 1985; Clifford et al., 1982). Lastly, it allows the study of plant signalling from the basal parts of the stem to the primary growth zones and its significance in non-seedling plants to be assessed.
The experiment described in this paper showed a marked influence of basal stem bending on stem elongation, demonstrating clear plant signalling. It is shown that the growth response was exclusively induced by the bent tissues, and was not due to side mechanical effects on the roots or at the point of attachment of the stem. No relation between the growth responses and the statical, energetical or positional parameters characterizing the global mechanical state of the bent tissues could be found.
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Materials and methods |
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Plants
Tomato (Lycopersicon esculentum Mill.) was retained as plant material as previous works have demonstrated a thigmomorphogenetic control of stem growth in this species (Gartner, 1994
; Depège et al., 1997). Tomato plants (cv. VFN8) were grown in a controlled environment (L/O 14/10 h, thermoperiod 25/18±1 °C, HR 55%) in a growth chamber. Light was provided by eight lamps (HQITS 250 wd, Osram®, France) providing a PAR of 300 µmol s-1 m-2 (measured on top of the canopy). Seeds were sown in vermiculite and watered manually every day. Care was taken not to spray water directly on the seedlings to prevent thigmostimulation (Wheeler and Salisbury, 1979). When the first leaf began to emerge, the seedlings were transferred to containers (eight plants per container) filled with 40 l of nutrient solution (Morizet and Mingeau, 1976). The nutrient solution was changed every week to limit the acidification, and was oxygenated by an aquarium pump. When the eighth internode (from the stem base) was 1 cm long, plants were transferred to a twin growth chamber for the bending experiment. At this stage, the hypocotyl and the first three internodes had finished their elongation (data not shown), the internodes four to eight were still elongating.
For each bending experiment, two plants were chosen, one as a control and one for the bending test. The two plants were selected for the similarity in their stage of development: same length of the eighth internode and particularly the same size of the eighth leaf (a classical index of the stage of development in tomato: Navarrete et al., 1997).
Experimental design
The bending experiments were performed in a growth cabinet with the same environmental conditions as during plants breeding (L/O 14/10 h, thermoperiod 25/18±1 °C, HR 55%).
The experimental design for the bending test is shown in Fig. 1. Each plant (i.e. treated and control) was attached in a vertical position to a fixed metal clamp (no degree of freedom in rotation or translation). The roots were bathed in a 2l pot (one plant per pot) containing oxygenated nutrient solution.
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The clamp was located 10 cm from the collar, in most cases in the upper part of the third internode. At the level of the clamp, the stem was surrounded with foam rubber in order to prevent wounding (the absence of wounding was systematically tested after each test). The base of the stem was surrounded with an inextensible strap linked to a motorized dynamometer, allowing controlled basal bending. Stem elongation was recorded continuously using LVDT, as described below.
Stem elongation measurement
Continuous stem elongation of each plant (bent and control) was monitored by a LVDT (linear voltage differential transducer) (L50 Chauvin Arnoux®, France): the eighth internode (from the stem base) was linked to a LVDT (Fig. 1
) by a counterbalanced system (local constructor, Chirain®, France).
One arm of the counterbalanced system was linked to the plant by a waterproofed cotton thread stuck on the eighth internode by an elastic plaster (not toxic for the plant), the other arm was linked to the LVDT bar by another waterproofed cotton thread. The LVDT bar was counterbalanced by an additional mass on the opposite arm. However, since the threads needed to be under tension, another mass (0.5 g) was added on the LVDT side to exert the minimal sufficient tension on the threads. There have been several reports that such an installation has only transient effects on the elongation of the stem (Fernandez and Wagner, 1994).
LVDTs were linked to an acquisition card RTI 815® (Analog Devices, Norwood, MA, USA). The data were transferred to a microcomputer implemented with Dasylab® software Version 1.5 (Dasytec, Germany) which converted mV into mm. So the stem elongations of the two plants (treated and control) were followed on screen throughout the experiment.
The stem elongation was recorded every second and the software computed an average value every 60 measurements. So, each point on the elongation graph corresponds to the mean elongation per minute.
Elongation rate could not be directly calculated by the difference between two successive points because of recording noise. For rate calculation, the elongation data have been smoothed by three successive sliding averages with a step of 21 min. The average value was put on the middle of each set of points. Then, the elongation rate was computed by the difference between two successive points on the smoothed curves.
Application of transient controlled bending
There are two ways to apply controlled bending: imposing the force or imposing a displacement. The second approach was chosen for experimental convenience and better control of the amount of bending: a 2 cm displacement of the stem base from its initial position was applied at a rate of 80 mm min-1.
Just before the application of bending, a photograph of the basal part of the stem was taken. Bending was then applied on one of the two plants (Fig. 3). The maximal applied force was recorded, a picture of the loaded stem was taken (care was taken to line up the bending plane with the plane of the camera), then the force was removed. A photograph of the stem after load removal was also taken to enable measurement of possible residual strains.
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Timing of the bending experiment
Several factors have been demonstrated to influence the thigmomorphogenetical response including daily variations of tomato sensibility to mechanical perturbation (Lefèvre et al., 1994) and frequency of the stimulation (Garner and Björkmann, 1996) using brushing. The authors chose to start with the simplest situation: a single transient bending applied always at the same hour of the day (between 16.30 h and 17.00 h).
As installation necessarily involves uncontrolled mechanical perturbations, preliminary studies were conducted to assess the time necessary for the plant to recover completely. They indicated that plants need at least 24 h to reach a stable elongation regime after installation (Fig. 2). Therefore a 1 d period was assigned for plant recovery after the experimental equipment (clamp, LVDT, strap, dynamometer) was in place before starting the bending test. Elongation was recorded continuously during this period. The similarity of the behaviour of the paired plants in terms of elongation growth and post-installation recovery was, therefore, assessed before the bending experiment. The transient bending was then applied, and the growth response was followed for two more days. Eighteen repetitions of this bending experiment were performed.
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Assessment of the effects of side perturbations
The displacement of the base of the stem induces flexure of the basal part between the strap and the clamp and it also induces mechanical perturbations of the roots (due to friction between the roots and the molecules of the nutrient solution). Many reports of the influence of root mechanical perturbation on the elongation of the shoot exist in the literature (Young et al., 1997; Masle, 1998).
There is also a direct interaction between a small part of the stem and the metal clamp, corresponding to the transmission of the load. The load transmitted to the clamp can be summarized by a bending moment and a shear force (Laroze and Barrau, 1987), and the bending moment is maximal at the clamp level. Therefore the zone at the edge of the clamp is highly stressed, and it has been argued in the literature (Goodman and Ennos, 1997) that the highly stressed zones might be responsible for most of the thigmomorphogenetic response. Additionally, in the type of experiment used here, this segment of stem is also the closest to the elongation zone. It was therefore necessary to determine the influence of each of these two mechanical perturbations on the growth response observed after bending.
In order to reproduce the mechanical perturbations of the roots during bending but without bending the basal part of the stem, a second type of experiment was designed and performed on another set of plants (5 repetitions). The pot itself was submitted to a 2 cm displacement with a displacement rate equal to the displacement rate during bending experiments (80 mm min-1), providing a similar velocity of relative displacement of the roots and the solution. The displacement of the pot was facilitated by placing it on a small roller (Fig. 4a), thereby preventing additional seismic mechanical perturbations due to friction of the pot on the table (which could have provoked jolts).
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To assess if the local interaction between the stem tissues and the metal clamp was responsible for the growth response, a third type of experiment was conducted. Five plants were submitted to the same resulting bending moment as during the bending experiment, but resulting from the application of a force (9.81 N) exerted much closer to the metal clamp (1 cm; Fig. 4c
). In that way, the part of the stem within the clamp was submitted to a similar resulting moment as during the bending experiment, but most of the basal part of the stem was not bent. It should be noticed, however, that in this case the transmitted shear force was higher. Five similar experiments were also executed with the same force as during bending experiments (i.e. providing the same shear force as during bending experiments to the clamp zone, but a lower bending moment).
Rheological behaviour of the basal part of the stem in bending
Elasticity:
By definition, a material is elastic if it recovers its initial shape after load removal (Laroze and Barrau, 1987). Elasticity was tested by comparison of the shape of basal part of the stem before loading and approximately 30 s after load removal.
The shape of the stem was described quantitatively by digitizing its central line. Digitizing was performed directly on the photographs by a 2D digitizing tablet (Summasketch Pro, Summasketch, USA) and a software of data acquisition called Saisumas (Moulia and Fournier, 1997).
Linearity of the load–deflection curve
In a subsample of six plants submitted to the transient bending test, the dial of the dynamometer was recorded continuously in order to establish load-deflection curves.
Global mechanical analysis:
The mechanical state of a structure can be characterized from two points of view: static (forces) and kinematic (deformation and trajectory). In these experiments, the bending cycle was characterized by six variables, specifying the mechanical transformation at the scale of the overall stem (global analysis). Statically by the force at maximal displacement Fm and the maximum bending moment. The maximum bending moment was computed directly by the product Fmxl (where l is the distance between the clamp and the applied force) as the hypothesis of small displacements was verified (data shown in the companion paper). Kinematically, the bending is classically quantified by the maximal deflection and by the deflection rate, but both are controlled and fixed to a constant value in these experiments. The inclination angle of the stem base (tangent to the central line) on the photograph was also measured, as this is related to possible graviperception (and also characterizes the global curving of the basal segment of the stem) (De Grado et al., 1997).
The amount of work given to the system, and the part stored as elastic strain energy were also estimated (but the calculation procedure will be presented later in the text as it is dependent on the observed rheological behaviour).
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Effect of transient bending on stem elongation
A representative example of the effect of bending on stem elongation is presented in Fig. 5
. The elongation of the two stems was very similar before the application of bending (Fig. 5A). In the 5 min following bending, the elongation of the bent stem stopped completely for 65 min (Fig. 5B). Then the elongation restarted, but with a rate inferior to the control (Fig. 5A). Finally after 550 min the bent stem recovered an elongation rate equal to the control (Fig. 5A). At this moment, the growth response was considered to be finished.
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Note that the time between the application of bending and the beginning of the growth response, and the duration of elongation cessation was measured directly on non-smoothed curves for more accuracy. The duration of the growth response was evaluated on elongation rate graphs.
The summary of the results of the 18 bending experiments is given in Table 1: in 17 cases (94%) there was a marked growth response. In 15 cases (83% of the total), there was a complete cessation of elongation lasting 60 min on average. In the two remaining cases, the elongation did not cease completely but was severely reduced (95%) for an hour.
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In most cases two different phases can be distinguished: growth cessation and ‘normal’ elongation rate recovery. The duration of these two phases were the two parameters retained to quantify the growth responses of plants in the following. It should be noted that the inter-stem variability was very different for these two parameters: almost no variability in the duration of growth cessation (CV= 6%), and a very large variability for the time for elongation rate recovery (CV=51%).
Effect of the mechanical perturbations of the roots and stem-clamp local
The mechanical perturbation applied to the roots did not, by itself, induce any measurable effect on stem elongation (Fig. 6A).
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Similarly, the local interaction between the stem and the clamp produced no obvious growth response, whether the clamp transmitted a similar bending moment (and a higher shear force) (see typical example in Fig. 6B
) or the same shear force and a smaller bending moment (data not shown).
Rheological behaviour of the basal part of the stem
A representative example of the comparison of the shapes of the stem before loading and after force removal, is given in Fig. 7A. The two sets of points were almost identical. The bending experiments were thus performed in the domain of elasticity of the tissues of the tomato stems.
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Figures 7B1
to B6 represent the force changes during bending for the six tested stems. It can be seen that in five cases (B2 to B6) there was a good linear relationship between the force and the displacement which provides evidence for linear elasticity. A closer look at the graphs revealed, however, that the first points were systematically above the straight line whereas the last points stayed systematically below. This provides evidence for the existence of a slightly more complex behaviour (probably some part of retarded elasticity). However, the linear regression fits quantitatively the sets of points very well, so that the approximation of a perfect elastic linear rheological behaviour can be made. In one case (Fig. 7B1) the relationship between the force and the displacement was not linear. Moreover, at fixed displacement, the force decreased rapidly due to stress relaxation phenomena. This rheological behaviour was observed in a very few cases during the bending experiments and these data were systematically removed. The two results allow the conclusion that in this experiment the bending tests were executed in the domain of linear elasticity of the stem tissues, and that no significant internal damages occurred within the stem tissues.
Relationship between global mechanical parameters and the growth responses
As stated previously, the only part of the growth response that showed significant variability in these experiments was the time for the recovery of an elongation rate similar to the control. Therefore this work concentrated on the relationship between this parameter of the growth response and the intensity of the loading (due to the variability in the mechanical structure of the basal part of the stem—diameters, type of tissues—there was also an inter-stem variability on the loading in this experiment).
The plot of the growth response versus the corresponding applied force is presented in Fig. 8. The correlation coefficient was -0.07 and did not differ significantly from 0 (Pr(>F)=0.7834). Thus, the force variability did not explain the variability of the growth response.
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As the lever arm was constant (10 cm) for the 18 stems submitted to bending, the maximal bending moment was proportional to the applied force for all the tested stems. So, the maximal bending moment did not explain the variability in growth response either.
The growth response was plotted versus inclination of the stem base (Fig. 9). The correlation coefficient was slightly higher (0.23), but remained very low and the probability for such a distribution to result from two independent variables was still 0.355. It can thus be concluded that the inclination of the stem base is unlikely to be directly involved in determining the variability of the growth responses.
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As the basal part of the stem was linearly elastic, there was no energy dissipated by viscous friction. So the elastic energy stored in the bent part at a given deflexion equals the amount of work W transmitted to the stem during the loading, that is
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Discussion |
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Effect of bending of the basal part of the stem on stem elongation; evidence for plant signalling
The bending experiments presented here induced small displacements and remained in the linear elastic domain of the stem, indicating no significant damage. Moreover, the effective mechanical perturbation was shown to be restricted to the basal, non-elongating, part of the stem, with no mechanical or positional side effects.
In these conditions, the transient decrease of the elongation rate in the upper parts of the stem gives clear evidence for plant signalling. Previous work using localized manual rubbing or flexing have already argued (Jaffe et al., 1980; Depège et al., 1997) that a stimulation of one internode induced growth responses in other internodes, but it was difficult to be sure that no side mechanical perturbation was involved (e.g. with manual flexing, the top of the stem and the leaves are submitted to a dynamic flexion under self-weight). From another source, an opposite result has been reported in older plants (but also different species) showing internodal segmentation on Bryonia, using similar uncontrolled stimulation techniques (Desbiez et al., 1982). Moreover, in both cases, the stimulated internodes were still in elongation. Our results demonstrate that, in mature vegetative tomato plants, a slight bending strictly restricted to the non-elongating basal part of the stem had a long-distance effect on stem elongation with no internodal segmentation.
Lag time for growth response
A clear but short lag time (8.3±3.7 min) was found between the onset of the bending and the cessation of growth. This lag time reflects the time to transport the signal to the elongating cells, plus the time for these cells to produce a growth response. The minimal travelling rate of the signal consistent with these data (i.e. corresponding to the hypothetical case where all the lag time would be due to the signal transport) can thus be obtained. In this experiment, the plants were on average 35 cm high. If it is assumed that the signal has to travel from at least the tissues just below the clamp all along the elongating internodes (so that each growing internode segment is reached by the signal) the minimal pathway length is approximately 25 cm (35 cm (total length)-10 cm (bent part)). Thus the signal (after mechanical perception in the non-elongating parts) was transferred to the elongating zones at a minimal rate of roughly 3 cm min-1. This is in fair agreement with the rate of propagation of a xylem hydraulic signal observed in tomato plant (Malone, 1994). However a direct experimental assessment of the involvement of such a signal would be required as other mechanisms have been argued in the literature (Jaffe and Biro, 1979). From another perspective, lag times for the kinetics of early events after mechanical perturbation have been also published (Jaffe, 1975). Using manual internode rubbing on bean stem elongation, a complete elongation cessation after 6 min was reported (i.e. very similar to this study's results). But in Jaffe's work rubbing was applied directly to the internode for which elongation was recorded. This can indicate that most of this lag time is determined in the vicinity of the responding zone, and is thus probably not related to the time of transport of the signal to the growth zone. The mechanisms involved in this local lag time are not clear, however, as the physiological events induced by mechanical perturbation are many, and the integrative mechanisms of thigmomorphogenesis remain largely to be elucidate.
Analysis of the growth response
The analysis of the growth responses allowed the distinction of two phases within the growth response: growth cessation and then growth recovery. The elongation cessation showed almost no variability between plants whereas there was a large variability of the growth recovery time between plants (from 120 to 1100 min). Such a biphasic growth response, and the fact that the complete stoppage of growth was an all or none response have been pointed out briefly (Jaffe et al., 1980).
Apart from one exception (data not shown), no ‘stored growth’ phenomena (Taiz, 1984) was observed: the decrease in elongation, induced by bending, was not compensated by further higher elongation rate. So, a single bending induced a reduction of the final stem height. But, in these conditions, a single bending reduced the stem height by 2 mm at the most. So it is likely that the significant reduction of final plant height in naturally windy conditions that has been clearly ascribed to thigmomorphogenetic response (Latimer et al., 1986) is due to repetitions of bending.
Biomechanical analysis: relation between the growth response and the parameters characterizing the global mechanical state of the bent stem
It is noteworthy that none of the global mechanical variables studied here were able to correlate significantly to the variability of the growth response. The force intensity was not directly related with the response. A global energetic approach did not prove more efficient. Nor did positional variables, like the inclination of the stem bases, thus allowing the authors to dismiss a possible involvement of some graviperception. Lastly, as this inclination was also related to the mean curvature of the basal part (as the distance to the clamp was constant), mean curvature was therefore not characterizing the stimulus either.
This is contrary to the results found earlier in bean seedlings (Jaffe et al., 1980). In their experiments, they have found a close relationships between daily elongation and (i) the force in a puncture-pressure and rubbing test (using their thigmostimulator), and (ii) the ‘bending angle’ (corresponding to our basal inclination) in a manual bending test. This experiment was originally designed in a way to be able to discriminate which of the two types of variables (force related or curvature related) was perceived, comparing static and angle variables in a single experiment (in bending, the bending moment and the maximal angle can be decorrelated if there is a variability in bending stiffness of the stems). It seems somewhat surprising that none of the variable could explain the variability of the response. Part of the discrepancy might come from the fact that in the earlier experiment (Jaffe et al., 1980) the mechanical perturbation was applied directly on the growing tissues in a seedling. For their manual bending experiment (in which the bending load and the side effects were uncontrolled) they were also imposing large curvatures, with maximal angles in the range of 10–50°, giving spatially averaged curvature of an order of magnitude of 10–2 mm-1, whereas in our experiment the order of magnitude of the curvature was in the range 10-3-10-4 mm-1. A complete comparison would, however, require data on stem diameters together with some rheological data (e.g. were these experiments within the elastic domain?). This points to the value of using this bending test on other materials to get a comparable database on several species, and clearly demonstrates the importance of a detailed mechanical characterization of the stimulus.
Meanwhile an open question remains: can the variability of response in these data be explained by some mechanical variable characterizing the mechanoperception and primary signal generation? It can be noted that all the previous parameters (describing the global mechanical state of the bent part) do not take into account the stem cross-sectional geometry and the way non-uniform mechanical fields are generated within the stem. The analysis of the local mechanical state of the bent part and the possible insights that can be obtained on the relation with the growth response are reported in a companion paper.
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Conclusion |
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The in situ thigmomorphogenetic bending test presented in this paper allows the application of a controlled quantified bending on a given segment of the plant stem and to measure the stem elongation continuously before and after the stimulus. The choice of a bending test rather than a puncture-pressing test (Jaffe et al., 1980
) makes it more related to the load encountered in naturally windy conditions and in agricultural practice.
The mechanical behaviour of the loaded stem can be qualified at the same time. Possible discrepancies in the observed biological responses due to differences in the direct physical effects of the mechanical perturbation on the loaded tissues (which depend on the load, but also on the rheology of the tissues and on stem geometry, both of which can vary between plants for many reasons (Moulia and Fournier, 1997) can be assessed. Lastly a procedure for studying possible side effects is presented, and it is shown that they were negligible in these studies, and hence that the observed growth response was exclusively due to the mechanical state of the bent part of the stem.
This test is thus a versatile tool for a mechanically controlled experimental study of thigmomorphogenesis. Using the test, this work demonstrated that a slight bending of the non-elongating parts can trigger a long-distance biphasic growth response in tomato. But no relation between the various variables globally characterizing the mechanical stimulus and the amount of the growth response could be found. The local analysis of the mechanical state of the bent part and the relation between the growth responses and the local mechanical parameters are given in the companion paper.
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Acknowledgments |
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This work was funded by the INRA (Institut National de la Recherche Agronomique, France), department of Bioclimatologie and by the Conseil Général d'Auvergne. We particularly thank Mr N Frizot for plants management and help in experiments. We also thank Mr B Adam and R Falcimagne (INRA Clermont-Ferrand, France) for the maintenance of the growth chambers and computer assistance, Mr M Crocombette (INRA Clermont-Ferrand, France) for realization of the plant containers and Mr C Bodet (INRA Clermont-Ferrand, France) for technical assistance. We thank Dr F Gastal (INRA- SEPF Lusignan, France) for very helpful advice on plant ecophysiology, Drs M Fournier (Engref, centre de Kourou, French Guyana) and L Bodé (CUST, Université de Clermont-Ferrand, France) for explanations and advice on mechanics and Dr F Ewers for English corrections.
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6 To whom correspondence should be addressed. Fax: +33 05 49 55 60 68. moulia{at}lusignan.inra.fr
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References |
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