Not only light quality but also mechanical stimuli are involved in height convergence in crowded Chenopodium album stands

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1 Research Not only light quality but also mechanical stimuli are involved in height convergence in crowded Chenopodium album s Hisae Nagashima 1,2 and Kouki Hikosaka 2,3 1 Nikko Botanical Garden, Graduate School of Science, The University of Tokyo, Tochigi, Japan; 2 Graduate School of Life Sciences, Tohoku University, Aoba, Sendai, Miyagi, Japan; 3 CREST, Japan Science and Technology Agency (JST), Tokyo, Japan Author for correspondence: Hisae Nagashima Tel: hisae@ceres.ocn.ne.jp Received: 12 March 212 Accepted: 22 May 212 doi: /j x Key words: biomass partitioning, flexing caused by wind, height convergence, light quality, mechanical stimuli, red to far-red (R FR) ratio, stem diameter growth, stem elongation. Summary In crowded s, height is often similar among dominant plants, as plants adjust their height to that of their neighbours (height convergence). We investigated which of the factors, light quality, light quantity and mechanical stimuli, is primarily responsible for stem elongation and height convergence in crowded s. We established s of potted Chenopodium album plants. In one, target plants were surrounded by artificial plants that were painted black to ensure that the light quality was not modified by their neighbours. In a second, target plants were surrounded by real plants. In both s, one-half of the target plants were anchored to stakes to prevent flexing by wind. The target plants were lifted or lowered by 1 cm to test whether height convergence was affected by the different treatments. Stem length was affected by being surrounded by artificial plants, anchoring and pot elevation, indicating that light quality, light quantity and mechanical stimuli all influenced stem elongation. Height convergence did not occur in the with artificial plants or in anchored plants. We conclude that light quality and mechanical stimuli are important factors for the regulation of stem growth and height convergence in crowded s. Introduction Stem growth is an important determinant in the competition for light amongst crowded plants (Schmitt et al., 1995; Dudley & Schmitt, 1996; Huber & Stuefer, 1997; Huber et al., 1998; Weinig, 2). Taller plants can expose a larger portion of their leaf area to sunlight and shade out competitors (Anten & Hirose, 1998; Hikosaka et al., 1999). However, with increasing plant height, the risk of lodging caused by wind increases (Casal et al., 1994) and biomass allocation to the leaves (Givnish, 1982) and roots (Maliakal et al., 1999) is reduced, which may decrease total resource acquisition. Being tall can therefore be both beneficial and detrimental to plant growth. Plants regulate stem growth depending on their environment. In crowded s, accelerated stem elongation has been observed (Schmitt et al., 1987; Geber, 1989; Weiner et al., 199; Weiner & Thomas, 1992; Weiner & Fishman, 1994; Nagashima, 1999). Despite this, most plants avoid overtopping their neighbours. Dominant plants, which expose their leaves to the canopy surface, have similar heights to their neighbours despite exhibiting a great variety in stem diameter and shoot mass. This phenomenon is termed height convergence (Weiner & Thomas, 1992; Weiner & Fishman, 1994; Nagashima & Terashima, 1995). Nagashima & Hikosaka (211) conducted an experiment with a of potted Chenopodium album plants, in which some of the plants were lifted and lowered to be taller and shorter, respectively, than neighbouring plants. The lifted plants exhibited a decreased stem elongation rate, whereas the lowered plants exhibited accelerated stem elongation. These results indicate that height convergence is a consequence of the regulation of stem growth, so as to maintain a similar height to neighbouring plants. Several environmental factors are known to influence stem growth. Of primary importance is light quality, which acts as an environmental signal for the onset of growth competition (Ballaré, 1999; Smith, 2; Pierik et al., 24). Because of the preferential absorption of red light by chlorophyll, light scattered by neighbouring plants has a reduced red to far-red (R FR) ratio. Phytochrome photoreceptors in plant stems detect this signal, encouraging the plants to accelerate stem elongation (Morgan & Smith, 1976, 1978, 1979). The quantity of light induces both positive and negative effects on stem elongation. As photosynthates are necessary for stem growth, high light levels may increase stem elongation. Alternatively, several studies have shown that high light levels can retard stem elongation and that moderate shade levels are better at stimulating elongation (e.g. Grime & Jeffrey, 1965; Lecharny & Jacques, 198; Holmes et al., 1982; Corré, 1983). Exposure to strong winds may reduce elongation of overtopping plants, probably through the 83

2 84 Research Phytologist mechanical effects of flexing and or through excess transpiration from the leaves (e.g. Latimer et al., 1986; Holbrook & Putz, 1989; Retuerto & Woodward, 1992; Henry & Thomas, 22). Plants in field s may shield each other from wind, but, when a plant grows above its neighbours, it may be more exposed to wind. It has been shown that plants also respond to mechanical stress, such as touching and rubbing, through the process of thigmomorphogenesis (Jaffe, 1973). Manual flexing of stems lowers stem elongation rates (e.g. Smith & Ennos, 23; Anten et al., 25, 26, 29). These studies indicate that mechanical stress is an important factor in the regulation of stem growth. Previous studies have used inventive methods to prove that phytochromes are involved in height regulation in plant s. Ballaré et al. (199) showed that plants did not grow to enhanced heights when covered with a far-red light filter, even when they became shaded by their neighbours. Transgenic plants, in which elongation in response to neighbours was blocked, exhibited decreased relative fitness when grown in competition with wild plants (Schmitt et al., 1995). However, no published work has indicated that plants regulate growth to a certain height solely in response to light quality. It is feasible that other factors, such as light quantity or physical stimuli, or both, can influence height growth in crowded s, possibly in combination with various other environmental factors. In the present study, experiments were undertaken to determine whether light quality, light quantity and mechanical stimuli are involved in height growth regulation and height convergence in field s. Two s of potted Chenopodium album plants were established. In one of the s, the target plants were surrounded by living plants, whereas, in the other, they were surrounded by artificial plants that were painted black. It has been shown previously that, in a containing live plants, both the R FR ratio and light quantity decrease with depth into the (Holmes & Smith, 1977; Ballaré et al., 1987; Gilbert et al., 1995), whereas, in a of artificial black plants, only light quantity decreases with depth with no change in the R FR ratio. If a decrease in the R FR ratio leads to an increase in stem growth, elongation would be expected to be greater in the containing live plants when compared with that in the containing artificial black plants. Selected target plants were anchored to stakes to minimize the effect of flexing caused by wind. If mechanical stimuli reduce stem growth in the s, anchored plants should exhibit increased stem elongation. If excess transpiration has a significant role in inhibiting elongation, the anchoring of plants should not increase stem elongation. In addition, selected pots of some target plants (including both anchored and nonanchored plants) were lifted, lowered or left unchanged to examine which environmental factor is primarily responsible for height convergence (Nagashima & Hikosaka, 211). If height convergence is primarily caused by a change in light quality, height convergence should not be observed in the containing artificial black painted plants. If height convergence is primarily caused by mechanical stimuli, height convergence should not be observed in anchored plants. If height convergence is primarily caused by light quantity, height convergence should be observed in both s containing live plants and containing artificial black plants. The experiments also measured the effects of light quality, light quantity and mechanical stimuli on plant architecture and biomass allocation. An earlier study has shown that accelerated elongation is associated with a reduction in diameter growth and biomass allocation to leaves and roots (Nagashima & Hikosaka, 211). We address the question of which environmental factor is primarily responsible for the changes of growth. Materials and Methods Plant materials Chenopodium album L. (Chenopodiaceae) is a broad-leaved summer annual that often colonizes disturbed habitats (Ohwi & Kitagawa, 1983; Grime et al., 1988) and shows great plasticity in stem growth in response to environmental conditions (Morgan & Smith, 1979). Experiment The experiment was conducted in an experimental plot within Nikko Botanical Gardens, Nikko, Japan (36 75 N, E) in 23. Mean monthly air temperatures during the experiment were 16.7, 2. and 21.4 C for the experimental months of June, July and August, respectively. Vinyl pots (n = 515), each 1.5 cm in diameter and 22.5 cm in height, were filled with sand and tightly arranged on a bench (width, 1.1 m; length, 5 m; height, 5 m), which was then placed outdoors. Seeds were obtained from plants used in previous experiments (Nagashima & Hikosaka, 211). Following cold stratification, c. 1 seeds were sown per pot on 19 May. The pots were watered once or twice per day as required. Seedling emergence was observed on 23 May. Seedlings were thinned to leave two or three per pot by the end of May and one per pot by 1 June, in order to reduce size differences among seedlings. The final plant density was plants m )2. Plants were fertilized with 2 ml of fivefold strength Hoagland s complete nutrient solution (21 mg N, 3.1 mg P, 23 mg K, 16 mg Ca, 4.9 mg Mg, 6.4 mg S,.16 mg Fe per pot per week; Hoagland & Arnon, 195) every week commencing on 2 June. On 18 July, when the measured leaf area index (LAI) was 1.9, the heights of all potted plants were measured, and 84 plants whose heights were around the mean were selected as target plants (stem length, 38.8 ± cm; stem basal diameter, 4.1 ± mm). They were randomly divided into two groups (Fig. 1). One group of plants was grown surrounded continuously by other live plants. The other group was grown surrounded by artificial plastic plants that had a similar shape and size to real plants, but were painted black (Supporting Information Fig. S1). The artificial plants were individually placed into sand-filled pots and packed to establish a. Half of the target plants in each had their stems anchored to a stake with wire at intervals of 1

3 Phytologist Research 85 (a) experiment. The portions of anchored plant stems that became elongated were also anchored to the stakes at intervals of 5 cm as necessary. Fingers were used to provide similar mechanical stimuli to the other target plants. The pots of target plants were randomized within each combination of treatments 1 wk after the start of the treatments. Two weeks after the start of the treatments, when LAI was 3.2, target plants were harvested and separated into various organs (leaf, stem and root). Stem length was measured (to the nearest 1 mm) from the base to the terminal shoot apex. Stem basal diameter was measured with callipers (to the nearest.1 mm) at the middle of the first internode in directions orthogonal to each other and averaged. Images of leaves and a scale for measurement were recorded with a digital camera (Coolpix 995, Nikon, Tokyo, Japan). Leaf area was measured using image analysis software (NIH Image v. 1.63, National Institutes of Health, Bethesda, MD, USA). Dry mass was determined following oven drying at 8 C for 3 d. (b) Fig. 1 Experimental design: (a) treatments and (b) pot arrangements. Half of the target plants were surrounded by Chenopodium album plants and the other half by artificial plants that were painted black (light quality treatment). Half of the target plants were anchored to stakes and the other half were not anchored (stem anchoring treatment). Target plants were lifted (Li), lowered (Lo) or remained at the same level as surrounding plants (U) (pot elevation treatment). cm to eliminate the effect of sway by wind. The nonanchored target plants were touched with fingers so that any mechanical stimulus they received was similar to that received by the anchored plants. In a further treatment, selected target plant pots were lifted by 1 cm (Li, lifted), lowered by 1 cm (Lo, lowered) or left at the same level (U, unchanged) as neighbouring pots. Seven pots were assigned to each combination of treatments. The height of the plastic plants was adjusted to the height of the nonanchored, unchanged target plants every 3 d during the course of the Determination of microenvironmental variables The vertical distribution of photosynthetically active radiation (PAR) in the s was measured with a quantum sensor (LI-19, LI-COR Inc., Lincoln, NE, USA) on 26 July, when weather conditions were overcast. Measurements were made at height intervals of 1 cm near each of seven nonanchored, unchanged target plants (seven replicates). Three measurements were taken at each height interval and were averaged to represent PAR at the height position of the plant. PAR was expressed as a relative value against a reference PAR measured above the canopy with another LI-19 sensor. The red ( nm) to far-red ( nm) (R FR) ratio of light coming from a horizontal direction was measured with a portable spectroradiometer (LI-18, LI-COR Inc.) at height intervals of 1 cm near each of five randomly chosen target plants after carefully removing the target plants (five replicates). Measurements were made between 1: and 14: h on 27 July under clear sky conditions. Measurements were taken for each height with the sensor facing the horizon in the direction of the four cardinal azimuths, and these four measurements per height were averaged. Wind speed in the s was measured with a portable wind meter (Kestrel 4, Nielsen Kellerman, Boothwyn, PA, USA) on 25 July, when the ambient average and maximum wind speeds were and 2.3 m s )1, respectively. Speeds were measured at three points at 1-cm intervals near nonanchored, unchanged target plants in each (seven replicates). A reference wind speed was measured above the canopy with another portable wind meter (Kestrel 4, Nielsen Kellerman). The daily average and maximum wind speeds during the experiment were 1.2 ±.5 and 3.1 ± 1.4 ms )1 (mean ± ard deviation), respectively, at the nearest weather station (Imaichi, 9 km from the garden). Data analyses The effects of the black artificial plants on the vertical distribution of PAR, R FR ratio and wind speed within the canopy were analysed using a two-way repeated measures analysis of variance

4 86 Research Phytologist (ANOVA) with plants as replicates after confirmation of parametric assumptions (JMP Statistical Software, SAS Institute Inc., Cary, NC, USA). The effects of painting artificial plants black, stem anchoring, pot elevation and their interactions on stem elongation, stem diameter growth and dry mass partitioning were analysed by a three-way ANOVA after confirmation of parametric assumptions. The Tukey Kramer honestly significant difference test was used for post hoc pairwise comparisons. The significance of height convergence in target plants was tested as follows: the mean value of the stem length of nonanchored, unchanged plants in each (which was regarded as a neighbour s height in each ) was subtracted from the apparent heights of target plants. The apparent height is the stem length + 1 and ) 1 cm for lifted and lowered plants, respectively. A normal distribution was assumed for the apparent height difference and the probability of stem elongation being 1 and ) 1 cm was calculated for lifted and lowered plants, respectively. If the probability was < 5%, the target plants were regarded as height converged. Results Microenvironments in the s PAR decreased with depth in both the containing live plants and that containing artificial black plants. The reduction in PAR was slightly smaller in the containing artificial black plants (P <.1, <.1 and 53 for the effects of height, black coloration and their interaction, respectively; Fig. 2a). The containing live plants showed a significant reduction in the R FR ratio with increasing depth in the canopy, whereas the containing artificial black plants showed only a slight reduction (P <.1 for all three effects; Fig. 2b). Wind speed decreased with increasing depth in both s, and the profile was not significantly different between the s (P <.1,.558 and.58 for the effects of height, black coloration and their interaction, respectively; Fig. 2c). Effects of environmental factors on stem elongation Stem elongation was affected significantly by all treatments: a large part of the variation was explained by black coloration, followed by pot elevation and stem anchoring (Tables 1, S1). The interaction effect was only significant for the interaction of black coloration and pot elevation. Stem length was greater in the containing live plants than in the containing artificial black plants for each combination of anchoring and pot elevation, indicating that reduced R FR ratio positively influenced stem elongation (Fig. 3a). Anchored plants exhibited increased stem length irrespective of the type of surrounding plants and pot elevation (Fig. 3a), indicating that reducing the mechanical stimuli also had a significant positive effect on stem elongation. The effect of anchoring had no interactive effects with other treatments, suggesting that it influences stem elongation independently and additively to other factors (Table 1). Lifted plants exhibited reduced elongation and lowered plants exhibited increased Relative PAR R/FR Relative wind speed (a) (b) (c) Canopy height Height (cm) Fig. 2 Microenvironments in the real plant (open circles) and the including artificial plants painted black (closed circles): (a) photosynthetically active radiation (PAR) relative to that c. 3 cm above the canopy; (b) red to far-red (R FR) photon ratio in horizontally incident light; (c) wind speed in the relative to that c. 3 cm above the canopy. Error bars represent ± SE. elongation in the containing live plants, as observed in a previous study (Nagashima & Hikosaka, 211). The response of stem length to pot elevation was insignificant in the containing artificial black plants (Fig. 3a). These results suggest that the effects of light quantity on elongation are small compared with those of light quality. The height difference between target plants and neighbouring plants was also measured (Fig. 4). In the containing live plants, unchanged plants without anchoring had heights similar to those of their neighbours. In the containing live plants, for both unanchored lifted and lowered plants, the height difference recorded between the target plant and its neighbours was reduced by 5 cm compared with the initial difference of 1 cm. This indicates that height convergence was occurring, as observed in a previous study (Nagashima & Hikosaka, 211). In the containing artificial black plants, where the height of the artificial plants was maintained at a similar height to that of unchanged plants, the unanchored lifted and lowered plants maintained a

5 Phytologist Research 87 Table 1 Three-way ANOVAs for measures with artificial black plants, stem anchoring and pot elevation as factors (see Fig. 1); F values are shown Artificial black plants Stem anchoring Pot elevation Artificial black plants stem anchoring Artificial black plants pot elevation Stem anchoring pot elevation Artificial black plants stem anchoring pot elevation Model df Stem length 173.7*** 28.9*** 47.3*** ** *** Diameter 14.4*** 7.3** ** Total mass 8.9** 5.6* 29.9*** *** Root mass 33.3*** ***.5 3.4* *** Stem mass * 18.5*** *** Leaf mass 13.2*** 6.4* 21.1*** *** Root total 34.4*** 4.9**.3 3.4* *** mass Stem total 83.7*** 12.1*** *** mass Leaf total 9.5** mass Lamina area 7.8** 6.8* SLA 6.6* ***.1 6.9** *** LAR ***. 4.3* *** LAR, leaf area ratio (lamina area total mass); SLA, specific leaf area (lamina area lamina mass)., P <.1; *, P <.5; **, P <.1; ***, P <.1. constant height difference with their neighbours throughout the experiment. These result suggests that differences in not light quantity but light quality are crucial for height convergence to occur. Anchored, unchanged plants in the containing live plants grew significantly taller than their neighbours (Fig. 4). Plants that had been lowered exhibited a reduction in the initial height difference with their neighbours, whereas plants that had been lifted maintained the initial height difference throughout the experiment. Similar trends were also found in anchored plants in the containing artificial black plants. These results suggest that height convergence does not occur when mechanical stimuli are eliminated. Stem diameter and biomass partitioning Stem diameter was affected by the black coloration and pot elevation treatments, but not by stem anchoring (Table 1). No interaction was found in any combination of the treatments. Plants in the containing artificial black plants tended to have a larger diameter than those in the containing live plants (Fig. 3b). Stem diameter tended to be larger and smaller in lifted and lowered plants, respectively (Fig. 3b). The ratio of length to diameter was influenced by all treatments (Table 1). The ratio tended to be higher in the containing live plants than in the containing artificial black plants, in anchored plants relative to nonanchored plants, and in lowered plants relative to lifted plants (Table S2). Interaction effects were detected, with the effects of stem anchoring and pot elevation tending to be greater in the containing live plants than in the containing artificial black plants (Tables 1, S2). Biomass and its partitioning among organs were affected by the treatments (Table 1). Total mass was greater in the containing artificial black plants than in the containing live plants, in anchored plants than in nonanchored plants, and in lifted plants than in lowered plants (Table S2). Biomass partitioning was influenced by the black coloration and pot elevation treatments, but not by anchoring (Table 1). There was an interaction effect between the black coloration and pot elevation treatments on root mass fraction (root total mass, RMF) and stem mass fraction (stem total mass, SMF). In the containing live plants, RMF tended to be greater in lifted plants than in both unchanged and lowered plants, whereas SMF exhibited the opposite response (Fig. 5). This response to pot elevation in RMF and SMF was not observed in the containing artificial black plants. Leaf mass fraction (leaf total mass, LMF) was not affected by pot elevation. These results suggest that biomass partitioning was affected primarily by light quality rather than by light quantity or mechanical stimuli. The specific leaf area (SLA) and leaf area ratio (LAR) were affected by pot elevation, and there was an interaction effect observed between the black coloration and pot elevation treatments (Table 1). SLA and LAR were higher in plants that had been lowered than in plants that had been lifted in the containing live plants (Table S2). There was an interaction effect observed between the black coloration and pot elevation treatments with responses to pot elevation being smaller in the containing artificial black plants than in the containing live plants. Discussion The results of this study clearly show that stem elongation is influenced by light quality, light quantity and mechanical stimuli. They also indicate that two of the factors, light quality and mechanical stimuli, are important environmental cues that induce height convergence, given that height convergence did not occur when the effects of light quality and mechanical stimuli

6 88 Research Phytologist Stem length (m) Stem basal diameter (mm) (a) c (b) d bc bcd b d Not anchored b cd c bcd a Anchored Real-plant d were eliminated. It is well known that light quality has a significant effect on stem growth in field s (Ballaré et al., 199; Ballaré, 1994), and it has been shown in an elegant experiment that, in a light quality gradient, plants keep up with their neighbours (Vermeulen et al., 28). The involvement of the other factors, however, has not been demonstrated previously. The results of this study suggest that stem elongation of plants in crowded s is governed by a complex interaction of different environmental factors. Anchored plants showed greater stem elongation, which is consistent with previous studies demonstrating that wind stimuli reduce elongation (e.g. Latimer et al., 1986; Holbrook & Putz, 1989; Henry & Thomas, 22) and that manual swaying affects stem length (e.g. Smith & Ennos, 23; Anten et al., 25, 26, 29). These results suggest that mechanical stimuli caused by wind action can be a significant constraint to stem elongation. Meanwhile, it has been argued that enhanced de bc e abc de bcd Not anchored d ab de a c bcd Anchored Black-painted Fig. 3 The length (a) and basal diameter (b) of stems of target Chenopodium album plants 2 wk after the start of the treatments (see Fig. 1). Plants were lifted (grey bars), lowered (black bars) or left unchanged (white bars). Different letters indicate significant differences (P <.5) between treatments according to the Tukey Kramer test (n = 7 in each group). Error bars represent + SE. Difference in apparent height from that of neighbouring plants (m) * * Not anchored * Anchored Real-plant *** Not anchored Anchored Black-painted Fig. 4 The difference in apparent height from that of neighbouring Chenopodium album plants 2 wk after the start of the treatments (see Fig. 1). Asterisks indicate the significance level of differences in apparent height before (,.1 and ).1 m for unchanged, lifted and lowered plants, respectively) and after treatments (*, P <.5; ***, P <.1; see the Materials and Methods section). Error bars represent ± SE. Unchanged, white bars; lifted, grey bars; lowered, black bars. evapotranspiration resulting from wind action may constrain overtopping (Nagashima & Hikosaka, 211). In the present study, however, anchored unchanged plants overtopped neighbours and produced greater biomass, suggesting that enhanced evapotranspiration was not a constraint. It should be noted that the wind speed during our experiment was relatively low. For example, we observed a maximum wind speed of 25 m s )1 in our previous study (Nagashima & Hikosaka, 211), whereas it was only 5 m s )1 in the present study. Therefore, it is still uncertain whether or not evapotranspiration is a constraint under strong winds. The pot elevation treatment influenced stem growth in the containing artificial black plants, suggesting that light quantity also affects stem elongation. This is consistent with previous work demonstrating that the photon fluence rate itself affects elongation (Ballaré et al., 1991). In the present study, stem length was longer in plants that had been lowered relative to those that had been lifted, despite having a smaller total biomass. This indicates that stem elongation is not simply related to the availability of photosynthates, but is also influenced by the reduction in light quantity. However, the effect of reduced light quantity on stem elongation was too small to cause height convergence, suggesting that it is less important than the effects of light quality and mechanical stimuli. In the present study, PAR was 15% higher in the black-painted than in the real plant. One may therefore consider that it is hard to distinguish to what extent the increased length of the target plants surrounded by living plants was caused by reduced PAR or R FR ratio and the lesser response to pot elevation in the black-painted might result from greater PAR. In our previous study, however, we used a lower plant density (77 vs 116 plant m )2 for previous and present studies, respectively) and a *

7 Phytologist Research 89 (a) Root (b) Stem (c) Leaf Proportion.5.3 c ab bc a a a.5.3 a a b bc c bc.5.3 ab ab b ab a a Realplant Blackpainted Realplant Blackpainted Realplant Blackpainted Fig. 5 The proportion of biomass of target Chenopodium album plants partitioned to the roots (a), stems (b) and leaves (c) 2 wk after the start of the treatments (see Fig. 1). Anchoring and nonanchoring treatments were pooled. Different letters indicate significant differences (P <.5) between each combination of pot elevation and light quality treatments according to the Tukey Kramer test (n = 14 in each group). Error bars represent + SE. Elevation treatments: unchanged, white bars; lifted, grey bars; lowered, black bars. smaller height difference between lifted and lowered plants (7 vs 1 cm). The relative PAR at the top of lowered plants was greater in the previous study than in the present black-painted plants (95% vs 6%, data not shown). In the previous study, therefore, height convergence occurred even when the variation in light quantity was smaller than in the black-painted in the present study. These results strongly indicate that height convergence occurs in response to light quality and mechanical stimuli, rather than light quantity. The light quality and mechanical stimuli treatments produced different effects on stem diameter growth and biomass allocation. Accelerated elongation in plants experiencing reduced R/FR ratio was associated with a decrease in diameter growth and an increase in biomass allocation to stems at the expense of that to roots, consistent with previous studies (Maliakal et al., 1999). Stem elongation in plants experiencing a reduction in mechanical stimuli was not accompanied by significant changes in diameter growth or biomass allocation, but simply by an increase in total biomass. Although both light quality and mechanical stimuli treatments affected significantly the length to diameter ratio, the effects of the former were greater (Table S1). These results suggest that stem shape and biomass allocation are more sensitive to variations in light quality than to mechanical stimuli. These results suggest that stem growth in field s is regulated primarily by two independent systems, with one responding to variations in light quality and the other responding to mechanical stimuli. This is supported by the observation that there was no interactive effect of light quality and mechanical stimuli on stem elongation, and that the effects on diameter growth and biomass allocation also varied between the light quality and mechanical stimuli treatments. These regulatory systems are interesting from an ecological aspect. The former system enables plants to avoid the effects of significant shading by neighbours, whereas the latter system may assist plants in avoiding the mechanical stresses that can occur when they overtop their neighbours. Indeed, in an experiment with Impatiens capensis, elongated plants at high density were more prone to fail mechanically and to show reduced reproduction (Huber et al., 211), suggesting that the control of mechanical robustness may influence fitness in elongating plants directly. These regulatory systems may also be consistent with constraints assumed in game-theoretical models for plant height (Givnish, 1982; Iwasa et al., 1985; Sakai, 1991; Falster & Westoby, 23). Height convergence is a result of the two regulatory systems and may not occur if either one of them is not effective. Conclusion This study presented three novel findings. First, variations in light quality, quantity and mechanical stimuli all influence stem elongation in a crowded in the field. Second, light quality and mechanical stimuli are crucial for height convergence of plants. If one of these environmental factors is not present, height convergence does not occur. Third, light quality and mechanical stimuli affect plant growth in different ways. Plants can detect the presence of neighbours and physical stresses on stems, each of which influences stem elongation independently. Regulatory systems are employed by plants to avoid both being shaded by neighbours and suffering from mechanical stresses, which could contribute to the success of individual plants under conditions of competition for light in field populations. Acknowledgements The authors thank anonymous reviewers, T. Hirose and M. Tateno for their valuable comments and suggestions, H. Takahashi for the experimental set-up, N. Osada, Y. Osone and H. Taneda for assistance with the experiment and for their valuable comments, and M. Aiba for statistical advice. This research was supported by fellowships from the Japan Society for the Promotion of Science (nos. 325 and 4172) and KAKENHI (no. 1261) to HN and by KAKENHI (nos and ) to KH.

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9 Phytologist Research 811 Weinig C. 2. Differing selection in alternative competitive environments: shade-avoidance responses and germination timing. Evolution 54: Supporting Information Additional supporting information may be found in the online version of this article. Fig. S1 Photograph of experimental s. Table S2 Stem dimensions, plant dry mass, dry mass partitioning, leaf area and growth parameters of target plants 2 wk after the start of the treatments Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing material) should be directed to the Phytologist Central Office. Table S1 Percentage of partial sum of squares in three-way ANOVAs for measures with artificial black plants, stem anchoring and pot elevation as factors Phytologist is an electronic (online-only) journal owned by the Phytologist Trust, a not-for-profit organization dedicated to the promotion of plant science, facilitating projects from symposia to free access for our Tansley reviews. Regular papers, Letters, Research reviews, Rapid reports and both Modelling/Theory and Methods papers are encouraged. We are committed to rapid processing, from online submission through to publication as ready via Early View our average time to decision is <25 days. There are no page or colour charges and a PDF version will be provided for each article. The journal is available online at Wiley Online Library. Visit to search the articles and register for table of contents alerts. If you have any questions, do get in touch with Central Office (np-centraloffice@lancaster.ac.uk) or, if it is more convenient, our USA Office (np-usaoffice@ornl.gov) For submission instructions, subscription and all the latest information visit