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Keywords:

  • aerenchyma;
  • flooding;
  • soil compaction

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  • 1
    We studied the differences in root strength of species with contrasting root structural types (the grass Paspalum dilatatum and the dicot Lotus glaber), and their relationship with tolerance to simulated cattle trampling under flooding conditions.
  • 2
    Root strength was analysed through measurement of the pressure required to cause root collapse. The responses of aerenchyma and plant mass to flooding and trampling were studied.
  • 3
    Root aerenchyma increased from 28·0 to 40·2% in P. dilatatum and from 12·9 to 19·7% in L. glaber under flooding conditions. The increase in aerenchyma did not affect root strength in the relatively trampling-resistant roots of P. dilatatum: roots cracked at >380 kPa in all treatments. In contrast, roots of L. glaber were weaker, cracking at 260 kPa; flooded roots with air spaces irregularly dispersed in the cortex cracked at 115 kPa.
  • 4
    Trampling, flooding or their combination did not affect the biomass of P. dilatatum. Conversely, the isolated effects of either trampling or flooding both decreased biomass accumulation in L. glaber. The combination of both treatments killed all Lotus plants.
  • 5
    In conclusion, root strength was positively associated with soil trampling tolerance. The effect of aerenchyma tissue generation on root strength varies among root structural types. Aerenchyma tissue increases root weakness in the less stable structural type of the dicot species, but had no effect on the strength of the grass.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Natural flooding and trampling by large herbivores are major disturbances that co-occur in many grassland regions of the world (McNaughton 1983; Soriano 1992; van der Wal & Brooker 2004). Flooding causes a drastic restriction of gas exchange between the soil and the atmosphere, leading to a rapid depletion of oxygen in the soil (Ponnamperuma 1984). Most flood-tolerant species are capable of generating additional aerenchyma tissue that allows the conduction of air to submerged tissues (Armstrong 1979). Animal trampling, co-occurring with grazing, might add another stress factor for plants, especially if flooding facilitates deeper penetration of the animal hoof into the soil (Warren et al. 1986; Finlayson et al. 2002). Trampling tolerance requires strong root structures to prevent root collapse and to ensure the functioning of the root system (Goodman & Ennos 2001). Both attributes – high root porosity and strong root structure – are thus desirable traits to enable plants to tolerate the simultaneous effects of soil trampling and flooding. However, an apparent trade-off between root mechanical strength and ventilating traits was proposed by Justin & Armstrong (1987) and reinforced by Engelaar, Jacobs & Blom (1993), who suggested that the generation of aerenchyma tissue associated with flood tolerance could reduce root strength. We address this issue.

Aerenchyma tissue configuration in the root cortex is highly variable among flood-tolerant species (Smirnoff & Crawford 1983; Justin & Armstrong 1987; Grimoldi et al. 2005). Four main root structural types (graminaceous, cyperaceous, Apium and Rumex) have been described, based on the cell cortical packing (cubic or hexagonal) and the spatial configuration of the aerenchyma tissue generated by flooding (sensuJustin & Armstrong 1987). The most contrasting root structures are the graminaceous and dicot types (Justin & Armstrong 1987). A cross-section of a graminaceous root resembles a bicycle wheel, with air spaces separated by arrays of intact cells disposed radially, surrounded by a ring of sclerenchymatic tissue (Smirnoff & Crawford 1983; Vasellati et al. 2001; McDonald, Galwey & Colmer 2002). In contrast, air spaces in dicot roots are irregularly dispersed throughout the cortex (Justin & Armstrong 1987; Grimoldi et al. 2005), and usually lack a ring of thick cells in the outer cortex (Visser et al. 2000). Functional differences between graminaceous and dicot roots have been described recently with regard to gas transport in relation to physical barriers in the outer layers of the cortex (Visser et al. 2000; McDonald et al. 2002). Those studies clearly demonstrated that the physical barrier prevented radial oxygen loss in the graminaceous type. However, the influence of root structure on mechanical strength and resistance to soil compression (as produced by cattle trampling) has not been addressed.

Our objective was to investigate the relationship between root structure, mechanical resistance to root compression, and tolerance to cattle trampling under flooding conditions. Based on previous studies, we selected the grass Paspalum dilatatum (Rubio, Casasola & Lavado 1995; Vasellati et al. 2001) and the dicot Lotus glaber (Striker et al. 2005) as representatives of the contrasting root types described above. We quantified the required pressure for root collapse (as a measure of mechanical strength) in control and flooded plants. We also studied aerenchyma and plant responses to flooding and simulated cattle trampling. To the best of our knowledge this is the first comparative study to deal with the connection between different root structural types and their tolerance to mechanical stress. We also present the first quantitative approach to elucidate if the generation of aerenchyma tissue affects root strength in two contrasting root types.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

plant material and study site

In September 2003, adult plants of similar size to the grass Paspalum dilatatum Poir. and the dicot Lotus glaber Mill. were extracted on the same collection day in grassland soil blocks from natural vegetation (0·3 × 0·3 × 0·25 m), taking the target individual as the centre of the block. The depth of the chosen block ensured collection of most of the underground system of the target plants because 65–70% of the root biomass in this grassland is concentrated in the first 0·1 m of the soil depth (Soriano 1992). Plants were taken from an extensive stand of a lowland grassland located in the Department of Pila, Province of Buenos Aires, Argentina (36°30′ S, 58°30′ W), defined phytosociologically as the community of Piptochaetium montevidensis, Ambrosia tenuifolia, Eclipta bellidioides and Mentha pulegium (Burkart, León & Movia 1990). This plant community is found in flat areas exposed annually to floods of varying intensity and duration (Soriano 1992). In summer floods remain for less than 3 weeks. After extraction, the individuals were immediately put in plastic containers and placed interspersed in the experimental garden of the Faculty of Agronomy at the University of Buenos Aires.

flooding and simulated cattle trampling

After 2 months’ acclimatization, we subjected each plant species to a 2 × 2 factorial completely randomized design of flooding and trampling treatments with five replicates. Flooding treatment was applied for 15 days at a water level of 6 cm above the soil surface, followed by a recovery phase of 30 days. The simulated cattle trampling was applied a few hours after flooding in four places (≈40% of soil block area) equidistant to the main axis of the target individual, but explicitly avoiding the centre of the plants. The flooding treatment was characterized by measurement of the oxygen diffusion rate at a soil depth of 5 cm with platinum micro-electrodes (Letey & Stolzy 1964); and the trampling treatment by measuring soil dry bulk density in soil core samples of 0·1 m depth following the methodology proposed by Taboada & Lavado (1988).

Trampling treatment was simulated with an apparatus adapted from Abdel-Magid, Trlica & Hart (1987) based on the principle of a second-class lever (Fig. 1). The apparatus was built to simulate the effect of one breeding cow of 400 kg, assuming an equal distribution of its weight on four hooves (82 cm2 per hoof). The static load at the hoof was then ≈120 kPa [(100 kg per 82 cm2) × (98·06 kPa cm2 kg−1)] (Fig. 1). In order to simulate the hoof impact of a treading cow (dynamic load), we measured the soil dry bulk density in continuously grazed sites and used these values as a reference to be reached by our system. After preliminary tests we determined that the hoof should be dropped from 0·25 m height (Fig. 1) to compact the soil to simulate cattle trampling at field conditions (both at hydric contents close to field capacity). In comparison, the hooves of adult cows exert dynamic pressures of ≈300 kPa (Scholefield & Hall 1986; Abdel-Magid et al. 1987).

image

Figure 1. System to simulate cattle trampling at the experimental garden. The static load at the hoof was ≈120 kPa. The hoof was dropped from 0·25 m high to cause soil compaction similar to cattle trampling. For further details see Materials and methods.

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anatomical analyses

The fractional porosity of roots and sheaths in P. dilatatum and of roots and stems in L. glaber was quantified at the end of the flooding period (day 15) using the pycnometer method (Sojka 1988), based on the weight increase that occurs when air spaces of plant tissues are replaced by water after maceration. In addition, samples of lateral roots taken 2–5 cm from root tips were taken from five plants per treatment, washed carefully, and immediately fixed in formalin–acetic acid–ethanol (FAA). Root sections ≈2 mm long were dehydrated and embedded in paraffin. Cross-sections 15–20 µm wide were cut with a rotary microtome and stained with safranin–Fast Green. For each plant, 10 sections were selected randomly for observation, and representative images were taken using an optical microscope (Zeiss Axioplan, Zeiss, Oberkochen, Germany) connected to an image analyser (Imagenation Px, Imagenation Corp., Beaverton, OR, USA).

root compressive strength

We measured the pressure required for root collapse in control and flooded plants of P. dilatatum and L. glaber (Fig. 2). Roots were compressed laterally with a blunt-ended brass shaft (19 × 11 mm) driven by the piston of a mini-cylinder connected to a pneumatic circuit (Duprat et al. 1995). A flow regulator (needle valve HOKE 1315G2B, Spartanburg, SC, USA) connected to the circuit ensured a low constant flux of pressurized air (1·06 l min−1 for all measurements), obtaining a slow compression of the root sample. The circuit was connected to a pressure transducer (ADZ Nagano S-010bar, Ottendorf, Germany). The pressure was recorded when the root cracked and small drops appeared on its surface. The force (F) to cause root collapse was calculated as: F = pressure in the cylinder (kPa) × internal piston area (cm2). In our system the internal piston area was 0·282 cm2. Root compressive strength (kPa) was then calculated as: F/[root diameter (cm) × compressed root length (cm)] (Niklas 1992). Root diameter and root collapse were observed with a binocular loupe with a reticule in the eyepiece. Measurements were made at the end of the flooding period (day 15) on similar lateral roots located in the first 5 cm soil depth in sections between 2 and 5 cm above the root apex. Each value represents the average of three subsamples per plant (n = 5). In both species, root diameters were similar between control and flooded treatments. During measurement, the root system was not separated from the shoot.

image

Figure 2. Equipment to evaluate root mechanical strength. Roots were laterally compressed with a blunt-ended brass shaft driven by the piston connected to a pneumatic circuit supplied by pressurized air. A flow-regulator valve connected to the circuit ensured the attainment of a low constant flux of air and slow compression of the root sample. The pneumatic circuit was connected to a pressure transducer, the output electric signal from which was recorded into a data logger. For further details see Materials and methods.

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survival and biomass

Plant survival was checked daily. Biomass harvests were carried out at the beginning of the experiment in additional randomly chosen individuals (day 0) and at the end of the recovery phase (day 45). Plant biomass was separated into shoot and root. Shoots were separated into green and dead material. The ratio of dead material to total shoot biomass was calculated for each species and treatment. In P. dilatatum, rhizomes were included in the root fraction. The crown of L. glaber was included in the shoot fraction. The material was weighed after oven drying for 72 h at 70 °C.

statistical analyses

No plants of L. glaber survived both trampling and flooding, and the design did not remain fully factorial. Therefore we evaluated differences for porosity and biomass between species using two different two-way anovas: one including species and flooding as main factors; another with species and trampling. For P. dilatatum, an additional Student's t-test was performed to compare control and trampling × flooding treatments. Root strength data were evaluated by one two-way anova with species and flooding as main factors. In all cases, a posteriori means comparisons were performed with Tukey's tests. All data sets were checked to satisfy anova assumptions; variables that involved percentages were (arcsine √x)-transformed. All results are presented as untransformed means of five replicates ± standard error.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

soil bulk density and oxygen diffusion rate

At the experimental garden, soil dry bulk density was affected significantly by trampling simulation (one-way anova, F3,16 = 4·5, P < 0·05), increasing 23% in relation to control blocks (1·25 ± 0·07 vs 1·02 ± 0·03 g cm−3). No differences were found in this parameter (t = 0·2, df = 8, P = 0·84) between blocks trampled by our simulation system and samples taken at field conditions from continuously grazed sites (1·27 ± 0·07 g cm−3). Flooding decreased the oxygen diffusion rate from 60 ± 1 to 6 ± 1 × 10−8 g cm−2 min−1 in the first 3 days of treatment; and to 0·6 ± 0·3 × 10−8 g cm−2 min−1 at day 15. When flooding was discontinued, all blocks recovered the original values during the first 5 days. Soil compaction produced by trampling reduced the oxygen diffusion rate to 75–83% in relation to control blocks.

anatomical traits and root mechanical strength

Root porosity differed among species (two-way anovas, F1,16 > 165·21, P < 0·001). Control plants of P. dilatatum presented higher values of root porosity than L. glaber (Table 1). In the grass, porosity corresponded to an extensive system of lysigenous aerenchyma tissue arranged radially in the root cortex, separated by rows of parenchymatic cells and surrounded by a ring of sclerenchymatic cells in the exodermis (Fig. 3a,b). In the dicot, constitutive porosity was related mainly to intercellular air spaces because of the cubic configuration of the cells in the medium and outer cortex, with few lysigenous lacunae. Also, L. glaber did not present a ring of thicker cells in the outer layers of the cortex (Fig. 3e,f). Flooding increased root porosity in both species (F1,16 = 41·30, P < 0·001; species × flooding: F1,16 = 2·64, P = 0·12; Table 1). In P. dilatatum the higher value of root porosity under flooding conditions was achieved without major changes in the root structure (Fig. 3a,bvs c,d). In L. glaber the higher root porosity was achieved by the development of lysigenous aerenchyma lacunae (Fig. 3g). Sheath porosity of P. dilatatum and stem porosity of L. glaber were also higher in flooding conditions (F1,16 = 28·37, P < 0·001; species × flooding: F1,16 = 2·67, P = 0·12; Table 1). Trampling reduced root porosity in both species (F1,16 = 8·64, P < 0·01; species × trampling: F1,16 = 0·11, P = 0·74; Table 1), while porosity of the aerial organs remained unchanged (F1,16 = 0·34, P = 0·56; Table 1).

Table 1.  Fractional porosity (%), quantified using the pycnometer method, of Paspalum dilatatum and Lotus glaber plants grown for 15 days under different treatments: control; trampling; flooding; trampling × flooding (T × F)
Porosity (%)Treatment
ControlTramplingFloodingT × F
  • Simulated trampling was applied at the beginning of the experiment; the flooding period lasted 15 days. Values are means ± SE of five replicates.

  • , No surviving plants.

Paspalum dilatatum
Roots28·0 ± 1·424·5 ± 2·440·2 ± 1·436·1 ± 3·9
Sheaths 8·2 ± 0·9 9·1 ± 1·219·7 ± 2·321·3 ± 2·7
Lotus glaber
Roots12·9 ± 1·1 7·5 ± 1·419·7 ± 1·7
Stems 6·8 ± 1·1 7·1 ± 0·912·9 ± 1·9
image

Figure 3. Transverse sections of roots of Paspalum dilatatum (a–d) and Lotus glaber (e–h) plants grown for 15 days under different treatments: control (a,e); trampling (b,f); flooding (c,g); trampling × flooding (d,h). Simulated trampling was applied at the beginning of the experiment. Plants of L. glaber subject to trampling in flooded soil did not survive. Arrows indicate the ring of sclerenchymatic tissue for mechanical protection in P. dilatatum. L., aerenchyma lacunae for root oxygenation. Bar, 100 µm for P. dilatatum; 80 µm for L. glaber.

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Root mechanical strength differed among species (two-way anova, F1,16 = 38·33, P < 0·001). The grass P. dilatatum presented higher values for this parameter than the dicot L. glaber (Fig. 4). The effect of flooding on this parameter differed between species, as indicated by the significant species × flooding interaction (F1,16 = 7·29, P < 0·01). For the grass, the root structure resembled a bicycle wheel, and required pressures higher than 380 kPa to cause collapse (Fig. 4). In this species, root structure was not weakened by the generation of new aerenchyma tissue (Tukey's test, P = 0·68), as 423 kPa radial pressure was required to cause root collapse (Fig. 4). Roots of the dicot L. glaber cracked at pressures of 260 kPa (Fig. 4). Under flooding conditions, the root structure of L. glaber was considerably altered and weakened by flood-induced anatomical changes (Tukey's test, P < 0·001), as only 115 kPa pressure was required to cause root collapse (Fig. 4).

image

Figure 4. Root strength of Paspalum dilatatum and Lotus glaber plants grown under control and flooding conditions for 15 days. Root diameter within species was similar between treatments (1·17 ± 0·10 and 1·09 ± 0·08 mm for P. dilatatum; 0·93 ± 0·07 and 0·82 ± 0·11 mm for L. glaber under control and flooding conditions, respectively). Values are means ± SE of five replicates.

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survival and biomass

Trampling and flooding produced differential responses among P. dilatatum and L. glaber for shoot biomass (two-way anovas; species × flooding: F1,16 = 6·23, P = 0·02; species × trampling: F1,16 = 6·33, P = 0·02). Flooding also provoked differences for root biomass among species (species × flooding: F1,16 = 4·64, P = 0·04). In the grass P. dilatatum shoot and root biomass, and the proportion of dead material, were similar among treatments (P > 0·5 for all variables; Fig. 5a), although shoot biomass tended to be slightly higher under flooding conditions. Flooded and trampled plants of this species did not differ from controls in shoot (t = 0·7, df = 8, P = 0·49) and root biomass (t = 0·1, df = 8, P = 0·92). In L. glaber, Tukey's tests revealed that trampling and flooding treatments significantly decreased shoot (P < 0·05) and root biomass (P < 0·01) in relation to control plants (Fig. 5b). Neither trampling nor flooding, as isolated disturbances, produced death of plants in this species. Remarkably, all plants of L. glaber subject to the simultaneous effects of trampling and flooding died within the first 6 days of the experiment, significantly more than in the control treatment (log-rank test, χ2 = 20, df = 3, P < 0·001). The proportion of dead material was higher under flooding (34 ± 5%; P < 0·05) and trampling (37 ± 6%; P < 0·05) compared with control conditions (19 ± 2%). Under flooding conditions, dead material was distributed equally through the plant; in contrast, trampled individuals presented dead stems only at the plant's periphery where they presumably received direct impacts from the simulated trampling system.

image

Figure 5. Shoot and root biomass of Paspalum dilatatum (a) and Lotus glaber (b) plants grown under different treatments: control (C); trampling (T); flooding (F); trampling × flooding (T × F). Simulated trampling was applied at the beginning of the experiment. The duration of experimental flooding was 15 days. Shoot biomass did not include dead material. †, No surviving plants. Values are means ± SE of five replicates.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

specific differences in root strength

The results show that the graminaceous root structural type of P. dilatatum, which requires higher mechanical stress to cause root collapse, is not weakened by the generation of additional aerenchyma tissue under flooding conditions. The cross-section of graminaceous roots resembles a bicycle wheel: air spaces are disposed radially, completely surrounded by a ring of sclerenchymatic tissue (Fig. 3a–d). Accordingly, the sclerenchymatic tissue appears to bring mechanical protection to the roots, as in other plant tissues (Reich et al. 1991; Niklas 1992; Niklas 1999). In addition, the particular radial alignment of the cells in the root cortex, parallel to the direction of the mechanical forces, might also contribute to enabling the formation of a physically stable structure. This idea was not suggested previously for internal architecture of herbaceous roots, but follows the same reasoning as proposed by Niklas (1999) for thick-walled cells within the leaf base of Chamaedorea, which possesses a remarkable rigidity owing to its special radial configuration. Under flooding conditions, the increase in root porosity of P. dilatatum is mainly due to changes in the proportion of root cortex (Grimoldi et al. 2005). Consequently, internal root architecture is basically the same among control and flooded plants (Fig. 3a,bvs c,d), allowing maintenance of root strength even after the registered increase in root porosity of plants subject to flooding conditions.

Much lower pressures are required to provoke the collapse of the dicot root structural type of L. glaber. Remarkably, the generation of aerenchyma tissue in response to flooding increases root weakness more than twofold for this species (Fig. 4). Evidence for differential root tolerance to compression under flooding conditions has not been presented previously, thus opportunities for comparison are minimal. Both the lower area of contact among cells of the cubic configuration (Justin & Armstrong 1987) and the lack of a ring for mechanical protection in the outer section of the cortex (Fig. 3e,f) could facilitate deformation of the root in front to radial compressive forces (Niklas 1992), helping to explain the lower mechanical stability of this structure. Under flooding conditions, the root architecture of the dicot species was altered largely due to the generation of aerenchymatic lacunae in the outer and middle sections of the cortex. The irregular disposition of the air spaces further reduced the contact among cortical cells (Fig. 3g), which appears to be closely related to the significant reduction in mechanical strength of the roots under flooding conditions. Therefore our results strongly suggest that, in dicot roots, the generation of additional aerenchyma tissue in the root cortex involves an evident trade-off among root oxygenation possibilities and the mechanical strength to compressive forces.

root strength mediates trampling tolerance in flooded soil

Tolerance to cattle trampling in flooded soil of P. dilatatum and L. glaber differs according to the root mechanical features discussed previously. For the grass P. dilatatum, we found that the required pressure for root collapse was clearly higher than that applied by our simulation system (similar to that of a real cow; Fig. 4). In accordance, soil trampling simulation under flooding conditions did not produce any deleterious effect in plant biomass of this species (Fig. 5a). For a grass species, this is the first report regarding the effect of this disturbance combination on the performance of individual plants. Conversely, trampling of soil a few hours after flooding was enough to damage the relatively weak roots of L. glaber. Similarly, Engelaar et al. (1993) reported the collapse of roots of Rumex palustris when hypoxia was applied in combination with a small soil-pore diameter (simulation of soil compaction). The unique report for trampling × flooding combination found deleterious effects for the dicot Plantago major (Engelaar & Blom 1995), a genus with a root structure similar to Lotus (Justin & Armstrong 1987; Grimoldi et al. 2005).

Plant biomass clearly reflected the differential tolerance of the target species to combined trampling and flooding. Trampling, flooding or the combination thereof did not affect total biomass accumulation in P. dilatatum. Former studies found that after 6 weeks’ flooding P. dilatatum had better water status (Insausti et al. 2001) and higher biomass accumulation in flooded conditions compared with well watered control plants (Rubio et al. 1995; Insausti, Chaneton & Soriano 1999). In our study, shoot biomass tended to be slightly higher in flooded conditions, but this seems reasonable given the short period of flooding (15 days). For L. glaber, both trampling and flooding effects provoked similar reductions of biomass accumulation (Fig. 5b). For flooded plants, the most probable explanation involves the reduction in the photosynthetic rate due to stomatal closure mediated by the anoxic environment at root level (Striker et al. 2005). Two different reasons appear to account for the responses of trampled but unflooded plants. First, the lower total biomass accumulation is likely to be a direct consequence of some degree of root damage under these conditions, and also of the reduction in root exploration due to mechanical impedance of the more compacted soil (Engelaar et al. 1995). Additionally, shoot biomass was partially damaged by the mechanical effect of the hoof: this fact was evident from the presence of some dead stems in the areas that suffered direct trampling impact. Graminoids and herbaceous dicots share dominance, with a similar proportion of standing biomass, in many grassland regions of the world (McNaughton 1983; Soriano 1992). As most graminaceous and dicot species show these contrasting features in root structural configuration, it is possible to predict that cattle trampling in flooded soil would provoke major alterations in the balance between species that could shift community dominance from dicots to the more trampling-tolerant graminaceous species. This hypothesis, and the implications for plant community dynamics, deserve further experimental work.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

We found a strong and positive connection among root mechanical strength and trampling tolerance, as influenced by soil flooding. The grass P. dilatatum combines a strong and porous root structure that enables higher tolerance to trampling in flooded soil. Conversely, the weak root structure of the dicot L. glaber does not tolerate mechanical compression, and plants could not survive trampling in soil-flooded conditions. This work highlights the fact that the effect of aerenchyma tissue generation in root mechanical strength is directly linked to the root structural type of the species. The generation of aerenchyma tissue in response to flooding increases root weakness in the less stable structural type of the dicot species, while the mechanical strength of the strong graminaceous type remains completely unaltered. To our knowledge, this is the first study that reveals the structural features that predispose root collapse when flooding induces the generation of additional aerenchyma tissue. Further experiments are now needed on other species and root structural types in order to improve understanding of the mechanisms that underlie root tolerance to soil mechanical stresses.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

We especially thank F. Mollard, V. Vasellati and G. Zarlavsky for their assistance. We thank Scott Wilson and two anonymous reviewers who helped to improve the manuscript, and P. Cipriotti for advice on data analyses. We also thank the Bordeau family, owners of Estancia Las Chilcas, who facilitated our work on their land. This study was funded by grants from the University of Buenos Aires (UBA G 070) and Academia Nacional de Agronomía y Veterinaria. G.S. was supported by a fellowship from University of Buenos Aires. This work complies with the ethics guidelines and current laws of Argentina.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
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