JXB Advance Access originally published online on August 22, 2006
Journal of Experimental Botany 2006 57(12):3123-3130; doi:10.1093/jxb/erl074
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© 2006 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)
RESEARCH PAPER |
Structural development and stability of rice Oryza sativa L. var. Nerica 1
Faculty of Life Sciences, University of Manchester, 3.614 Stopford Building, Oxford Road, Manchester M13 9PT, UK
*To whom correspondence should be addressed. E-mail: r.ennos{at}manchester.ac.uk
Received 9 March 2006; Accepted 6 June 2006
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Abstract |
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The structural development of glasshouse-grown rice Oryza sativa L. var. Nerica 1 was studied in relation to its stability against lodging. The morphology and mechanical properties of both the stem and roots were examined from tillering, 4 weeks after transplantation up to maturity, together with plant weight distribution and anchorage strength. The ‘factors of safety’ against root and stem failure were subsequently calculated throughout development. Rice plants showed similar morphology to wheat, although they possessed around twice as many tillers per plant and 10 times as many coronal roots. The mechanics of anchorage were also similar. The strength and rigidity of individual tillers increased throughout development as the plants grew taller and heavier and were around 15 times greater than in wheat. By contrast, individual root bending strength, the number of roots, and the anchorage strength levelled off earlier, and anchorage strength was only around twice that in wheat. Consequently, while the self-weight safety factor against stem failure was much higher than in wheat, increasing until late on in development from around 30 to 150, the self-weight safety factor against root anchorage failure was similar to wheat, decreasing from around 15 to 5. Consequently, plants subjected to anchorage tests always failed in their root system rather than their shoot system. The results suggest that, in the field, rice plants would be more likely to undergo root lodging than stem lodging, and that breeding efforts to reduce the incidence of lodging should act to strengthen the rather weak coronal roots.
Key words: Anchorage, biomechanics, lodging, rice, safety factor, structural development
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Introduction |
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Lodging, which is the permanent displacement of the stem of a free-standing crop plant from the vertical (Pinthus, 1973; Berry et al., 2004), has attracted the attention of scientists particularly on rice for over 80 years. Over 70 years ago (Ramaiah and Mudaliar, 1934), it was shown that the more lodging-resistant types of rice had stems with a thicker band of sclerenchyma at the periphery of the stem than lodging-susceptible strains. More recently, Chaturvedi et al. (1995) showed that lodging-tolerant submerged lowland rice varieties had more vascular bundles, both peripheral and in the inner section of the outer layers, than susceptible varieties.
These studies did not show any causal link, via mechanical strength, between anatomy and lodging susceptibility. By contrast, Ooawa et al. (1993) carried out studies on differences in physical and chemical characteristics of culms associated with lodging resistance in paddy rice. They discovered that differences in the degree of lodging and the lodging index were largely dependent on variation in the breaking strength of the basal internode. The chemical differences were related to the physical characteristics, pointing to the fact that the latter derives from the former.
The resultant effects of these studies have always been the selection of shorter varieties with more rigid straws and culms. As in wheat (Berry et al., 2004) these efforts have allowed breeders to increase the yield without increasing lodging susceptibility, despite the greater weight of the panicle. However, lodging remains a problem in cereals and, more recently, attention has started to be focused on the mechanics of roots and root systems vis-à-vis lodging. According to Crook and Ennos (1994), one of the reasons for the incomplete success of plant breeding programmes in reducing the occurrence of lodging may be that the mechanical basis of lodging has not been fully explored; breeders have tended to rely on a more empirical approach to selection. Analysing the basis of lodging further, they stated that the susceptibility of a variety to lodging will depend on three factors: (i) the size and dynamics of the forces to which it is subjected (Pinthus, 1973); (ii) the bending strength of the stem and its resistance to lodging (Neenan and Spencer-Smith, 1975); and (iii) the anchorage strength of the root system.
Other scientists worldwide have implicated the root system and its failure as being responsible for lodging in rice. Terashima et al. (1994, 1995) worked on the eco-physiological characteristics related to lodging tolerance in direct-sown rice and found out that rice varieties resistant to lodging developed more roots in deeper soil layers than lodging-susceptible varieties, and that the contribution of unit root weight to lodging tolerance was higher in deeper than in shallower soil layers. Thus their work confirmed that, in rice, a greater ability to form roots with a higher bulk density in subsoil was one of the most important characteristics for root lodging tolerance.
The implication of roots, root systems, and soil factors in lodging phenomena became clearer with the studies of Ennos (1991) and Crook and Ennos (1993, 1994) on the mechanics of lodging in winter wheat, studies put into context by Ennos's review of the mechanics of root anchorage (Ennos, 2000). The two studies by Crook and Ennos (1993, 1994) looked into the mechanics of lodging and related them to the stem and root characteristics associated with lodging in winter wheat. From these studies, it was clear that lodging resistance was not related to the strength and stiffness of the stems, which were usually adequate to withstand the force to which they were subjected, but rather was dependent on a cone of rigid coronal roots which emerge from around the stem base. A theoretical model of anchorage in the studies suggested that lodging resistance should be dependent on the diameter of the root–soil cone, coronal root bending strength, and soil shear strength. The earlier study by Ennos (1991) on spring wheat also found that the root system was weaker than the shoot system in wet soil, notwithstanding the differences in the root morphology between the two types of wheat.
The root system in rice is different from that in wheat. Izumi et al. (1996) showed the branching pattern in rice roots to be largely herringbone type. Though, this does not particularly indicate the specific anchorage system in rice, the time-course changes in the growth of the roots vis-à-vis the branching system may influence the resistance or otherwise to root lodging in rice. A clearer picture of the influence of the root system of rice and its nature was presented by Ogata and Matsue (1996), who found that the thickness of the crown roots at different periods after sowing is influential in determining lodging resistance in direct-sown rice. It is, therefore, imperative to obtain a better understanding of the mechanics of root anchorage and lodging resistance in rice.
Lately, lodging has been responsible for limiting the yield and the potential for increases in yields as well as losses in eventual and harvestable yields in cereals globally (Berry et al., 2004). This has been implicated in wheat (Laude and Pauli, 1956; Weibel and Pendleton, 1964; Crook and Ennos, 1994, 1995), oats (Pendleton, 1954), barley (Day, 1957), grain sorghum (Larson and Maranville, 1977), and rice (Basak et al., 1962; Setter et al., 1997). In rice, yield losses can be as much as 1% for every 2% of the crop lodged (Setter et al., 1997) and grain quality is also affected. The prevalence of lodging in rice is very variable, as it depends on a wide range of factors, and in bad years yield losses in Japan can reach up to 50% (Setter et al., 1997). Unfortunately, the most sophisticated research has been concentrated on barley, oats, and especially wheat (Berry et al., 2004).
The research described in this paper was aimed at making the first step in understanding how to prevent root lodging in rice by investigating its structural development and stability using the techniques pioneered for wheat by Crook and Ennos (1993, 1994).
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Materials and methods |
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Planting
The Nerica 1 variety of upland rice Oryza sativa L. which was supplied by the West African Rice Development Association at IITA, Ibadan, Nigeria, was used in the two trials carried out. The seeds were sown in a plastic tray lined with a thin layer of John Innes compost no. 3, and were well watered. Plants were grown in a greenhouse which was heated to an average daytime temperature of 26 °C, falling to 12 °C at night. Supplementary lighting was supplied to give a 16 h day and maximum irradiance was essentially that of full sunlight. The seedlings, which emerged 3 d after sowing, were watered daily and observed for good growth until about 2 weeks when they were transplanted into polystyrene trays measuring 78 cmx38 cmx21 cm and filled with John Innes compost no. 3. The compost was mixed with horticultural grit of 3–6 mm size at a 3:1 ratio to aid drainage. This soil-based compost should mimic a relatively weak natural soil. Each tray contained 10 rice plants transplanted at a spacing of 20 cmx12.5 cm. There were 18 such trays in the first trial and 15 in the second. Each tray, which had perforations underneath to allow drainage, was watered on a daily basis, while nutrients (Phostrogen) were applied at weekly intervals starting from 12 weeks after transplanting.
Two trials were carried out: an exploratory first trial, in which the techniques developed for previous studies of wheat (Crook and Ennos, 1993, 1994) were customized for rice and the mechanics of lodging investigated; and a second trial in which the development was studied more regularly and methodically, as in the study of Crook et al. (1994) of the structural development of wheat.
Stem and root morphology and mechanical properties
Shoot morphology:
At the 4th week after planting (WAT), five plants, randomly selected from the whole population were carefully uprooted, ensuring that all the soil particles were removed without damaging the roots. The plants' roots were immediately immersed in water and plants were taken to the laboratory for morpho-physical studies. Parameters taken per plant were plant height, centre of gravity, stem cross-sectional area at 5 cm height, number of leaves, fresh shoot weight, number of adventitious roots, and number of tillers. These data were also measured at 11 subsequent fortnightly harvests, spanning six key growth stages: tillering (4 WAT), stem elongation (10 WAT), booting (18 WAT), anthesis (20 WAT), grain filling, (22 WAT), and maturity (26 WAT). Details of the methods used are given below
On arrival in the laboratory, about 0.5 h after uprooting, the number of tillers and leaves were recorded, and the coronal root system was cut away from the stem, both the root systems and the bases of the stems being kept immersed in water to maintain turgor until just before measurements were made. Each shoot system was separated into its individual tillers and the plant height, fresh weight, and centre of gravity were recorded. Plant height was determined by measuring the distance from the base to the tip of the last leaf using a ruler. The centre of gravity was determined by placing separate tillers across an outstretched index finger and moving the tiller along the finger until the balance point was reached—the height of the centre of gravity being the distance from the base of the stem to the balance point (Crook and Ennos, 1994). Plant fresh weight was determined using a top loading balance (Denver Instrument no. 60665). Stem diameter was measured 5 cm from the base in two (right-angled) directions: the long axis and short axis of an approximate ellipse. The cross-sectional area of the stem base was then calculated 5 cm from the base using the formula
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Mechanical properties of the stems:
Under the applied force of wind, bending failure of the stem in most cereals is most likely to occur towards the base of the stem (Ennos, 2000; Berry et al., 2004). For this reason, mechanical tests were limited to the lower region of the stems. The main tillers of five plants at each harvest were prepared for mechanical testing. Basal sections, 10 cm long, were cut from the tillers so that the bending strength and rigidity could be measured using a universal testing machine (Instron model 4301, fitted with a 100 N load cell).
Each section was then subjected to a three-point bending test. Stem sections were placed on two metal supports set 60 mm apart, while a blunt rubber probe of diameter 20 mm, attached to the crosshead of the Instron, was moved down at a speed of 50 mm min–1, touching the stem midway between the supports and bending it. A force/displacement graph was simultaneously recorded on an interfacing computer and used to calculate the mechanical properties of the stem section. The bending strength, S, of a uniform beam is given by
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Morphology and mechanical properties of the roots:
The coronal root system which had been removed was washed carefully to clean away the soil and then prepared for mechanical testing; the thick coronals were cut away from the plant base and the length of rigid root, with a diameter of 
0.5 mm, was measured. Roots with rigid basal regions 25 mm long (Crook and Ennos, 1993, 1994) were then subjected to three-point bending tests on the Instron to determine their bending strength. In this case, the supports were set 20 mm apart, the probe diameter was 4 mm and the crosshead speed of the Instron was 20 mm min–1. Over this distance the roots were not noticeably curved. Roots were out of the water >5 min before being tested.
For each plant, the number and bending strength of rigid roots 25 mm long were recorded and the sum of bending strength of all the roots was calculated.
Anchorage mechanics and anchorage strength:
In the first trial, the anchorage mechanics of rice were examined visually in plants from 12 to 26 WAT using the trenching method developed by Coutts (1983) for trees, and used in wheat by Crook and Ennos (1993). The movements of the roots and soil during the overturning of five plants were observed when plants were pulled over parallel to a vertical trench cut alongside the stem, so revealing a ‘cross-section’ of the root system.
In the second trial starting from the eighth week after transplanting (8 WAT) until maturity, eight plants were randomly selected every fortnight for anchorage tests. Soil cores, each with a diameter of 103 mm, were used to remove the rice plants from the polystyrene trays while ensuring that disturbance to the root system and the surrounding ball of earth was minimal (Crook and Ennos, 1993). The plants were taken to the laboratory where they were watered overnight to bring the soil medium to field potential. This gave simulated soil conditions typical of waterlogged conditions after rain, when anchorage failure is most likely to occur; it produced soil strengths measured using a shear-vane of 14.9 ±4.6 kPa in the soil cores. The plants were then subjected to anchorage tests using the universal testing machine, Instron, fitted with a lodging meter which enabled it to deliver a rotational force centred around the base of the stem at soil level (Ennos, 1991; Crook and Ennos, 1993). The apparatus simultaneously pushed the plant over and measured the restoring anchorage moment supplied by the root system, applying a force at a height of 17 cm from the stem base and pushing it over at a rotational velocity of 50° min–1. The interfacing computer calculated two measurements of anchorage: (i) the stiffness, given as the initial slope of the moment/angular deflection curve; and (ii) the strength, given as the restoring moment provided by the plant at a deflection from the vertical of 30°. This deflection was chosen to make the results comparable with those for wheat (Crook and Ennos, 1993).
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Results |
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Shoot morphology
The plants displayed consistent increases in plant height, centre of gravity, and fresh shoot weight right up to maturity (Fig. 1), although height growth did tend to flatten off after around 12–15 WAT. The centre of gravity remained at around 30% of plant height. There were similar steady increases in the numbers of leaves and tillers, and in the total cross-sectional area of the tillers (Fig. 2).
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Mechanical properties of the stems
The bending strength and flexural rigidity of individual stems also increased steadily with time in the rice plant (Fig. 3).
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Morphology and mechanical properties of the roots
Figure 4 shows the number of adventitious roots produced per plant, as well as the bending strength of both the individual roots and the sum of root strengths of each plant over time. The number of the roots produced per plant increased over time before levelling off at about 18 WAT. By contrast, the individual root strength levelled off after only 8 WAT, with a slight, but not significant, decline at maturity. Consequently the total root bending strength also rose to a maximum at 18 WAT, and fell slightly but not significantly thereafter, being influenced more by the number of roots than by individual root strength.
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Anchorage tests
Observation of the cross-sections of the root systems of rice plants showed that plants showed very similar anchorage failure to winter wheat (Crook and Ennos, 1993); rotation occurred around a point 3–5 cm below ground and on the windward side of the plant (opposite the direction of push-over force). Consequently, as the plant was pushed over, the soil on the leeward side and underneath the plant was compressed and sheared and the stem bent a little towards the leeward side. Anchorage failure occurred before the stems failed. After the test, plants did not return to their original upright position, but leaned at an angle of 10–30° to vertical, pointing to the fact that anchorage failure had occurred.
Figure 5 shows the anchorage stiffness and anchorage strength of plants over time. Resistance to the anchorage failure increased with time until around 14 WAT (Fig. 5b), after which it levelled off. The trend for anchorage stiffness (Fig. 5a) was similar except that there was an unexplained peak at 14 WAT. The large standard deviation suggests that this was due to a single rogue point.
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Self-weight moments and factors of safety
The results of the morphological and mechanical tests on the plants were combined to calculate two further sets of parameters: the self-weight moments of individual tillers and whole plants; and self-weight safety factors of the plant against shoot and root lodging which were first devised by Crook and Ennos (1994).
Push over forces caused by wind on a plant will be transmitted to the ground and may eventually cause the stem to bend permanently. Once a plant is leaning, the weight of the aerial parts of the stem will help to push the plant over further. Given the mass distribution of the stem and the angle of its inclination, an estimate of its ‘self-weight’ moment, MP, of a single tiller can be calculated according to the following equation (Crook and Ennos, 1994)
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is the angle of inclination of each stem from the vertical. The self-weight moment of individual tillers was calculated at an inclination of 30° from the vertical for the rice plant (Fig. 6a). The self-weight moment for the entire plant was also calculated by summing the values for each tiller (Fig. 6b). Both parameters increased with age of the plant throughout the study period.
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For a plant to remain structurally intact, both the stem and the anchorage system must be strong enough to withstand the overturning moments generated by the wind and by the weight of the plant. Estimates of safety factors against the self-weight component of the turning moment were calculated by combining measurements on stem and anchorage strength with the calculated self-weight moments (Crook and Ennos, 1994).
Self-weight safety factors against stem failure
The ability of each stem to avoid breaking depends upon the basal bending strength. The safety factor against stem breakage (SFS), is expressed by the following equation
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°, from the vertical. Values of the safety factor for the main tillers were calculated for a stem inclination from the vertical of 30° (Fig. 7a). The self-weight safety factors against stem breakage in rice increased from around 30 to 150 as the plant advanced in age before levelling off at around 21 WAT.
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Self-weight safety factors against anchorage failure
Self-weight safety factors against anchorage failure, SFA, were also calculated according to the following equation
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where SA is the anchorage moment at ° from the vertical and MP is the self-weight moment generated per plant at ° from the vertical. Figure 7b shows the safety factor against anchorage failure at an inclination of 30° from the vertical during plant development. The safety factors against anchorage failure, unlike those obtained for the safety factors against stem failure, decreased after about 12 WAT. The safety factors for the anchorage failure were also much lower than for stem failure, falling from 14 to around 5. Note that there are no error bars for the safety factors against anchorage failure, because anchorage strength and plant morphological data had been collected from different plants, but are likely to be around 50% of the mean values.
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Discussion |
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The results of this study show that, unsurprisingly, the Nerica 1 variety of rice has broadly similar patterns of development and broadly similar mechanics to the two winter wheat varieties studied by Crook et al. (1994), but that there were, in some cases, large quantitative differences between the two cereals that should affect the likelihood of stem and root lodging. These results also suggest that the techniques developed for the wheat study are appropriate, at least to start with, to investigate the effect of differences in genotype and management on the lodging resistance of rice.
Although the pattern of development of the shoot system of the rice was similar to that of wheat (Crook et al., 1994), there were also some striking differences which should affect the likelihood of stem lodging. Despite being of similar height and mass, the tillers were around 15 times stronger than those of wheat. This difference is largely due to the greater cross-sectional area and hence strength of the elliptic stems in rice compared with the narrow circular stems of wheat. Consequently, the self-weight safety factor of the tillers against stem failure was high, rising from around 30 at 8 WAT to around 150 at maturity. By contrast, in wheat, the factor of safety fell from around 15 to around 5 (Crook et al., 1994). This suggests that this upland rice variety would be far less likely to suffer from stem lodging than winter wheat, especially as rice grows in tropical regions that have much lower winds.
The anchorage system of rice was also broadly similar to that in wheat (Crook et al., 1994). As in wheat, plants were anchored by a mass of rigid adventitious roots that emerged from the base of the tillers to form an inverted crown shape. Anchorage failure in both plants occurs in the same way; plants rotate about a point on the windward side of the crown, and the crown and the soil it surrounds compresses the soil beneath it. However, once again there were quantitative differences in both strength and development. The roots of the rice plants were actually only around one-third as strong in bending as those of wheat. However, the number of roots was around 10–15 times that seen in wheat. As a consequence, by harvest rice had an anchorage two to three times as strong as that of wheat. Because of the extra number and weight of the tillers, however, the factor of safety of rice against root lodging was similar to that of wheat and showed the same pattern over time; in both cases, factors of safety fell from around 15 to 3 or 4. This suggests that the rice plants, like wheat, are more likely to be susceptible to root lodging than stem lodging.
Of course, it must be remembered that this is only a preliminary and small-scale study, and there are many caveats that must be put on any conclusions drawn here. First, it is not possible to extrapolate from this study directly to all rice varieties since there are huge differences between deepwater, paddy, and upland rice.
Secondly, the rice plants were grown in Manchester, UK, and therefore by necessity were grown in unnatural greenhouse conditions, which must have greatly affected their morphology and mechanics. Apart from the relatively low light and temperatures, which slowed development, the soil was by no means natural. The most important difference, however, was probably the lack of wind in the greenhouse. Plants grown in windier conditions show thigmomorphogenetic responses, which generally make them shorter and stronger (Ennos, 1997). Crook and Ennos (1996) have shown, for instance, that supporting the stems of wheat plants against stem sway reduces stem strength by around 20% and anchorage strength by about one-third. Further research should be carried out on plants grown in field conditions.
Thirdly, the analysis of safety factors used here only takes into account the self-weight of the stems, and ignores the usually larger aerodynamic forces on plants and the dynamic forces caused by stem sway. Although Crook and Ennos (1994) showed that there was reasonable correlation between the simple self-weight safety factors and lodging resistance in wheat, subsequent research has developed much more sophisticated analyses of lodging based on the dynamics and aerodynamics of plants (Baker et al., 1998; Farquar and Meyer-Phillips, 2001; Spatz and Zebrowski, 2001; Berry et al., 2004; Py et al., 2005). Future quantitative work analysing the effect of genotype and management on rice lodging should also use these sorts of models.
Nevertheless, from this research, two important points seem to have emerged. The factor of safety of rice against stem lodging is far greater than that in wheat, so rice plants should be very unlikely to stem lodge. By contrast, rice shows similarly low factors of safety against root lodging as wheat, so like wheat it is likely to be relatively prone to root lodging. To avoid the occurrence of lodging, therefore, efforts should be channelled towards increasing the anchorage strength rather than stem strength. These can be approached in two ways: (i) rice plants with stronger and stiffer roots can be bred, without reducing the number of roots per plant; and (ii) the soil medium could be altered to make it stronger.
The findings of this research also point to areas in which further research could improve our understanding of lodging in rice and help suggest how it can be prevented. Further studies could be carried out to compare rice varieties with contrasting lodging susceptibilities, and to investigate the effects of different soil types on the rice anchorage system and, in effect, on its lodging susceptibility. There is also a need for field trials that will help in further understanding how plants lodge, the environmental factors that affect plant stability, and how to prevent lodging in the future.
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Acknowledgements |
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We would like to thank Dr O Osiname of the West African Rice Development Association for the supply of the Nerica 1 rice variety seeds. We are also grateful to Thurston W Heaton and his staff at the Botanical Unit of Owens Park, University of Manchester, Manchester, UK, for assisting in looking after the plants in the greenhouse. Dr Oladokun was funded by a Royal Society international short visit grant.
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References |
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