Báo cáo khoa học: "Factors affecting the direction of growth of tree roots"

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affecting the direction of growth of tree roots Factors M.P. Coutts EH25 9SY, Scotland, U.K. Forestry Commission, Northern Research Station, Roslin, Midlothian, originally growing in many directions. Introduction were In practice, both the direction of growth and differential development contribute to The direction of growth of the main roots the final form. of a tree is an important determinant of the There is scant information about the form of the root system. It affects the way principal controls over the orientation of the system exploits the soil (Karizumi, tree roots. Most studies deal with herba- 1957) and has practical significance for ceous species, and even for them experi- the design of containers and for cultivation mental work and reviews have generally systems which can influence tree growth been confined to geotropism of the seed- and anchorage. This review discusses the ling radicle. The direction sensing appara- way in which root orientation is esta- tus lies in the root cap (Wilkins, 1975). blished and how it is modified by the envi- The structure of the root cap is variable, ronment. but there is no essential difference be- The form of tree root systems can be tween those of herbaceous species and classified in many ways but the common- trees. Work on herbaceous species there- est type in boreal forests is dominated by fore has a strong relevance for trees, but horizontally spreading lateral roots within certain differences must be noted. For about 20 cm of the ground surface (Fayle, example, any correlative effects between 1975; Strong and La Roi, 1983). A vertical the taproot and laterals may be modified taproot may persist or may disappear in trees by the size, age and complexity of during development. Sinker roots are their root systems. Furthermore, the roots more or less vertical roots which grow of herbs, and especially of annuals, may down from the horizontal laterals. They have evolved optimal responses to sea- are believed to be important for anchorage sonal conditions, whereas the young tree and for supplying water during dry peri- must build a root system to support it phy- ods. Roots which descend obliquely from sically and physiologically for many years. the tap or lateral roots are also present An example of response to temporary and the distinction between these and influences is given by soybean, in which sinkers is a matter of definition. Dif- the lateral roots grow out 45 cm horizon- ferences in root form could arise from dif- tally from the taproot, then turn down verti- cally during the summer (Raper and Bar- ferences in root direction or from differen- ber, 1970), possibly in response to tial growth and survival of roots which
inclined at an angle (plagiogeotropic). or high temperature (Mitchell and drought are Russell, 1971). A forest tree could not sur- The angle bei:ween the lateral root and the vive on a root system so restricted lateral- plumb line is called the liminal angle, and is known to vary with species (Sachs, ly. 1874). Billan et al. (1978) even found dif- ferences in the liminal angles between two Orthogeotropic roots provenances of Pinus taeda L.: the pro- venance from the driest site had the small- In both herbs and trees the seedling est angle (i.e., the most downwardly di- taproot (or radicle) is usually positively rected lateral roots). They also found that geotropic. If the root is displaced from its the liminal angle of the upper laterals was vertical (orthogeotropic) position, the tip about twice that of those lower on the bends downwards. The signal for the taproot, a finding in general agreement direction of the vector of gravity is given by with Sachs’ (1874) observations on herbs. the sedimentation of starch grains onto the floor of statocytes in the central tissues of The responses of plagiotropic roots to the root cap. This signal results, in an gravity have been demonstrated by reo- unexplained way, in the production and rienting either entire plants growing in redistribution of growth regulators, in- containers (S;achs, 1874; Rufelt, 1965), or cluding indole-3-acetic acid and abscisic individual roots (Wilson, 1971When Wil- acid (ABA), which become unevenly (1971) displaced horizontal Acer son distributed in the upper and lower parts of rubrum L. roots to angles above the hori- the root. Unequal growth rates then occur roots bent downwards. When zontal, the in the upper and lower sides of the zone of below the horizontal, the roots displaced extension, resulting in corrective curva- did not curve, they continued to grow in ture. There are many reviews of geotrop- the direction in which they had been ism (Juniper, 1976; Firn and Digby, 1980; placed. Such roots are described as being Jackson and Barlow, 1981; Pickard, 1985) weakly plagi!otropic (Riedacker et al., and the mechanism will not be discussed 1982). However, some species show an further here. upward curvai:ure of downwardly displaced roots (strong plagiotropism). In his review, The detection of and response to gravity Rufelt (1965) concluded that the liminal rapid. The presentation time for the are angle is determined by a balance between seedling radicle of Picea abies L. is only positive geotropism and a tendency to 8-10 min (Hestnes and Iversen, 1978) grow upwards, e.g., a negative geotro- and curvature is often completed in a mat- pism. ter of hours. Orthogeotropic taproots retain their response to gravity indefinitely, Certain correlative effects between the although 2 m long roots of Quercus robur of the taproot and the growth and tip (L.) responded slowly displace- to more orientation of 1LL have been described. In ment and had a longer radius of curvature Theobroma cacao L., if the taproot is ex- than shorter, younger roots (Riedacker et cised below very young laterals, some of al., 1982). them will bend downwards, increase in size and vigour, and become positively geotropic replacement roots, i.e., roots Plagiogeotropic and diageotropic roots which replace the radicle. However, if the taproot is cut below laterals more than 7 d (1° L) grow from First order lateral roots old, they do not change in growth rate or the taproot horizontally (diageotropic) or
son-Flanagan and Owens, 1985), yet the orientation; their behaviour has become growth can remain unaltered direction of fixed (Dynat-Nejad, 1970; Dynat-Nejad over repeated cycles of growth and dor- and Neville, 1972). Experiments by these mancy. Furthermore, loss of the entire root workers, which included decapitation of tip generally gives riseto replacement the taproot tip and blocking its growth by roots which have the same gravitropic re- coating it with plaster, showed that the sponses as the mother root, indicating that progressive development of a rather the programme lies in the subapical por- stable plagiotropism by the lateral roots tion. Work on the acquisition of the plagio- was related to the growth rate of the geotropic growth habit by lateral roots taproot, but not to that of the lateral roots requires further development and exten- themselves. Experiments on C7. robur in- sion to other species. Plagiogeotropism is dicated that the behaviour of the lateral even less well understood than geotro- roots in that species is determined even pism of the radicle, on which much more earlier than in T. cacao, at the primordial work has been done, but the experiments stage (Champagnat et al., 1974). Riedac- on correlative control indicate that in the ker et al. (1982) largely confirmed this developed tree root system, it is unlikely work. They found that if the tip of the that the vertical roots influence the orienta- taproot was blocked rather than cut, the tion of existing plagiogeotropic laterals. growth of new laterals above the blockage was enhanced and they became weakly Lateral roots of second and higher orthogeotropic. However, it took time for orders of branching and diminishing dia- the roots to acquire this response and, in meter become successively less responsi- Q. suber L., the lateral roots grew ve to gravity. Since gravity is sensed by 20-30 cm and developed thicker tips be- the sedimentation of the amyloplasts in fore turning downwards. It is not entirely the root cap, higher order roots may have clear whether such a response was also caps too small to enable a geotropic re- induced in lateral roots already present at sponse. Support for this idea comes from the time the taproot tip was blocked. work on Ricinus. The first order lateral roots grow 15-20 mm horizontally from When the tip of a main vertical or hori- the taproot, then turn vertically down- zontal root is injured, replacement roots wards. Moore and Pasieniuk (1984) found are free from apical dominance effects that the development of this positive re- and curve forwards, to become parallel to sponse to gravity was associated with the main root, instead of growing at the increased size of the root cap. The gra- usual liminal angle, or angle with respect dual development of a gravitropic re- to the mother root (Horsley, 1971 ). In this sponse in laterals of Q. suber might also way, the direction of growth of the main be associated with growth of the root cap. root axes is maintained, both outwards, The ectomycorrhizal roots of conifers, away from the tree, and in the vertical which are ageotropic, have poorly de- plane. veloped caps and the cap cells appear to be digested by the fungal partner (Clowes, The way in which the direction of root 1954). Whether there are important with respect to gravity becomes growth anatomical differences between the root fixed, or programmed, has not been stu- caps of the larger, first order plagiotropic died. Although gravity is sensed by the lateral roots of trees, and the caps of cap, the programme must lie elsewhere, taproots and sinkers, has not been de- because the cap dies when the root termined. becomes dormant (Wilcox, 1954; John-
The orientation of root initials the next order to positions opposite the primary xylem strands. Thus, if a line two Root orientation is determined first by the drawn through these strands in transverse direction in which the root initial is facing section, the ’primary xylem line’, is vertical, before it emerges from the parent root roots will emerge pointing only upwards and, subsequently, by curvature. The 1L L and downwards (Fig. 1 a). This vertical orientation is present in the 1Lat its maintain a direction of growth away from junction with the taproot (Fig. 1b). In prac- the plant, an obvious advantage for soil tice, many branches on 1Lat a distance exploration and for providing a framework for anchorage. Noll (1894) termed this from the tree :are produced in the horizon- tal plane, as observed by Wilson (1964) in growth habit of roots exotropy. The laterals are initiated in vertical files related to the A. rubrum, therefore twisting of the root position of the vascular strands in the apex must occur. Wilson noted a clock- taproot. The taproot of Q. robur, for wise twisting (looking away from the tree) in A. rubrum. Twisting is also common in example, has 4-5 strands (Champagnat et al., 1974), and the existence of 4-5 files Picea sitchensis (Bong.) Carr. Many roots of laterals ensures that the tree will have which were sectioned showed partial rota- roots well distributed around it. In conifers, tion of the axis, followed by corrections the taproot is usually triarch or tetrarch, in the opposite direction (Coutts, whereas the laterals are mostly diarch, unpublished). Examination of 24 roots, e.g., Pseudotsuga menzesii (Mirb.) Franco 2-5 m long, showed that the primary (Bogar and Smith, 1965), Pinus contorta xylem line was more commonly oriented horizontally than vertically, favouring the (Douglas ex Louden) (Preston, 1943). In some species, the files of laterals are aug- initiation of horizontal roots. As the root mented by adventitious roots from the twists, the next order laterals can arise in stem base and trees produce additional any direction. main roots the base of by branching near The angle of initiation may account for the 1 ° L (see Coutts, 1987). the production of sinker roots from laterals. In an unpublished study on P. sit The diarch condition of most of the la- chensis, sinkers were defined as roots teral roots of conifers restricts branches of
downwards at angles of less than growing sinker roots with a stricter verti- ve 45° to the vertical 12-15 cm from their cal orientation than those of Picea, and point of origin, while roots at angles within they may therefore originate in a different 45° of the horizontal were called side way. roots. An examination of 50 roots of each It is not known whether sinker roots are type on 10 yr old trees showed that the weakly plagiotropic, their direction being angle of growth was strongly related to the mainly a matter of the direction of initia- angle of initiation, and thus to the angle of tion, or whether the tip becomes positively the primary xylem line (Fig. 2). geotropic, perhaps by some process of Sinkers and side roots were predomi- habituation. Observations on Pinus resi- in a downward and in nantly initiated nosa Ait. indicate that the sinkers may a horizontal direction, respectively. Roots have special geotropic properties: lateral of both types tended to curve sligh- roots from them emerge almost horizontal- tly downwards after they emerged from ly, but then turn sharply downwards the 1° L. Some species, e.g., Abies, ha- (Fayle, 1975).
Surface roots of compacted vermiculite (Wilson, a zone 1971), but roots growing downwards at 45° into compacted but penetrable layers Many 1°L curve gently downwards with did not deflect. Wilson (1967) found that distance from the tree (Stein, 1978; Eis, when horizontal roots of A. rubrum 1978), but some, which may originate encountered vertical barriers, they de- from the upper part of the taproot and flected along them, sometimes with the therefore have the largest liminal angles, root tip distorted laterally towards the bar- grow at the soil surface, in or beneath the rier (Fig. 3a). On passing the barrier, the litter. Many surface roots are 2° L and 3° L roots deflecteci back towards the original (Lyford, 1975; Eis, 1978). Surface roots angle. The correction angle varied with the grow up steep slopes as well as downhill initial angle of incidence between root and (McMinn, 1963). Presumably they are pro- barrier, and with barrier length. Barrier grammed to grow diageotropically, but length in the range 1-7 cm had only a their orientation is modified by the environ- small effect on correction angle. Riedacker ment. The remainder of this review deals (1978) obtained similar results with the with environmental effects. roots of Popu cuttings and barriers up us l to 7 cm long. With barriers 10-12 cm long, nearly half of the roots continued growth in Mechanical barriers the direction of the barrier. Roots made to deflect downwards at barriers inclined to Barriers which affect root orientation in- the vertical, made upward corrective cur- clude soil layers with greater mechanical vature; they were slightly less influenced impedance than that in which the root has by barrier length than horizontal roots. been growing, and impenetrable objects in Orthogeotropic: taproots of Q. robur seed- the soil. Downwardly directed roots can lings deflected past a series of 2 cm long deflect upwards to a horizontal position on barriers, maintaining a remarkably vertical encountering compacted subsoil, but turn orientation overall (Fig. 3b). Replacement down if they enter a crack of hole (Dexter, taproots formed after injury appeared to 1986). Horizontal roots or A. rubrum be insensitive t:o barrier length. deflected upwards when they encountered The mechanism by which roots make corrective curvatures after passing bar- riers is not known. Large variation in cor- rection angles has been reported, and it is possible that barrier length is less impor- tant than the time for which the root has been forced to deflect. The mechanism is an important one for maintaining exotropic growth. Light and temperature Light from any direction increase the can graviresponsiveness of the radicle and lateral roots of some herbaceous species (Lake and Slac:k, 1961Light is sensed by
tips of growing taproots and sinkers the root cap (Tepfer and Bonnet, 1972). are killed when the water table rises, and which elicit a response vary Wavelengths regeneration takes place when it falls with plant species, e.g., Zea (Feldman and during drier periods. Such periodic death Briggs, 1987) and Convolvulus (Tepfer and regrowth produce the well-known and Bonnett, 1972) respond to red light and show some reversal in far red, where- ’shaving brush’ roots on many tree spe- as the plagiotropic roots of Vanilla turn cies. In spite of poor soil aeration, the tips downwards only in blue light (Irvine and of taproots and sinkers maintain a gen- erally downward orientation. This could be Freyre, 1961). ). because periods of growth coincide with There is little information on trees. Iver- periods when the soil is aerated. However, and Siegel (1976) found that when P. sen in an experiment on P. sitchensis grown abies seedlings were lain horizontally in out of doors in large containers of peat, the light, subsequent growth of the radicle main roots which grew down at 0-45° in darkness was reduced, but curvature from the vertical did not deflect when was unaffected. Lateral roots of P. sit approaching a water table maintained chensis showed reduced growth and 26 cm below the surface (Coutts and downward curvature in low levels of white Nicholl, unpublished). The roots pene- light (Coutts and Nicholl, unpublished). trated 1-5 cm into the waterlogged soil Such responses indicate that care must be and then stopped growing. This behaviour exercised when using root boxes with contrasts with certain herbaceous species. transparent windows for studies on the Guhman (1924) found that the taproots direction of growth. In the field, light may and laterals of sunflower grew diageotropi- help regulate the orientation of surface cally in waterlogged soil, and Wiersum roots, just as it does for Aegeopodium rhi- (1967) observed that Brassica and potato zomes, which respond to a 30 s exposure roots grew upwards towards better aer- by turning downwards into the soil (Ben- ated zones. The finest roots of trees may net-Clark and Ball, 1951 ). also grow upwards from waterlogged soils, The of roots is influenced growth corn as found for Melaleuca quinquenerva At soil by temperature. temperatures (Cav.) Blake by Sena Gomes and Koz- above and below 17°C, plagiotropic prima- lowski (1980), and for flooded Salix (see ry roots become angled more steeply Gill, 1970). However, the emergence of downwards (Onderdonk and Ketcheson, roots above flooded soil does not neces- 1973). No information is available for sarily mean that the roots have changed trees. direction, they may have been growing upwards prior to flooding. Little is known about the response of roots to waterlogging. Arm- plagiotropic Waterlogging and the soil atmosphere strong and Boatman (1967) considered that the shallow horizontal root growth of Molinia in bogs was a response to water- has drastic effect Waterlogging soil a on aeration and logged conditions, but did not present consequently on tree root observations on growth in well-drained development (Kozlowski, 1982). Waterlog- soil. The proliferation of the surface roots ged soils are characterised by a lack of of trees on wet sites may be a result of oxygen, increased levels of carbon dioxide and ethylene, together with many other compensatory growth rather than a chemical changes (Armstrong, 1982). The change in orientation.
The direction of growth of plant organs Dessication is influenced by C0 For example, the . 2 diageotropic rhizomes of Aegeopodium The curvature of roots towards moisture is deflect upwards in the presence of 5% called hydrotropism. Little work has been C0 (Bennet-Clark and Ball, 1951),and 2 done on it and Rufelt (1969) questioned this response has been supposed to help whether the phenomenon exists. Sachs maintain their position near the soil sur- (1872) grew various species in a sieve of face. Ycas and Zobel (1983) measured moist peat, hanging inclined at an angle in the deflection of the plagiotropic radicle of a dark cupboard. When the seedling roots corn exposed to various concentrations of emerged into water-saturated air, they 0 C0 and ethylene. Substantial effects , 22 grew downwards at normal angles, but in on the direction of growth were obtained drier air they curved up through the small- only with C0 Roots in normal air grew at . 2 est angle towards the moist surface of the an angle of 49° to the vertical, whereas in peat. Sachs concluded that they were 11 % C0 they deflected upwards to an 2 responding to a humidity gradient. Loomis angle of 72°. The minimum concentration and Ewan (1935) tested 29 genera, in- of C0 required to cause measurable 2 cluding Pinus, by germinating seeds be- deflection was 2%. Concentrations of tween layers of wet and dry soil held in 2-11% C0 are above those found in well- 2 various orientations. In most plants tested, drained soils but, in poorly draining, for- including Pinus, no consistent curvature ested soils, Pyatt and Smith (1983) fre- towards the wet soil occurred. In species quently found 5-10% C0 at depths of 2 ° which gave a positive result, the 1 L were 35-50 cm. However, concentrations were unaffected, only the radicle responded. usually less than 5% at a depth of 20 cm Some of the non-responsive species had and would presumably have been lower responded in Sachs’ system, an anomaly still nearer the surface, where most of the which may be explained by problems of present. In Ycas and Zobel’s roots were methodology. The containers of wet and at non-toxic (1983) experiments, ethylene dry soils in Loomis and Ewan’s experi- concentrations had little effect on the ments were placed in a moist chamber direction of corn root growth, and only and the vapour pressure of the soil atmo- small effects on corn had been found by sphere may well have equilibrated during Bucher and Pilet (1982). In another study, the course of the experiment. orthogeotropic pea roots responded to Jaffe et al. (1985) studied hydrotropism ethylene by becoming diageotropic but the in the pea mutant, ’Ageotropum’, which roots of three other species did not has roots not normally responsive to gravi- respond in this way (Goeschl and Kays, ty. Upwardly growing roots which emerged 1975). from the soil surface continued to grow upwards in a saturated atmosphere but, at It appears the though downwardly as relative humidities of 75-82%, they bent roots of trees do not deflect on growing downwards to the soil. No response took soil. This failure encountering waterlogged place if the root cap was removed and it to deflect is consistent with the conclusion was concluded that the cap sensed a of Riedacker et al. (1982) that the positive humidity gradient. geotropism of tree roots is difficult to alter. These results have implications for the There is not enough information on plagio- behaviour of tree roots at the soil surface geotropic roots to say whether soil aera- and where horizontally growing roots tion affects their orientation.
the sides of drains. For species, appear to originate from root pri- encounter when P. sifchensis roots grow mordia which happen to be angled down- example, from the side of a furrow made by spaced- wards. Their georesponsiveness is un- furrow ploughing, they turn downwards on known. The gravitropism of taproots is a stable feature and the vertical roots of emerging into litter or overarching vegeta- tion. Experiments to investigate this be- trees do not seem to deflect from water- haviour shewed that horizontal roots which logged soil layers, unlike the roots of cer- tain herbs. They have been made to emerged from moist peat into air at a rela- tive humidity of 99% grew without de- deflect only on encountering impenetrable flecting, but at 95% they deflected down- barriers. wards to the peat (Coutts and Nicholl, The direction of growth of first order unpublished). This behaviour could have laterals around the tree in the horizontal been a hydrotropic response, but roots plane is set by the position of the initials which grew out from the peat at angles on the taproot. The direction of growth is above the horizontal into air at 95% humi- maintained away from the tree by correc- dity, also turned downwards, rather than tive curvatures, when the root is made to upwards towards the nearest moist sur- deflect by obstacles in the soil. If the tip is face. This suggests that localised water killed, replacement roots also curve and stress at the root tip had induced a posi- continue growth in the direction of the tive geotropic response. It is relevant to main axis. In the vertical plane, geotropic note that water stress induces the forma- responses of the laterals are subject for a tion of ABA in root tips (Lachno and Baker, short period to correlative control by the tip 1986; Zhang and Davies, 1987), and ABA of the taproot. Work on broadleaved spe- has been implicated in geotropism. An cies indicates that during that period, the explanation of geotropism induced by lateral root apex becomes programmed to water stress could also apply to the down- grow at a particular angle to the vertical. ward curvature of otherwise ageotropic This angle can be modified by the environ- roots already mentioned, but not to ment: temperature, light and humidity can upward curvatures in Sachs’ experiments. alter the graviresponsiveness of lateral It is in any case unlikely that roots growing roots. It is not certain whether hydrotropic in soil exhibit hydrotropism because the responses occur nor whether the lateral vapour pressure difference, even between roots of trees respond to soil aeration or moist soil and soil too dry to support root deflect from waterlogged soil. The way in growth, is so small (Marshall and Holmes, which the growth of main lateral roots is 1979) that roots would be unlikely to maintained near the soil surface, even in detect it. A positive geotropic response by roots growing uphill, is not properly roots in dry soil would be likely to direct understood. Thin roots of more than first them to moister layers lower down. order, including mycorrhizas, have small roots caps and do not appear to respond to gravity. Conclusions Acknowledgment The seedling radicle, and roots which replace it after injury, are usually positively I thank Dr. J.J. Philipson for his helpful com- geotropic. Sinker roots, at least in one ments on the manuscript.
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