Plants Roots and the Soil

This book focuses on vascular plants and their interactions with soils. It has long been appreciated that plants influence the properties of soils and that soil type can, in turn, influence the type of plant that grows. This knowledge of plant/soil interactions has been put to use by humans in their agriculture and horticulture. For example, Pliny The Elder quotes Cato as writing 'The danewort or the wild plum or the bramble, the small-bulb, trefoil, meadow grass, oak, wild pears and wild apple are indications of a soil fit for corn, as also is black or ash-coloured earth. All chalk land will scorch the crop unless it is extremely thin soil, and so will sand unless it is extremely fine; and the same soils answer much better for plantations on level ground than for those on a slope' (Rackham, 1950). Similarly, long before the nitrogen-fixing abilities of rhizobia were documented scientifically, Pliny The Elder noted that lupin 'has so little need for manure that it serves instead of manure of the best quality', and that 'the only kinds of soil it positively dislikes are chalky and muddy soils, and in these it comes to nothing' (Rackham, 1950).

This close association of soils and plants has led, too, to an ongoing debate as to the role of plants in soil formation. Joffe (1936) wrote that 'without plants, no soil can form' but others such as Jenny (1941, reprinted 1994) demonstrated that vegetation can act as both a dependent and an independent variable in relation to being a soil-forming factor. Ecologists find it useful to work with vegetation types and plant associations comprising many individual plant species; these associations are frequently linked to soil associations, and in this regard, at this scale, the vegetation is not an independent soil-forming factor. However, it is also appreciated that within a vegetation type, different plant species may have effects which lead to local variations in soil properties and where plants do act as a soil-forming factor. For example, in mixed temperate forests the pH of litter extracts of different species may range from 5.8 to 7.4, leading to different types of humus from the different species and hence different rates of mineral leaching. Similarly it is well documented that the planting of coniferous trees on several areas in Europe has increased rates of soil acidification in some areas, and resulted in podsol formation on soils that were previously earths (Hornung, 1985).

Although the focus of much plant and soil science has been on the return of leaves to the soil both as a stock of C in the soil and as a substrate for soil organisms, root returns to soil are larger than shoot returns in several regions. For example, early work by ecologists such as Weaver in the USA demonstrated that several grasses produced more organic matter below ground than above ground (Weaver et al., 1935). This interest in carbon inputs to soils has been re-ignited with the current debate over sequestration of C by vegetation in an attempt to mitigate the greenhouse effect induced by rising CO2 concentration of the atmosphere. For example, observations of deep-rooted grasses introduced into the grasslands of South America have demonstrated that they can sequester substantial amounts of carbon (100-500 Mt C a-1 at two sites in Colombia) deep in the soil (Fisher et al., 1994). Roots and their associated flora and fauna are the link between the visible parts of plants and the soil, and are the organs through which many of the resources necessary for plant growth must pass. As part of the system that continually cycles nutrients between the plant and the soil, they are subject to both the environmental control of the plant and the assimilatory control of the plant as a whole.

This chapter examines the close connection between the root and shoot systems of vascular plants and what is known about the co-ordination of activities between the two systems. It also describes some of the main features of the interaction between roots and soils as a prelude to more detailed examination of changes to soil properties in the vicinity of roots in later chapters.

1.1 The evolution of roots

Roots and shoots are considered by most botanists to be entirely separate organs, although some developmental processes are shared, and some inter-conversion can occur (Groff and Kaplan, 1988). Raven and Edwards (2001) sought to define what constitutes a root of a vascular plant to distinguish it from a shoot, and concluded that the distinguishing features were 'the occurrence of a root cap, a more defined lineage of cells from the apical cell(s) to tissues in the more mature parts of the roots, the essentially universal occurrence of an endodermis, a protostele (i.e. a solid cylinder of xylem) sometimes with a pith, and endogenous origin of lateral roots from roots' (Table 1.1). These same features are shown diagrammatically in Fig. 1.1. Others (e.g. Gifford and Foster, 1987) have highlighted the uniqueness of roots because of their bidirectional meristem that produces both an apical root cap and subapical root tissues (see section 2.4.1).

The general structure and function of roots and shoots are so different that the two organs are often conveniently separated for the purposes of research. Functionally, roots absorb water and nutrients, and anchor the plant, while shoots photosynthesize and transpire, and are the site of sexual reproduction (Groff and Kaplan, 1988). Usually both root and shoot must occur together for a plant to function and grow, although there are some exceptions to this generalization. Roots and shoots gradually acquire their distinguishing features during the differentiation and growth of the embryo sporophyte, but are not usually recognizable until the apical meristems are differentiated.

The fossil record for the evolution of roots is less helpful than that for shoots, but recognizable root-like structures start to appear in Early Devonian times (410-395 million years ago). Fossilized remains of many early land plants are fragmentary, and delicate structures such as root caps may not have been preserved, so that evolutionary sequences are often difficult to date with certainty. Gensel et al. (2001) use the terms 'rootlike' and 'rooting structures' to describe fossil structures which resembled roots and were positioned such that they may have anchored the plant to a substrate; whether they also functioned as absorbers of water and nutrients is unknown. Raven and Edwards (2001) suggest that Lower

Table 1.1 The characteristics of early vascular plants (above and below ground) and those of shoots and roots of extant plants

Characteristic Early vascular plants

Shoot of extant plants

Root of extant plants

Primary xylem Protostele

Root cap Endodermis in organs lacking secondary thickening Origin of branches

Hairs

Absent

Absent (apparently)

Superficial organ

'Axis hairs'; mycorrhizas on below-ground parts

Protostele in some pteridophytes; pith present in other vascular plants

Absent

Usually absent; present in many pteridophytes and some spermatophytes

Branch shoots are of superficial origin, while roots originating from shoots can be endogenous or exogenous Varied 'shoot hairs' usually present

Non-medullated protostele (except in some monocoyledons with central Pith) Present

Present in almost all cases and sometimes supplemented by an exodermis (an endodermis-like hypodermis) Branch roots arise endogenously, while shoots originating from roots can be endogenous or exogenous

'Root hairs'; often supplemented by mycorrhizas

Adapted from Raven and Edwards, 2001.

Anemia Plant Diagramatic Daigr

Fig. 1.1 Diagrammatic representation of a typical dicotyledon showing the characteristic properties of roots and shoots: (a) longitudinal view, (b) transverse section of a root, and (c) transverse section of a stem. The shoots bear leaves and daughter shoots that originate exogenously while lateral roots arise far from the root apex and are endogenous in origin. The arrangement of the cortex (C), phloem (Ph) and xylem (X) is shown. (Redrawn and reproduced with permission from Groff and Kaplan, The Botanical Review; New York Botanical Garden, 1988.)

Fig. 1.1 Diagrammatic representation of a typical dicotyledon showing the characteristic properties of roots and shoots: (a) longitudinal view, (b) transverse section of a root, and (c) transverse section of a stem. The shoots bear leaves and daughter shoots that originate exogenously while lateral roots arise far from the root apex and are endogenous in origin. The arrangement of the cortex (C), phloem (Ph) and xylem (X) is shown. (Redrawn and reproduced with permission from Groff and Kaplan, The Botanical Review; New York Botanical Garden, 1988.)

Devonian sporophytes had below-ground parenchymatous structures which performed the functions of roots (anchorage, nutrient and water uptake), but they did not have root caps or an endodermis. Traces of dichotomous root-like structures 5-20 mm in diameter, 10-90 cm long, and penetrating into the substratum to nearly 1 m have been found in fossils of the late Early Devonian (375 million years ago) (Elick et al., 1998), thus allowing the mining of nutrients from the rocks which supplied the increasing biomass of plants at this time. The distinguishing features of roots of vascular flowering plants (angiosperms) first appeared in several plant types such as lycopodia and some bryophytes in Mid Devonian times in a period of rapid plant diversification (Raven and Edwards, 2001). Brundrett (2002) suggests that as plants colonized the land they would have faced powerful selection pressure to increase the surface area of their absorptive surfaces in soil to parallel that occurring in their photosynthetic organs; interception of light and CO2 would thereby be in balance with that of nutrients and water.

A possible evolutionary sequence of shoots and roots is shown in Fig. 1.2 (Brundrett, 2002) in which the evolution of roots emerged as a consequence of the differentiation of underground stems (rhizomes) into two specialized organs: (i) thicker perennial stems that form conduits to distribute water and nutrients, serve as stores and support above-ground structures; and (ii) thinner, longer structures to absorb water and nutrients. Root hairs may have evolved from the rhizoids of earlier plants to increase the volume of substrate available for exploitation, with mycorrhizal fungi also co-evolving with roots (Brundrett, 2002). The available evidence also suggests that while roots evolved first among the lycopsids, they also evolved on at least one other occasion during the evolution of vascular land plants. The suggestion that roots may have gradually evolved from shoots is supported by the observed

Fig. 1.2 A diagrammatic representation of the possible evolution of stems, rhizomes, leaves and roots from the thallus of an early bryophyte-like terrestrial plant, using a hypothetical final example with a woody trunk. (Reproduced with permission from Brundrett, New Phytologist; New Phytologist Trust, 2002.)

Fig. 1.2 A diagrammatic representation of the possible evolution of stems, rhizomes, leaves and roots from the thallus of an early bryophyte-like terrestrial plant, using a hypothetical final example with a woody trunk. (Reproduced with permission from Brundrett, New Phytologist; New Phytologist Trust, 2002.)

developmental and genetic similarities of shoot and root cell division and differentiation in Arabidopsis, although it is also possible that evolutionary convergence of the genetic mechanism occurred after evolution of the root (Dolan and Scheres, 1998).

1.2 Functional interdependence of roots and shoots

1.2.1 Balanced growth of roots and shoots

The different morphologies, anatomies, physiologies and functions of roots and shoots have frequently led to their being considered as two separate systems within the entire plant. Nevertheless, while each system grows and functions as a discrete site for the capture of specific resources (carbon dioxide, light, water and nutrients), the two systems are coupled together and their functions have to form an integrated system. Early explorations of this coupling led to theories based essentially on the size or weight of the two organs. Hellriegel in 1883 in a 'basic law of agriculture' (quoted by van Noordwijk and de Willigen, 1987) wrote that 'The total above-ground growth of plants is strongly dependent on the developmental stage of the root. Only when the root can fully develop will the above-ground plant reach its full potential'. From such writings came the notion that the size of both systems might be inter-related and the simpler notion that big shoots were associated with big root systems. Mayaki et al. (1976), for example, sought to determine a relation between rooting depth and plant height of soyabean as a means of estimating irrigation requirements, and shortly after dwarfing genes were introduced into cereal crops it was hypothesized that their root systems might be shallower as a result (e.g. Lupton et al., 1974 for wheat). Such simple morphological equilibria were demonstrated to be non-existent.

In a set of experiments designed to investigate the equilibrium between root and shoot growth, Troughton (1960) and Brouwer (1963) observed that characteristic equilibria were attained depending on the conditions prevailing. Their experiments demonstrated the following:

(1) When root growth is limited by a factor to be absorbed by the root system, then root growth is relatively favoured; conversely, when the limiting factor has to be absorbed by the shoot, its growth is relatively favoured.

(2) Disturbance of the ratio of root:shoot brought about by either root removal or defoliation leads to changes in the pattern of growth so that the original ratio is rapidly restored (Fig 1.3).

(3) Transfer of plants from one environment to another causes changes in the pattern of assimilate distribution so that a new characteristic root:shoot ratio is established over a period.

The realization that disturbance led to plant activities that restored root:shoot balance, and that it was a combination of both growth and the activities of the root and shoot systems in capturing resources that determined the new equilibrium, led to the concept of a 'functional equilibrium' (Brouwer, 1963, 1983). According to this concept, the root and shoot respond not to the size of each other, but to the effectiveness with which the basic resources are obtained from the environment by the complementary organ. Photosynthate, then, is

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