Mineral nutrients

It takes more than light, CO2 and water to make a plant. Mineral resources are also needed. The mineral resources that the plant must obtain from the soil (or, in the case of aquatic plants, from the surrounding water) include macronutrients (i.e. those needed in relatively large amounts) -nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg) and iron (Fe) - and a series of trace elements -for example, manganese (Mn), zinc (Zn), copper (Cu), boron (B) and molybdenum (Mo) (Figure 3.18). (Many of these elements are also essential to animals, although it is more common for animals to obtain them in organic form in their food than as inorganic chemicals.) Some plant groups have special requirements. For example, aluminum is a necessary nutrient for some ferns, silicon for diatoms and selenium for certain planktonic algae.

Green plants do not obtain their mineral resources as a single package. Each element enters the plant independently as an ion or a molecule, and each has its own characteristic properties of absorption in the soil and of diffusion, which affect its accessibility to the plant even before any selective processes of uptake occur at the root membranes. All green plants require all of the 'essential' elements listed in Figure 3.18, although not in the same proportion, and there are some quite striking differences between the mineral compositions of plant tissues of different species and between the different parts of a single plant (Figure 3.19).

Many of the points made about water as a resource, and about roots roots as foragers as extractors of this resource, apply equally to mineral nutrients. Strategic differences in developmental programs can be recognized between the roots of different species (Figure 3.20a), but it is the ability of root systems to override strict programs and be opportunistic that makes them effective exploiters of the soil. Most roots elongate before they produce laterals, and this ensures that exploration precedes exploitation. Branch roots usually emerge on radii of the parent root, secondary roots radiate from these primaries and tertiaries from the secondaries. These rules reduce the chance that two branches of the same root will forage in the same soil particle and enter each other's RDZs.

roots and the dynamics of water depletion zones macronutrients and trace elements

Essential to restricted groups of organisms

(a) Boron - Some vascular plants and algae

(b) Chromium - Probably essential in higher animals

(c) Cobalt - Essential in ruminants and N-fixing legumes

(d) Fluorine - Beneficial to bone and tooth formation

(e) Iodine - Higher animals

(f) Selenium - Some higher animals?

(g) Silicon - Diatoms

(h) Vanadium - Tunicates, echinoderms and some algae

Esse

ntial for most organisms

Essential to most living organisms

Essential to animals

1 H

He

3 Li

Be

(a) 5 B

6 C

7 N

8 O

9 F

10 Ne

11 Na

12 Mg

Si

15 P

16 S

17 Cl

18 Ar

19 K

20 Ca

21 Sc

22 Ti

23 V

24 Cr

25 Mn

26 Fe

27 Co

28 Ni

29 Cu

30 Zn

31 Ga

32 Ge

33 As

(f)34 Se

35 Br

36 Kr

37 Rb

38 Sr

39 Y

40 Zr

41 Nb

42 Mo

43 Tc

44 Ru

45 Rh

46 Pd

47 Ag

48 Cd

49 In

50 Sn

51 Sb

52 Te

53 I

54 Xe

55 Cs

56 Ba

57 La

72 Hf

73 Ta

74 W

75 Re

76 Os

77 Ir

78 Pt

79 Au

80 Hg

81 Tl

82 Pb

83 Bi

84 Po

85 At

86 Rn

87 Fr

88 Ra

89 Ac

Lanthanons

58 Ce

59 Pr

60 Nd

61 Pm

62 Sm

63 Eu

64 Gd

65 Tb

66 Dy

67 Ho

68 Er

69 Tm

70 Yb

71 Lu

Actinons

90 Th

91 Pa

92 U

93 Np

94 Pu

95 Am

96 Cm

97 Bk

98 Cf

99 Es

100 Fm

101 Md

102 No

103 Lr

Figure 3.18 Periodic table of the elements showing those that are essential resources in the life of various organisms.

Roots pass through a medium in which they meet obstacles and encounter heterogeneity - patches of nutrient that vary on the same scale as the diameter of a root itself. In 1 cm of growth, a root may encounter a boulder, pebbles and sand grains, a dead or living root, or the decomposing body of a worm. As a root passes through a heterogeneous soil (and all soils are heterogeneous seen from a 'root's-eye view'), it responds by branching freely in zones that supply resources, and scarcely branching in less rewarding patches (Figure 3.20b). That it can do so depends on the individual rootlet's ability to react on an extremely local scale to the conditions that it meets.

There are strong interactions between water and nutrients as resources for plant growth. Roots will not grow freely into soil zones that lack available water, and so nutrients in these zones will not be exploited. Plants deprived of essential minerals make less growth and may then fail to reach volumes of soil that contain available water. There are similar interactions between mineral resources. A plant starved of nitrogen makes poor root growth and so may fail to 'forage' in areas that contain available phosphate or indeed contain more nitrogen.

Of all the major plant nutrients, nitrates move most freely in the soil solution and are carried from as far away from the root surface as water is carried. Hence nitrates will be most mobile in soils at or near field capacity, and in soils with wide pores. The RDZs for nitrates will then be wide, and those produced around neighboring roots will be more likely to overlap. Competition can then occur - even between the roots of a single plant.

The concept of RDZs is important not only in visualizing how one organism influences the resources available to another, but also in understanding how the architecture of the root system affects the capture of these resources. For a plant growing in an environment in which water moves freely to the root surface, those nutrients that are freely in solution will move with the water. They will then be most effectively captured by wide ranging, but not interactions between foraging for water and nutrients

Figure 3.19 (a) The relative concentration of various minerals in whole plants of four species in the Brookhaven Forest, New York. (b) The relative concentration of various minerals in different tissues of the white oak (Quercus alba) in the Brookhaven Forest. Note that the differences between species are much less than between the parts of a single species. (After Woodwell et al., 1975).

Figure 3.19 (a) The relative concentration of various minerals in whole plants of four species in the Brookhaven Forest, New York. (b) The relative concentration of various minerals in different tissues of the white oak (Quercus alba) in the Brookhaven Forest. Note that the differences between species are much less than between the parts of a single species. (After Woodwell et al., 1975).

Figure 3.20 (a) The root systems of plants in a typical short-grass prairie after a run of years with average rainfall (Hays, Kansas). Ap, Aristida purpurea; Aps, Ambrosia psilostachya; Bd, Buchloe dactyloides; Bg, Bouteloua gracilis; Mc, Malvastrum coccineum; Pt, Psoralia tenuiflora; Sm, Solidago mollis. (After Albertson, 1937; Weaver & Albertson, 1943.) (b) The root system developed by a plant of wheat grown through a sandy soil containing a layer of clay. Note the responsiveness of root development to the localized environment that it encounters. (Courtesy of J.V. Lake.)

Figure 3.20 (a) The root systems of plants in a typical short-grass prairie after a run of years with average rainfall (Hays, Kansas). Ap, Aristida purpurea; Aps, Ambrosia psilostachya; Bd, Buchloe dactyloides; Bg, Bouteloua gracilis; Mc, Malvastrum coccineum; Pt, Psoralia tenuiflora; Sm, Solidago mollis. (After Albertson, 1937; Weaver & Albertson, 1943.) (b) The root system developed by a plant of wheat grown through a sandy soil containing a layer of clay. Note the responsiveness of root development to the localized environment that it encounters. (Courtesy of J.V. Lake.)

intimately branched, root systems. The less freely that water moves in the soil, the narrower will be the RDZs, and the more it will pay the plant to explore the soil intensively rather than extensively.

The soil solution that flows through variations between soil pores to the root surface has a nutrients in their biased mineral composition compared freedom of with what is potentially available. This movement is because different mineral ions are held by different forces in the soil. Ions such as nitrate, calcium and sodium may, in a fertile agricultural soil, be carried to the root surface faster than they are accumulated in the body of the plant. By contrast, the phosphate and potassium content of the soil solution will often fall far short of the plant's requirements. Phosphate is bound on soil colloids by surfaces that bear calcium, aluminum and ferric ions, and the rate at which it can be extracted by plants then depends on the rate at which its concentration is replenished by release from the colloids. In dilute solutions, the diffusion coefficients of ions that are not absorbed, such as nitrate, are of the order of 10-5 cm2 s-1, and for cations such as calcium, magnesium, ammonium and potassium they are 10-7 cm2 s-1. For strongly absorbed anions such as phosphate, the coefficients are as low as 10-9 cm2 s-1. The diffusion rate is the main factor that determines the width of an RDZ.

For resources like phosphate that have low diffusion coefficients, the RDZs will be narrow (Figure 3.21); roots or root hairs will only tap common pools of resource (i.e. will compete) if they

Figure 3.21 Radioautograph of soil in which seedlings of mustard have been grown. The soil was supplied with radioactively labeled phosphate (32PO-) and the zones that have been depleted by the activity of the roots show up clearly as white. (After Nye & Tinker, 1977.)

are very close together. It has been estimated that more than 90% of the phosphate absorbed by a root hair in a 4-day period will have come from the soil within 0.1 mm of its surface. Two roots will therefore only draw on the same phosphate resource in this period if they are less than 0.2 mm apart. A widely spaced, extensive root system tends to maximize access to nitrate, whilst a narrowly spaced, intensively branched root system tends to maximize access to phosphates (Nye & Tinker, 1977). Plants with different shapes of root system may therefore tolerate different levels of soil mineral resources, and different species may deplete different mineral resources to different extents. This may be of great importance in allowing a variety of plant species to cohabit in the same area (coexistence of competitors is discussed in Chapters 8 and 19).

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