Roots as water foragers

For most terrestrial plants, the main source of water is the soil and they gain access to it through a root system. We proceed here

Figure 3.16 Changes in the concentrations of nutrients in plant material grown at twice-ambient atmospheric CO2 concentrations, based on 25 studies on leaves of a variety of plants (colored bars) and five studies of wheat grains (gray bars). Black lines indicate the standard errors. (After Loladze, 2002.)

Figure 3.16 Changes in the concentrations of nutrients in plant material grown at twice-ambient atmospheric CO2 concentrations, based on 25 studies on leaves of a variety of plants (colored bars) and five studies of wheat grains (gray bars). Black lines indicate the standard errors. (After Loladze, 2002.)

0

ta

ü a a

diameters hairs

■a

cells

ceo

r

^ ( A ^

fiit

1

i i

i i i

ii i

1

1000 I

i i i

0.01 0.001 ii i

0.0001 Pore size (im)

1

I I I

1

0

1 2

3 4 1

15 6

7 pF

Water drains away freely

Available water

Available water to many native species

Water unavailable

Figure 3.17 The status of water in the soil, showing the relationship between three measures of water status: (i) pF, the logarithm of the height (cm) of the column of water that the soil would support; (ii) water status expressed as atmospheres or bars; (iii) the diameter of soil pores that remain water-filled. The size of water-filled pores may be compared in the figure with the sizes of rootlets, root hairs and bacterial cells. Note that for most species of crop plant the permanent wilting point is at approximately -15 bars (-1.5 X 106 Pa), but in many other species it reaches -80 bars (-8 X106 Pa), depending on the osmotic potentials that the species can develop.

Figure 3.17 The status of water in the soil, showing the relationship between three measures of water status: (i) pF, the logarithm of the height (cm) of the column of water that the soil would support; (ii) water status expressed as atmospheres or bars; (iii) the diameter of soil pores that remain water-filled. The size of water-filled pores may be compared in the figure with the sizes of rootlets, root hairs and bacterial cells. Note that for most species of crop plant the permanent wilting point is at approximately -15 bars (-1.5 X 106 Pa), but in many other species it reaches -80 bars (-8 X106 Pa), depending on the osmotic potentials that the species can develop.

(and in the next section on plant nutrient resources) on the basis of plants simply having 'roots'. In fact, most plants do not have roots - they have mycorrhizae: associations of fungal and root tissue in which both partners are crucial to the resource-gathering properties of the whole. Mycorrhizae, and the respective roles of the plants and the fungi, are discussed in Chapter 13.

It is not easy to see how roots evolved by the modification of any more primitive organ (Harper et al., 1991), yet the evolution of the root was almost certainly the most influential event that made an extensive land flora and fauna possible. Once roots had evolved they provided secure anchorage for structures the size of trees and a means for making intimate contact with mineral nutrients and water within the soil.

Water enters the soil as rain or melting snow and forms a reservoir in the pores between soil particles. What happens to it then depends on the size of the pores, which may hold it by capillary forces against gravity. If the pores are wide, as in a sandy soil, much of the water will drain away until it reaches some impediment and accumulates as a rising watertable or finds its way into streams or rivers. The water held by soil pores against the force of gravity is called the 'field capacity' of the soil. This is the upper limit of the water that a freely drained soil will retain.

There is a less clearly defined lower limit to the water that can be used in plant growth (Figure 3.17). This is determined by the ability of plants to extract water from the narrower soil pores, and is known as the 'permanent wilting point' - the soil water content at which plants wilt and are unable to recover. The permanent wilting point does not differ much between the plant species of mesic environments (i.e. with a moderate amount of water) or between species of crop plants, but many species native to arid regions can extract significantly more water from the soil.

As a root withdraws water from the soil pores at its surface, it creates water-depletion zones around it. These determine gradients of water potential between the interconnected soil pores. Water flows along the gradient into the depleted zones, supplying further water to the root. This simple process is made much more complex because the more the soil around the roots is depleted of water, the more resistance there is to water flow. As the root starts to withdraw water from the soil, the first water that it obtains is from the wider pores because they hold the water with weaker capillary forces. This leaves only the narrower, more tortuous water-filled paths through which flow can occur, and so the resistance to water flow increases. Thus, when the root draws water from the soil very rapidly, the resource depletion zone (RDZ; see Section 3.2.1) becomes sharply defined and water can move across it only slowly. For this reason, rapidly transpiring field capacity and the permanent wilting point plants may wilt in a soil that contains abundant water. The fineness and degree of ramification of the root system through the soil then become important in determining the access of the plant to the water in the soil reservoir.

Water that arrives on a soil surface as rain or as melting snow does not distribute itself evenly. Instead, it tends to bring the surface layer to field capacity, and further rain extends this layer further and further down into the soil profile. This means that different parts of the same plant root system may encounter water held with quite different forces, and indeed the roots can move water between soil layers (Caldwell & Richards, 1986). In arid areas, where rainfall is in rare, short showers, the surface layers may be brought to field capacity whilst the rest of the soil stays at or below wilting point. This is a potential hazard in the life of a seedling that may, after rain, germinate in the wet surface layers lying above a soil mass that cannot provide the water resource to support its further growth. A variety of specialized dormancy-breaking mechanisms are found in species living in such habitats, protecting them against too quick a response to insufficient rain.

The root system that a plant establishes early in its life can determine its responsiveness to future events. Where most water is received as occasional showers on a dry substrate, a seedling with a developmental program that puts its early energy into a deep taproot will gain little from subsequent showers. By contrast, a program that determines that the taproot is formed early in life may guarantee continual access to water in an environment in which heavy rains fill a soil reservoir to depth in the spring, but there is then a long period of drought.

Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment