General

The long-distance transport of water and solutes - mineral elements and low-molecular-weight organic compounds - takes place in the vascular system of xylem and phloem. Long-distance transport from the roots to the shoots occurs predominantly in the nonliving xylem vessels. Coniferous trees lack the continuous system of xylem vessels, and depend on tracheides which are non-living conducting cells ranging in length from 2 to 6 mm (Tyree and Ewers, 1991). In annual plant species too long-distance transport in the xylem vessels may be interrupted by tracheides, for example at the root-shoot junction (Aloni and Griffith, 1991) or in the nodes of the stem. These structures pose an internal resistance to xylem volume flow but simultaneously permit an intensive xylem-phloem solute transfer (Section 3.3.4).

Xylem transport is driven by the gradient in hydrostatic pressure (root pressure) and by the gradient in the water potential. As a reference the water potential of pure free water is defined as having a water potential of zero. Accordingly, values for water potential are usually negative. The gradient in water potential between roots and shoots is quite steep particularly during the day when the stomata are open. Values become less negative in the following sequence: atmosphere » leaf cells > xylem sap > root cells > external solution. Solute flow in the xylem from the roots to the shoots is therefore unidirectional (Fig. 3.1). However, under certain conditions in the shoots a

  1. 3.1 Direction of long-distance transport of mineral elements in nonliving xylem vessels and in the phloem in roots.
  2. 3.1 Direction of long-distance transport of mineral elements in nonliving xylem vessels and in the phloem in roots.

Table 3.1

Accumulation and Long-Distance Transport of 45Ca, 22Na and 42K in Maize Seedlings"'*

Content («eq per 12 plants (24 h) ')

Plant part

45Ca

22Na

42K

Shoot

2.20

0.01

9.07

Endosperm

0.18

0.04

2.38

24-27 cm root

0.01

0.06

0.35

21-24 cm root

0.01

0.09

0.85

18-21 cm root

0.01

0.18

1.30

15-18 cm root

0.01

0.46

1.58

12-15 cm zone of

supply

0.40

1.28

1.93

9-12 cm root

0

0.03

0.40

6- 9 cm root

0

0.02

0.38

3- 6 cm root

0

0.02

0.45

0- 3 cm root

0

0.01

0.75

Total

2.82

2.20

19.44

"Based on Marschner and Richter (1973).

'Each seedling was supplied with 1 meq 1_I of labeled nutrient solution to the root zone 12-15 cm from the root tip. The remainder of the root system was supplied with the same solution in which the nutrients were not labelled.

"Based on Marschner and Richter (1973).

'Each seedling was supplied with 1 meq 1_I of labeled nutrient solution to the root zone 12-15 cm from the root tip. The remainder of the root system was supplied with the same solution in which the nutrients were not labelled.

counterflow of water in the xylem may also occur, for example, from low-transpiring fruits back to the leaves (Lang and Thorpe, 1989; Section 3.4).

In contrast to the xylem, long-distance transport in the phloem takes place in the living sieve tube cells and is bidirectional. The direction of transport is determined primarily by the nutritional requirements of the various plant organs or tissues and occurs, therefore, from source to sink (Chapter 5). In addition, phloem transport is an important component in cycling of mineral nutrients between shoots and roots (Section 3.4) and for signal conductance of the nutritional status of the shoots (Section 2.5.6). Mineral elements can also enter the phloem in the roots and thus be translocated bidirectionally. The translocation of different mineral elements taken up by a particular zone of the root varies markedly during long-distance transport from the zone of supply, as shown in Table 3.1 for maize seedlings. For the reasons already mentioned, long-distance transport from the zone of supply to the root tip must take place in the phloem. Whereas 45Ca is rapidly translocated to the shoot, the translocation of 22Na toward the shoot is severely restricted. The steep gradient in the 22Na content of the root sections in the direction of the shoot (basipetal) reflects resorption by the surrounding root tissue and is a typical feature of so-called natrophobic plant species (Section 10.2). Some 22Na has also been translocated via the phloem to the root tip. In contrast, 42K is quite mobile both in the xylem and in the phloem, and a markedly high proportion of the potassium taken up in more basal root zones is translocated via the phloem toward the root tip, which acts as a sink for this mineral nutrient.

Table 3.2

Xylem Volume Flow (Pressurized Exudation at 100 kPa) and Mineral Element Concentrations in the Xylem Sap of Soil-Grown Nodulated Soybean During Reproductive Stage"

Table 3.2

Xylem Volume Flow (Pressurized Exudation at 100 kPa) and Mineral Element Concentrations in the Xylem Sap of Soil-Grown Nodulated Soybean During Reproductive Stage"

Parameter

Plant development stages

Full pod extension

Early-mid podfill

Late podfill

Early leaf yellowing

Sap volume (ml (50 min)"1 per plant)

1.43

1.13

0.94

0.43

Mineral element

concentration

K (mut)

6.1

5.0

4.0

2.4

Mg (him)

3.8

2.6

1.9

1.2

Ca (mm)

4.8

3.9

3.9

2.2

P(mm)

2.5

1.6

0.9

0.4

S (mm)

1.8

1.6

2.1

1.5

B (mm)

1.0

1.5

1.6

3.2

Zn (mm)

23.0

29.0

32.0

42.0

Cu (jam)

2.7

3.6

2.8

6.9

During long-distance transport, mineral elements and organic solutes are transferred between the xylem and phloem by extensive exchange processes, referred to as loading and unloading. The transfer is mediated by specific cells called transfer cells (Pate and Gunning, 1972). Despite this interchange, and internal cycling, mineral nutrients, such as phosphorus, supplied to only one part of the root system (lateral or seminal roots) are transported preferentially to those parts of the shoots that have direct vascular connections with particular root zones (Stryker et al., 1974). This distribution pattern is especially important for the mineral nutrition of trees that are supplied with fertilizer in a localized area of the root system.

Natural Detox

Natural Detox

Are you looking for a full total body detox? If so, then you might want to try a 10 day detox or some refer to it as the 2-week detox. A 10-day detox is a full body detox that usually means taking several different steps to reach your total body transformation. It might involve a change in diet, exercise and more.

Get My Free Ebook


Post a comment