Effect of Transpiration Rate on Uptake and Translocation

The rate of water flux across the root (short-distance transport) and in the xylem vessels (long-distance transport) is determined by the root pressure and the rate of transpiration. An increase in the transpiration rate may, or may not, enhance the uptake and translocation of mineral elements in the xylem. Enhancement can be achieved in various ways, as shown in Fig. 3.5. Scheme A is true for mineral elements such as boron and silicon, except in the case of wetland rice (Section 10.3.2). Scheme C may be important for soil-grown plants (Section 15.2), particularly in saline substrates (Section 16.6). Whether or not transpiration affects uptake and translocation rate of mineral elements depends predominantly on the following factors:

Mineral Passive Transport
  1. 3.5 Model for possible enhancement effects of high transpiration rates on the uptake and translocation of mineral elements in roots. A. 'Passive' transport of mineral elements through the apoplasm into the stele. B. More rapid removal of mineral elements released into the xylem vessels (Emmert, 1972). C. Increase in the mass flow of the external solution to the rhizoplane and eventually into the apparent free space of the cortex, favoring active uptake into the symplasm. E, Endodermis; X, xylem; arrow, water flux (A to C see text).
  2. 3.5 Model for possible enhancement effects of high transpiration rates on the uptake and translocation of mineral elements in roots. A. 'Passive' transport of mineral elements through the apoplasm into the stele. B. More rapid removal of mineral elements released into the xylem vessels (Emmert, 1972). C. Increase in the mass flow of the external solution to the rhizoplane and eventually into the apparent free space of the cortex, favoring active uptake into the symplasm. E, Endodermis; X, xylem; arrow, water flux (A to C see text).
  3. Plant age. In seedlings and young plants with a low leaf surface area, enhancement effects of transpiration are usually absent; water uptake and solute transport in the xylem to the shoots are determined mainly by the root pressure. As the age and size of the plants increase, the relative importance of the transpiration rate, particularly for the translocation of mineral elements increases.
  4. Time of day. In leaves up to 90% of the total transpiration is stomatal. During the light period, transpiration rates and thus the potential enhancement of uptake and translocation of mineral elements are much higher than during the dark period. Short-term transient falls in the translocation rates of mineral elements at the onset of the dark period reflect the change from transpiration-mediated to root pressure-mediated xylem volume flow (Crossett, 1968). A consistent and synchronous diurnal pattern in transpiration rate and uptake rate of potassium and nitrate (Le Bot and Kirkby, 1992) is probably caused by changes in carbohydrate availability in the roots or feedback control of uptake.

Nodulated legumes show a particular diurnal pattern in shoot transport of fixed nitrogen. The sharp decrease in transpiration-driven xylem volume flow during the dark period is compensated for by a sharp increase in the concentration of fixed nitrogen (as ureides, see Chapter 7) in the xylem sap, thus keeping the total xylem transport rate of fixed nitrogen constant throughout the light/dark cycle (Rainbird etal., 1983).

3. External concentration. It is well known that an increase in the concentration of mineral elements in the nutrient medium may enhance the effect of transpiration rate on the uptake and translocation of mineral elements. This is most likely the result of transport as shown in schemes A and C in Fig .3.5. Usually, translocation rates are more responsive to different transpiration rates than are uptake rates, as shown for sodium in Table 3.5. The effect of transpiration on potassium is negligible in comparison with that

Table 3.5

Effect of Transpiration Rate of Sugar Beet Plants on Uptake and Translocation of Potassium and Sodium from Nutrient Solutions"'6

Potassium Sodium

External Low High Low High concentration (mM) transpiration transpiration transpiration transpiration

"Based on Marschner and Schafarczyk (1967) and W. Schafarczyk (unpublished). ^Transpiration in relative values: low transpiration = 100; high transpiration = 650.

on sodium. This difference corresponds to the differences in the uptake isotherms of these elements at increasing external concentrations (Fig. 2.22). At low external concentrations the nitrate flux in the xylem of maize plants is also not affected by decreasing the transpiration rate to 50%, and a reduction in transpiration rate to 20% being required before a major decline in nitrate flux becomes apparent (Shaner and Boyer, 1976).

4. Type of mineral element. Under otherwise comparable conditions (e.g. plant age and external concentration), the effect of transpiration rates on the uptake and transport follows a typically defined ranking order of mineral elements. It is usually absent or only low for potassium, nitrate and phosphate but may become significant for sodium (Table 3.5) or calcium. As a rule, transpiration enhances the uptake and translocation of uncharged molecules to a greater extent than that of ions. There is a close relationship between transpiration rate and uptake rates of certain herbicides (Shone et al., 1973). The uptake and translocation of mineral elements in the form of molecules is of great importance in the cases of boron (boric acid; Fig. 2.13) and silicon (monosilisic acid; Jones and Handreck, 1965; but see Section 10.3). A close correlation between transpiration and the uptake of silicon is shown for oat plants in Table 3.6.

There is perfect agreement between silicon content measured in plants and that predicted from the transpiration values (water loss times silicon concentration in the soil solution). Silicon accumulation in the shoot dry matter may therefore be a suitable parameter for calculations of the water use efficiency (WUE; kg dry matter produced/ kg water transpired) in field-grown cereals grown under rainfed conditions (Walker and Lance, 1991). However, this parameter is unsuitable, for example, in plants grown at different irrigation regimes (Mayland et al., 1991) as well as in plants grown in nutrient solution (Jarvis, 1987), or when different genotypes within a species such as barley are compared (Nable et al., 1990b).

Even in plants where close correlations between transpiration and silicon uptake are found, roots are not freely permeable to the radial transport of silicon. In wheat in endodermal cells large silicon depositions are found increasing from apical to basal root zones (Hodson and Sangster, 1989c) and such large deposition in the endodermis is typical for field-grown cereals (Bennett, 1982).

Table 3.6

Measured and Calculated Silicon Uptake in Relation to Transpiration (Water Consumption) of Oat Plants0

Table 3.6

Measured and Calculated Silicon Uptake in Relation to Transpiration (Water Consumption) of Oat Plants0

Harvest

Transpiration

Measured uptake

Calculated Si uptake*

after days

(ml per plant)

(mg per plant)

(mg per plant)

44

67

3.4

3.6

58

175

9.4

9.4

82

910

50.0

49.1

109

2785

156.0

150.0

"From Jones and Handreck (1965).

''Silicon concentration in the soil solution: 54 mg P1.

"From Jones and Handreck (1965).

''Silicon concentration in the soil solution: 54 mg P1.

The absence of effects of reduced transpiration rates on the root to shoot transport of mineral nutrients may indicate a high proportion of xylem to phloem transfer in the stem tissue, or a corresponding increase in xylem sap concentrations of the mineral nutrients. Alternatively, the involvement of a nontranspirational component of xylem transport, namely the so-called Munch-counterflow has been stressed recently (Tanner and Beevers, 1990). This component is based on recycling of water derived from solute volume flow in the phloem from shoots to roots (Section 3.4.4) and the release of this water, and some of the solutes (recycled fraction) into the xylem. According to Tanner and Beevers (1990) the calculated amounts of water recycled in this manner vary between 9% (at high transpiration rates) and 30% (at low transpiration rates) of the total water uptake by the roots of maize plants. Values in this high order have been questioned for various reasons by Smith (1991). Nevertheless, recycling of water in plants must be taken into account in relation to recycling of mineral nutrients in general and distribution of calcium within the shoot in particular (Section 3.4.3).

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Responses

  • Leighton
    What is the effect of transpiration on nutrient uptake by plants?
    7 years ago
  • Haddas
    How minerals affects transpiration?
    7 years ago
  • Richard
    What is the rate of xylem translocation determined by?
    7 years ago
  • ari
    How does transpiration correlates with translocation?
    1 year ago

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