External and Internal Factors

As a rule, an increase in the external ion concentration leads to an increase in the concentration of ions in the xylem exudate. However, the relative concentration falls as the external concentration is increased (Table 2.36). This concentration gradient ('Concentration factor') between the external solution and the xylem exudate decreases and can even fall below 1 in the case of calcium. The exudation volume flow shows a somewhat different pattern, and is maximal at 1.0 mM external concentration (Table 2.36). At 0.1 mM this flow is limited by the ion concentration in the xylem. In contrast, at 10.0 mM, the flow is limited by the water availability (i.e., the low water potential in the external solution) and the small concentration gradient between the external solution and the xylem. The increase in the exudate concentration of the mineral nutrients, with

Table 2.36

Relationship between External Concentration, Exudate Concentration, and Exudate Volume

Flow in Decapitated Sunflower Plants

Table 2.36

Relationship between External Concentration, Exudate Concentration, and Exudate Volume

Flow in Decapitated Sunflower Plants

External solution KNO3 + CaCl2 (mM each)

Exudate (mM)

'Concentration factor'

Exudation volume flow (ml (4 h)-1)

k+

Ca2+

no3-

k+

Ca2+

no3-

0.1

7.3

2.8

7.4

73

28

74

4.0

1.0

10.0

3.2

10.7

10

3.2

10.1

4.5

10.0

16.6

4.2

10.3

1.7

0.4

1.0

1.6

the rise in external concentration from 1.0 to 10.0 itim, does not compensate for the decrease in the exudation volume flow. Thus, in contrast to the accumulation in roots (hyperbolic function of the external concentration, see e.g., Fig. 2.11), the rate of root pressure-driven shoot transport of mineral nutrients can decline at high external concentrations.

An increase in the root zone temperature has a much greater effect on the exudation volume flow than on the ion concentration in the exudate (Table 2.37). This is consistent with the expectation that a root behaves as an osmometer: Temperature determines the rate of ion transport in the symplasm (plasmodesmata, Section 2.7) and the release into the xylem, and water moves accordingly along the water potential gradient. There are, however, distinct differences between a root and a simple osmometer. An increase in the root temperature results in an increase in the potassium concentration but a decrease in the calcium concentration of the exudate. This shift in the potassium/calcium ratio might reflect temperature effects either on membrane selectivity or on the relative importance of the apoplasmic pathway of radial transport of calcium and water (Engels et al., 1992). Similar shifts in the potassium/calcium translocation ratio are also observed at different soil temperatures (Walker, 1969). This temperature effect may have important implications for the calcium nutrition of plants and might explain the enhancement of calcium deficiency symptoms in lettuce at elevated root temperatures, despite a slight increase in the calcium concentration of the leaf tissue (Collier and Tibbits, 1984).

Table 2.37

Temperature Effect on Exudation Volume Flow and on Potassium and Calcium Concentration in the Exudate of Decapitated Maize Plants0

Exudate

Table 2.37

Temperature Effect on Exudation Volume Flow and on Potassium and Calcium Concentration in the Exudate of Decapitated Maize Plants0

Exudate

Temperature

(°C)

Exudation volume flow (ml (4 h)"1)

K+/Ca2+

8

5.3

13.4

1.5

8.9

18

21.9

15.2

1.0

15.2

28

31.7

19.6

0.8

24.5

"Concentration of KN03 and CaCl2 in the external solution: ImM each.

"Concentration of KN03 and CaCl2 in the external solution: ImM each.

Table 2.38

Effect of Root Respiration on Exudation Volume Flow and Ion Concentration in the Exudate of Decapitated Maize Plants"

Table 2.38

Effect of Root Respiration on Exudation Volume Flow and Ion Concentration in the Exudate of Decapitated Maize Plants"

Exudate con

centration (mM)

Exudation volume flow

K+ Ca2+

Treatment6

(ml (3 h)_1)

o2

26.5

16.6 1.8

n2

5.7

15.2 1.7

"Concentration of KN03 and CaCl2 in the external solution: 0.5 raM each. ^Respiration treatment consisted of bubbling oxygen or nitrogen through the external (nutrient) solution.

"Concentration of KN03 and CaCl2 in the external solution: 0.5 raM each. ^Respiration treatment consisted of bubbling oxygen or nitrogen through the external (nutrient) solution.

The rate of release of ions into the xylem is closely related to root respiration (Table 2.38). A lack of oxygen strongly depresses the exudation volume flow but not the concentrations of potassium and calcium in the exudate. Oxygen deficiency seems to affect ion release into the xylem and root hydraulic conductivity to the same degree.

As in ion accumulation in root cells, maintenance of the cation-anion balance is necessary in the xylem exudate (Allen et al., 1988; Findenegg et al., 1989). Therefore, the accompanying ion may affect the transport rate even at low external concentrations (Table 2.39). When KN03 is supplied, the exudation flow rate is almost twice as high as the flow rate when an equivalent concentration of K2S04 is added. Since the potassium concentration in the exudate is similar in both treatments, the transport rate of potassium supplied as K2SO4 is only about half the rate of potassium supplied as KNO3.

In contrast to the potassium concentration, the concentrations of nitrate and sulfate in the exudate exhibit a large difference (18.1 and 0.6 meq P1, respectively) between the treatments. The corresponding difference in negative charges in the exudate is approximately compensated for by elevated concentrations of organic acid anions. In the K2S04 treatment, however, the rate-limiting factor is probably the capacity of the

Table 2.39

Flow Rate and Ion Concentration in the Xylem Exudate of Wheat Seedlings"

Treatment

Parameter KNO3 K2S04

Table 2.39

Flow Rate and Ion Concentration in the Xylem Exudate of Wheat Seedlings"

Treatment

Parameter KNO3 K2S04

Exudation flow rate (/u\ h_1

per 50 plants) f)

372

180

Ion concentration (¡ueq ml~

Potassium

23.3

24.5

Calcium

9.1

9.5

Nitrate

18.1

0.0

Sulfate

0.2

0.8

Organic acids

9.6

25.8

"Seedlings were supplied with either KNO3 (1 mM) or K2S04 (0.5 mM) in the presence of 0.2 mM CaS04. From Triplett et al. (1980).

"Seedlings were supplied with either KNO3 (1 mM) or K2S04 (0.5 mM) in the presence of 0.2 mM CaS04. From Triplett et al. (1980).

Table 2.40

Relationship between Photoperiod, Carbohydrate Content of Roots, and Uptake and Translocation of Potassium in Decapitated Maize Plants".

Table 2.40

Relationship between Photoperiod, Carbohydrate Content of Roots, and Uptake and Translocation of Potassium in Decapitated Maize Plants".

Photoperiod (h)

12/126

24/0

Carbohydrate in roots (mg) Total potassium uptake (meq)

Potassium translocation in exudation volume flow (meq) Exudation volume flow (ml (8 h)-1) Relative decline in flow rate within 8 h (%)

122 (48) 1.3 1.0 30.3 60

328 (226)c 5.0 3.5 88.5 12

"Data per 12 plants.

'Hours of light/hours of darkness. This pretreatment with different day lengths was for one day (i.e., the day prior to decapitation).

"Numbers in parentheses denote carbohydrate content after 8 h (decapitation).

"Data per 12 plants.

'Hours of light/hours of darkness. This pretreatment with different day lengths was for one day (i.e., the day prior to decapitation).

"Numbers in parentheses denote carbohydrate content after 8 h (decapitation).

roots to maintain the cation-anion balance by organic acid synthesis; this leads to a decrease in the rate of potassium and calcium release into the xylem and a corresponding decrease in exudation flow rate.

Release of ions into the xylem and the corresponding changes in root pressure are also closely related to the carbohydrate status of the roots (Table 2.40). Variation in the length of the photoperiod for one day before decapitation affects the carbohydrate status of the roots and correspondingly the rate and duration of exudation volume flow after decapitation. Both the uptake and translocation rate of potassium in roots with a high carbohydrate content are considerably greater than in roots that are low in carbohydrate. The higher translocation rate is closely related to the exudation volume flow. In roots with a low carbohydrate content, reserves are rapidly depleted after decapitation and there is a corresponding decline in the rate of exudation volume flow within 8 h. This depletion of carbohydrates in the roots of decapitated plants is one of the factors which limits studies on exudation volume flow.

However, release of ions into the xylem and exudation volume flow are not necessarily related to the carbohydrate status of the roots but can show endogenously regulated, distinct diurnal fluctuations which are also maintained in plants transferred to continuous darkness (Ferrario et al., 1992a,b). Hormonal effects may be involved in this endogenous rhythm as, for example, abscisic acid (ABA) strongly enhances exudation volume flow (Fournier etal., 1987).

2.9.2 Xylem Exudate, Root Assimilation and Cycling of Nutrients

Analyses of xylem exudates provide valuable information about assimilation of mineral nutrients in the roots, for example, or the capacity of roots for nitrate reduction or in legumes for N2 fixation. In soybean and other tropical legumes the proportion of ureides (Chapter 7) to total nitrogen in the xylem exudate reflects the nodule activity and is also a suitable indicator in field-grown plants of the relative contribution of N2 fixation to the nitrogen nutrition of legumes (Peoples et al., 1989). In non-legumes, analysis of the forms of nitrogen in the xylem sap can provide valuable information on the assimilation of nitrogen in the root and the importance of the various organic and inorganic fractions in long distance transport (van Beusichem, 1988). This also holds true for analysis of the binding forms of heavy metals in the xylem exudate (Cataldo et al., 1988). Analysis of xylem exudate also provides insight into transport of hormones (e.g., ABA, cytokinins) or related compounds like polyamines (Friedman et al., 1986) as signals from roots to the shoots (Chapter 5). The discovery of unexpectedly high concentrations of sugars in xylem exudates of annual species (Cataldo et al., 1988; Canny and McCully, 1989) may represent sugars leaked from the phloem and swept into the xylem stream before retrieval into the phloem (McCully and Mallett, 1993). These results demonstrate the potential of xylem sap analysis for modeling not only mineral nutrient cycling but also the nitrogen and carbon economy of plants.

There are, however, several factors to be considered in the interpretation of xylem exudate analyses. Xylem sap collected from decapitated plants (exudate) represents only the root pressure component of xylem volume flow. For evaluation of the transpirational component exudates have to be collected either under vacuum at the cut stump, or by increasing the external pressure in the root zone (pressure chamber). With both methods xylem volume flow is increased and mineral nutrient concentration usually decreased. However,with these methods calculated transport rates to the shoot might be quite different from the results found in intact plants (Salim and Pitman, 1984; Allen etal., 1988). Furthermore, irrespective of the collection method, the xylem sap also contains shoot-derived nutrients recycled in the phloem and reloaded (or leaked) in the roots into the xylem (Section 2.5.6). Recycling of water may also have to be considered in interpreting of xylem sap analysis (Tanner and Beevers, 1990; Chapter 3). The fractions of recycled nutrients can be particularly high in the case of potassium, nitrogen and sulfur, and may lead to misinterpretations, for example, on the capacity of roots for reduction of nitrate and sulfate. The proportion of recycled nutrients in the xylem sap depends on various factors such as plant species and nutritional status in general and shoot demand in particular as shown for potassium in Table 2.41.

When shoot demand is high potassium transport in the xylem exudate strongly increases and net translocation is also higher (sequential harvests), but net uptake is unaffected (Table 2.41). Accordingly, root content of potassium is about 20% higher in the plants with low shoot demand. The differences between net translocation and xylem

Table 2.41

Role of Shoot Demand on Net Uptake, Net Translocation and Flux of Potassium in the Xylem Exudate of Maize"

Table 2.41

Role of Shoot Demand on Net Uptake, Net Translocation and Flux of Potassium in the Xylem Exudate of Maize"

Potassium (/¿mol g 1 root fresh wt h ')

Shoot

Net uptake

Net translocation

Xylem exudate

demand*

0-3 days

0-3 days

day 3

High

2.26

1.83

8.55

Low

2.28

1.17

2.46

"Engels and Marschner (1992b).

6Shoot demand altered by the shoot base temperature, see Table 2.15.

"Engels and Marschner (1992b).

6Shoot demand altered by the shoot base temperature, see Table 2.15.

transport of potassium reflect differences in recirculation of potassium which, against expectation, were higher in plants with high shoot demand and presumably related to the role of potassium in xylem transport of nitrate (Section 3.2). This is an example of the important information xylem sap analysis can provide for regulation of xylem loading and recycling of mineral nutrients, if combined with other methods like phloem sap analysis (Allen et al., 1988; Van Beusichem et al., 1988) or measurements of net uptake and net changes in nutrient contents in roots and shoots.

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