Of xylem sap of barley and rice grown hydroponically under varied concentrations of iron

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As shown in the previous section 4.1, the concentration of PS was much higher than that of Fe in xylem sap of -Fe barley. The ratio of the concentration of PS/Fe varied depending on Fe nutritional status of the plant. In order to obtain data for the PS/Fe ratio as affected by the Fe status of the plant, the xylem sap from grasses grown with various concentrations of Fe (0, 1, 10, 100 ^M) in hydroponic culture was collected and analyzed. Iron sources in the hydroponics media for barley and rice were EDTA-Fe (Kawai et al., 1993) and citrate-Fe (Alam et al., 2003), respectively.

Table 14-1. Concentration of phytosiderophores and amino acids in xylem sap of Fe-sufficient or Fe-deficient barley grown in nutrient solution._

Amino Acid Fe-sufficient Fe-deficient

Table 14-1. Concentration of phytosiderophores and amino acids in xylem sap of Fe-sufficient or Fe-deficient barley grown in nutrient solution._

Amino Acid Fe-sufficient Fe-deficient

Phytosiderophores (PS)

19±2

453±43

Aspartate

129±6

51±2

Threonine

349±36

624±16

Serine

607±13

763±25

Asparagine+Glutamate

1218±100

3868±149

Glutamine

2638±90

1805±95

Cysteine

0.5±0.1

4±1

Proline

32±4

202±32

Glycine

41±5

43±9

Alanine

386±41

380±28

Valine

217±25

433±20

Methionine

6±0.1

28±2

Isoleucine

30±4

33±2

Leucine

26±2

39±4

Tyrosine

20±2

8±1

Phenylalanine

54±2

11±0.3

Total

5771±98

8746±26

Values represent the mean ±SE of 2 replications.

Values represent the mean ±SE of 2 replications.

Table 14-2. Amounts and ratios of phytosiderophores and Fe of xylem sap of barley and rice. Xylem sap was collected for 3 hours from 36 plants of barley or 96 plants of rice grown hydroponically in a greenhouse. Iron sources for the cultivation were EDTA-Fe for barley and citrate-Fe for rice.

Barley

PS (nmol plant-1

' h-1)

Fe (nmol plant-1

' h-1)

PS/Fe ratio

Fe-deficient

4.57

0.0122

375

Fe 1 ^M

1.65

0.0709

23.3

Fe 10 ^M

1.37

0.382

3.59

Fe 100 ^M

0.689

0.944

0.730

Rice

PS (pmol plant-1

' h-1)

Fe (pmol plant-1

' h-1)

PS/Fe ratio

Fe-deficient

171

0.774

221

Fe 1 ^M

42.6

13.5

3.16

Fe 10 ^M

40.1

20.1

2.00

Fe 100 ^M

17.4

36.4

0.478

The amount of PS and Fe and the PS/Fe ratios in xylem sap of barley and rice are shown in Table 14-2. The PS/Fe ratio in xylem sap varied depending on the Fe status of the plants. Generally, the PS/Fe ratio decreased according to the increase in Fe concentrations of the medium. Tiffin (1966) reported that citrate/Fe ratios ranged between 1 and 3. The PS/Fe ratios fluctuated in a broader range than those of the citrate/Fe ratio. The PS/Fe ratio could be even less than 1 in the plants grown under Fe abundant condition (100 ^M).

Based on the obtained data, it was not possible to conclude that PS were the major carriers for Fe in the xylem of grasses.

The concentrations of metal micronutrients of xylem sap of barley are shown in Figure 14-7. Manganese and Zn concentrations decreased when Fe was added to the medium. However, there was no distinct relationship between the PS concentration and the concentrations of Mn, Zn, and Cu in xylem sap. It seemed that the concentrations of Mn, Zn, and Cu were not related to the concentration of Fe or PS in xylem sap. The function of several chelators for the uptake of metal micronutrients from rhizosphere by the root has been discussed (Linsay 1974; Treeby et al., 1989; Zhang, 1993). The role of PS in plant tissue for the translocation of metal micronutrients also needs further investigation.

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