Quantification of zinc transport via the phloem to the grain in rice plants (Oryza sativa L.) at early grain-filling by a combination of mathematical modeling and ⁶⁵Zn tracing

Figures & data

Figure 1 Schemes of zinc-65 (⁶⁵Zn) translocation to the grain and glume via the xylem and phloem, when ⁶⁵Zn was applied through the culm cuts above (A1) and below (A2) the flag-leaf node, from the flag leaf (B) and from the roots (C). The schemes were described principally after Tanaka et al. (2007). The parameter “a” is the ratio of the ⁶⁵Zn imported into the glume to that imported into the grain via the xylem. The parameter “b” is the ratio of ⁶⁵Zn imported into the glume via the phloem to that imported into the grain. The parameter “t” is the proportion of the ⁶⁵Zn deposited in the grains via the phloem to the total ⁶⁵Zn in the grains.

Table 1 Partitioning of radioactivity in the top parts of rice (Oryza sativa L.) 72 hours after the application of zinc-65 (⁶⁵Zn) via the root-bathing culture solution

Plant part	Radioactivity (cpm ± SE, n = 3)	Partitioning of radioactivity within top parts (%)
Panicles	2631 ± 210	52.9
Glumes (10 pieces)	25 ± 8
Grains (10 pieces)	893 ± 100
Flag leaf
Leaf blade	4 ± 2	0.1
Leaf sheath	995 ± 330	20.2
Other leaves
Leaf blades	58 ± 24	1.2
Leaf sheaths	389 ± 34	7.8
Culms	863 ± 394	17.4

The radioactivity showed an excess over the background activity. SE, standard error.

Table 2 Partitioning of the radioactivity exported from the zinc-65 (⁶⁵Zn)-fed flag leaf blade in the rice plant (Oryza sativa L.) parts 48 hours after the initiation of ⁶⁵Zn application to the flag leaf blade

Plant part	Radioactivity (cpm ± SE, n = 5)	Partitioning of radioactivity among plant parts other than the flag-leaf blades (site of ^65Zn feeding (%)
Panicles	267 ± 47	63.2
Glumes (10 pieces)	N.D.
Grains (10 pieces)	85 ± 11
Sheath of the ^65Zn-fed flag leaf	77 ± 1	18.3
Other leaves	N.D.	0.0
Culm	33 ± 4	7.8
Roots	45 ± 5	10.7

N.D., not detected; SE, standard error. The radioactivity showed an excess over the background activity.

Table 3 The radioactivities (cpm) of zinc-65 (⁶⁵Zn) in 10 glumes and 10 grains administered for 2, 3, 4 and 6 h via a culm cut above or below the flag-leaf node in rice (Oryza sativa L.)

	Above		Below
^65Zn feeding (h)	Glumes	Grains	Glumes	Grains
2	27.3 ± 3.2	20.4 ± 1.4	11.8 ± 0.7	15.6 ± 1.8
3	25.8 ± 10.7	23.0 ± 11.5	26.1 ± 5.5	42.9 ± 1.9
4	6.6 ± 1.1	19.9 ± 11.7	37.1 ± 17.6	50.4 ± 3.1
6	32.7 ± 13.2	73.4 ± 30.0	87.5 ± 24.1	144.9 ± 24.5

Table 4 Calculated ratios of zinc-65 (⁶⁵Zn) or cadmium-109 (¹⁰⁹Cd) radioactivity in glumes/grains hours after their application to the cut stem, flag leaf blade and root-bathing culture solution in rice (Oryza sativa L.)

	Ratio for ^65Zn (present study)			^109Cd (Tanaka et al. 2007)
Site of ^65Zn or ^109Cd application		above	below		above
Culm cut above or below the flag-leaf node (the ratio corresponding to “a”)	2h	1.35 ± 0.29	0.78 ± 0.13*	1.3h	21.9 ± 5.1
	3h	1.18 ± 0.17	0.61 ± 0.13*	2h	9.9 ± 3.2
	4h	0.52 ± 0.17	0.72 ± 0.19	3h	2.7 ± 0.7
	6h	0.45 ± 0.01	0.58 ± 0.11	4.5h	2.1 ± 0.5
Flag leaf (the ratio corresponding to “b”)				6h	0.037 ± 0.010
				12h	0.028 ± 0.012
	24h	0		24h	0.022 ± 0.006
	48h	0		48h	0.019 ± 0.006
Roots (the ratio corresponding to “c”)	24h	0.059 ± 0.022		24h	0.46 ± 0.27
	48h	0.036 ± 0.009		48h	0.16 ± 0.03
	72h	0.028 ± 0.006		72h	0.22 ± 0.08

Data are means ± standard error, n = 3. The marks (*) at 2 and 3 h indicate that the ratios of radioactivity in glumes/grains caused by the treatment with ⁶⁵Zn below the flag-leaf node were significantly lower (P < 0.05) than those of radioactivity caused by the treatment with ⁶⁵Zn above the flag-leaf node as determined by Student’s t-test.

Table 5 Parameters for zinc (Zn) and cadmium (Cd) transports into the grains and glumes of newly absorbed Zn and Cd and their percent distribution (shown in parentheses) based on the models involving xylem transport to the grains of rice (Oryza sativa L.)

Parameters	Zn (%)		Cd (%)
To grains
via phloem (t)	0.93	(90)	0.93	(78)
via xylem (1 – t)	0.07	(7)	0.07	(6)
To glumes
via phloem (bt)	0	(0)	0.02	(2)
via xylem (a(1 – t))	0.03	(3)	0.17	(14)
Whole seed	1.03	(100)	1.19	(100)

Parameter “a”, ratio of ⁶⁵Zn imported into the glume to that imported into the grain via the xylem; “b”, ratio of ⁶⁵Zn imported into the glume via the phloem to that imported into the grain; “t”, proportion of ⁶⁵Zn deposited in the grains via the phloem to the total ⁶⁵Zn in the grains. The choice of the representative parameters of “a” and “b” is described in the Discussion section.

Tanaka K, Fujimaki S, Fujiwara T, Yoneyama T, Hayashi H 2007: Quantitative estimation of the contribution of the phloem in cadmium transport to grains in rice plants (Oryza sativa L.). Soil Sci. Plant Nutr., 53, 72–77.

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