Silicon-accumulating plants in the plant kingdom

Si-accumulator in plant kingdom

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Chapter 5

Silicon-accumulating plants in the plant kingdom 5.1. CRITERIA FOR DISCRIMINATING SIACCUMULATING PLANTS FROM NON-ACCUMULATING PLANTS The mineral composition of plants varies with the plant species and the growth environment. Striegel (1912) compared the mineral compositions of various plant species growing on the same soil and found the largest variation in the contents of Si and Ca. In the monocot species (all gramineous plants) tested, the Si content was high and the Ca content low. In dicot species (including various families), however, the Ca content was high and Si content low. There was a clear difference in a ratio of Ca to Si between monocots and dicots. In an attempt to characterize Si-accumulating plant species in the plant kingdom, Takahashi et al. (1976-1981) made an extensive survey on the mineral composition of nearly 500 plant species ranging from Bryophyta to Angiospermae, Table 5.1 shows the water soluble Si contents in the soils from which samples were collected. Si-accumulating plants can be discriminated from non-accumulating plants, using two criteria (Si content and Si/Ca ratio, Table 5.2). The Si content of 0.5% is used as the critical value. This value is based on the following supposition. If the Si concentration in the soil solution (1-12 ppm) is 10 ppm Table 5.1 Content of water soluble Si in soils where plant samples were collected Sampling sites Water soluble SiO., (mg/lOOg dry soil) Nippon Shinyaku Botanical Gardens* 3.9 Kyoto Prefectural Botanical Gardens' 2.1 Okayama University Experimental Farm* 2.7 Hiroshima Pref. Agric. Experiment 2.2 Station* National Institute of Genetics Farm** 7.9 * alluvial soil ** volcanic ash soil

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Table 5.2 Criteria for Si-accumulating plants Type Si-accumulator Si content (%) >1.0 Si/Ca ratio >1.0 Degree of Si accumulation

+

^____ Intermediate 1-0.5 1-0.5

Si-excluder <0.5 <0.5

±

and water requirement (300-700 ml per 1 g dry matter production) is 500 ml, the content of Si taken up by passive absorption on a dry weight basis will be 0.5%. Since the plants that accumulate Si tend to have a low Ca concentration, the Si/Ca ratio is used as the second criterion. Plants with a Si content and Si/Ca ratio higher than 1.0% and 1.0, respectively, are defined as Si accumulators (Table 5.2). By contrast, plants with a Si content lower than 0.5% and Si/Ca ratio lower than 0.5, are defined as Si excluders (non-accumulating plants). Plants with a Si content and Si/Ca ratio in between Si accumulators and excluders are intermediate type plants. For example, if a plant has a Si content higher than 1.0%, but the Si/Ca ratio is below 1.0, this plant belongs to the intermediate type. 5.2. CHARCTERISTICS OF SILICON ACCUMULATORS AND THEIR DISTRIBUTION IN PLANT KINDOM From an analysis of the mineral composition, 175 species collected from Nippon Shinyaku Botanical Gardens were classified into three groups based on their Si content (Table 5.3). In group A (shown by + in Table 5.3), the Si content was more than 1.5%. This group includes monocots I, Pteridophyte I, and Bryophyte. In group B (shown by - in Table 5.3), the Si content was lower than 0.25%. This group includes monocots II, dicots II, Gymnosperm, and Pteridophyte II. In group C (shown as ± in Table 5.3), the Si content and Si/Ca ratio was 0.86% and 0.54, and dicots I were included (Table 5.3). These results suggest that the plants in group A take up Si actively (active uptake type), but the plants in group B reject the uptake of Si (rejective uptake type). Plants in group C may take up Si passively (passive uptake type). According to the criteria shown in Table 5.2, some monocots, some Pteridophytes and Bryophytes tested are Si accumulators (Table 5.3). Among the monocots, most Si accumulators belong to Cyperacea and Gramineae (for details, see Appendix-3A).

Si-accumulator in plant kingdom

65

Table 5.3 Mineral composition of 175 plant species collected from Nippon Shinyaku Botanical Gardens Classification of Number Si/Ca B Si Mg K Ca plant species of (ppm) (%) (%) (%) (%) species Angiospermae Monocots Dicots Gymnospermae Pteridophyta Bryophyta Total

I II I II I II

147 22 40 8 77 12 10 4 2 175

0.50 1.88 0.20 0.86 0.23 0.13 1.88 0.20 3.46 0.58

1.66 0.65 1.78 1.76 1.87 1.20 1.13 0.80 1.06 1.57

0.24 0.14 0.25 0.17 0.27 0.11 0.26 0.27 0.26 0.23

2.70 2.39 3.18 2.34 2.57 1.15 2.17 1.64 0.93 2.52

19.2 3.0 13.7 9.5 27.7 26.7 8.5 28.9 7.6 18.9

0.61 3.12 0.14 0.54 0.15 0.17 3.68 0.23 3.31 0.76

+ ± + +

Table 5.4 shows the contents of Si and Ca in 45 species of Pteridophyta collected from Kyoto Prefectural Botanical Gardens. All species in Lycopsida and Equisetopsida were Si accumulators, while both Si accumulators and non-accumulators were included in Filicales, the largest order of Filicopsida (Table 5.4, for details, refer to Appendix-3B). There were no families that had both Si accumulators and non-accumulators (see Appendix-3B). The analysis of 10 plant species in Cyperaceae collected from Nippon Shinyaku Botanical Gardens revealed that Cyperus and Carex are Si accumulators while Scirpus is a Si non-accumulator (Table 5.5, for details see Appendix-3A). Table 5.6 shows the contents of Si and Ca in 211 gramineous species collected from Kyoto Prefectural Botanical Gardens and National Institute of Genetics Farm. All Gramineae species are Si accumulators, but the degree of Si accumulation differs among subfamilies. The degree of Si accumulation was in the order of Bambusoideae>Pooideae>Panicoideae> Eragrostoideae (Table 5.6, for details, see Appendix-3C, D). Commelinaceae and Juncaceae, which are close to Gramineae and Cyperaceae were also analyzed for their Si and Ca contents. These samples were collected from Kyoto Prefectural Botanical Gardens and Hiroshima Prefectural Agricultural Experiment Stations. The results showed that Commelinaceae had a relatively high Ca content, but some species had a higher Si content than Ca content (Table 5.7, for details, refer to Appendix-3E, F), suggesting that Si accumulators also exist in addition to Si excluders and intermediate t)rpe in this family. Species o^ Juncaceae contain small amounts of Si and Ca

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Table 5.4 Contents of Si and Ca in 45 species of Pteridophyta collected from Kyoto Prefectural Botanical Gardens Si/Ca Ca% No. of Si% Species Si accumulator 2.68 L38 3.01 23 8.42 Lycopsida 0.55 4.60 2 Equisetopsida 2.79 2.18 5.81 2 Filicopsida Marattiales 1.35 Marattiaceae 1.66 1.23 1 Filicales 1.04 4.17 Osmundaceae 2 4.01 2.12 1.11 Blechnaceae 1 2.35 0.84 3.05 Pteridaceae 4 2.67 1.70 1.98 Thelypteridaceae 4 3.15 1.20 1.52 Athyriaceae 1.86 7 Si non-accumulator Filicopsida Filicales Dryopteridaceae Davalliaceae Polypodiaceae Total

22

0.26

1.26

0.21

17 2 3 45

0.27 0.37 0.12 1.66

1.42 1.24 0.80 1.33

0.18 0.44 0.15 1.25

and some of them are ranked as excluder but on average Juncaceae intermediate tjrpe.

is

Table 5.5 Contents of Si and Ca in Cyperaseae coWecteA from Nippon Shinyaku Botanical Gardens Si/Ca Ca% Si% No. of Species 3.44 0.60 Cyperaseae 1.62 10 Si accumulator 3.37 0.64 Cyperus 3 2.06 4.85 0.60 Carex 5 1.91 Si excluder 0.41 0.52 Scirpus 2 0.21

Si-accumulator in plant kingdom Table 5.6 Contents of Si and Ca in Gramineae No. of species S i % Bambusoideae Oryzeae* 73 7.36 Others** 3.91 78 Pooideae*"^' 2.69 22 Panicoideae*'^ 30 2.73 Eragrostoideae * * 8 1.73 Total 211 4.73 * sampled from National Institute of Genetics ** sampled from Kyoto Prefectural Botanical Gardens

67

Ca%

Si/Ca

0.44 0.69 0.63 0.90 0.84 0.63

17.9 5.8 4.6 3.4 2.5 9.4

There are no species belonging to Si accumulators in dicots collected (Table 5.3), but the Si content has been reported to be high in Urticaceae. In addition, it was observed that cucumber cultured hydroponically with high Si concentration gave a high Si content. To characterize the degree of Si accumulation in these families, samples of Urticaceae and Cucurbitaceae were collected from Kyoto Prefectural Botanical Gardens and Experimental Farm of Okayama University. These plant species contained over 1% Si, especially species Cucurbitaceae showed a higher Si content (Table 5.8). As these species had a much higher Ca content than Si content, they belong to the intermediate type according to the criteria described above (see Appendix-3G, H for details). Table 5.7 Contents of Si and Ca in Commelinaceae* and Juncaceae " Ca% No. of Si% Species

Si/Ca

0.56 2.32 Commelinaceae 28 1.23 1.52 Si accumulator 4 1.83 2.90 0.50 Intermediate 2.61 17 1.24 0.15 Si excluder 1.90 7 0.25 1.12 Juncaceae 12*" 0.29 0.33 Collected from Kyoto Prefectural Botanical Gardens Collected from Hiroshima Prefectural Agricultural Experiment Station Number of cultivars ofJuncus

Chapter

68

MonocotyJedoneae

Dicotyledoneae

Gramineae Cyperacea

'Cucurbitale Commelinaceae Urticales Angiospermae

Equisetopsida

Filicopsida

Gymnospermae

Lycopsida

Reridophyta

Bryophyta Si-accumulator f

j

Chlorophyta

Si-excluder

^?^i Intermediate

Figure 5.1. Distribution of Si-accumulators in phylogenetic tree.

5

Si-accumulator in plant kingdom Table 5.8 Ccontents of Si and Ca in Cucurbitaceae"^ and Urticaceae*"^ No. of species Si % Ca % Cucurbitaceae 8 2.09 4.40 Urticaceae 5 1.03 4.64 * collected from Okayama University Experimental Farm ** collected from Kyoto Prefectural Botanical Gardens

69

Si/Ca 0.47 0.32

Figure 5.1 shows the distribution of Si accumulators in the phylogenetic tree constructed based on these data. Silicon is highly accumulated in Bryophyta, and Lycopsida and Equisetopsida of Pteridophyta, but decreased from Filicopsida in Pteridophyta to Gymnospermae and Angiospermae. However, high Si accumulation is seen again in Cyperaceae and Gramineae in monocots (Figure 5.1). It is well known that some plant species accumulate a large amount of Na, Al, Mn, Se, etc. However, only the distribution of Si accumulators is fitted well to the phylogenetic tree. Accumulation of Na, Al, Mn, and Se is related to soil factors such as salinity, acidity, reduction degree, and parent material, and their accumulation is the result of adaptation in these special soil environments. By contrast. Si is always abundant in soil. Therefore, Si accumulation depends on whether the plant takes up Si or not. From this point of view. Si accumulation is an advantageous trait for plants, and this trait is considered to be preserved. 5.3 VARIETY DIFFERENCE IN SILICON CONTENT SI-ACCUMULATING AND INTERMEDIATETYPE SPECIES

IN

THE

There are wide variations in Si content with the species as described above. Takahashi et al. (1981b) further investigated the variation in Si content among different varieties in the same species. The mineral content in the leaves of 38 Oryza perennis varieties growing in the same soil (Experimental Farm of the Table 5.9 Mineral content of the leaves of 38 varieties from Oryza perennis Average Range Si (%) 7.67 10.60 - 5.38 K(%) 2.25 3.19-1.36 Ca (%) 0.39 0.57 - 0.26 Mg(%) 0.12 0.20-0.08 P(%) 0.16 0.34-0.06 S(%) 0.14 0.32-0.07

Chapter 5

70

National Institute of Genetics) were analyzed. The Si content in leaves varied from 5.38 to 10.60% and was 7.67% on average (for details see Appendix-3C). The variation in Si content was rather small compared with that of K, Ca, Mg, P, S (Table 5.9). The relationship between Si content and the content of other elements except P was not clear. In the case of P, a weak negative correlation was observed. Recently, Ma et al. (2002c) analyzed the Si content of barley grain of about 400 varieties. The hull of gramineous grain usually contains a large amount of Si as does the leaves, and can also be used for the analysis of Si accumulation. The grains of two varietal groups were used; 274 standard varieties (SV)

o JO

500

1000

1500

2000

2500

3000

3500

4000

4500

4000

4500

Si content of bariey grain (mg/kg) 60 BCCUS

2 50 i

40

'S 30 u

I 20 3

10 -1

500

1

1000

i_

1500

2000

2500

3000

3500

Si content of barley grain (mg/kg) Figure 5.2 Frequency distribution of Si content of barley grains from Standard Variety (SV) and Barley Core Collection of United State (BCCUS).

Si-accumulator in plant kingdom

71

Table 5.10 Difference between Si contents of covered and hull-less barley grain and of two-row and six-row barley grains Si content (mg/kg) SV BCCUS Covered barley 2640.9±468.0 2439.8±439.5 Hull-less barley 116.0±44.8 26.8±23.4 Two-row Six-row

2219.3±624.8 1902.9±1013.2

2027.8±868.0 2287.2±1078.6

selected at the Barley Germplasm Center of the Research Institute for Bioresources, Okayama University, and 135 varieties from the Barley Core Collection of United State. These barley gains were collected from the plants growing on the same soils. The Si content of barley grain showed a large difference, ranging from 0 to 0.36% in SV and from 0 to 0.34% in BCCUS (Figure 5.2, for details, see Appendix-4A, B). The Si content was much lower in hull-less barley than in covered barley (Table 5.10). This is because most Si was localized in the hull (Table 5.11). The Si content of the hull was between 1.53 and 2.71% in the varieties tested. The Si content of two-row barley was similar to that of six-row barley (Table 5.10), suggesting that the Si content is not affected by spike row. The Si content of barley grain also did not differ with the origin of barley. Further analysis of 210 cultivars from East Asia also showed similar trends as SV and BCCUS (see appendix 4-C). The results of the studies on rice and barley indicate that Si content also varies with the variety in the same species. However, the mechanisms responsible for the variations remain to be examined in the future. The variation might result from different capacity of uptake by the roots, and/or accumulation. Table 5.11 Localization of Si in barley grainI SVNo. Si content (%) Total Hull 39 0.25±0.03 1.99±0.23 55 0.22±0.02 2.49±0.23 110 0.31±0.01 2.71±0.45 137 0.21±0.01 1.92±0.13 211 0.23±0.01 1.99±0.20 213 0.31±0.00 2.21±0.04 223 0.18±0.00 1.53±0.07

Hulled grain 0.10±0.01 0.05±0.00 0.06±0.00 0.05±0.00 0.03±0.00 0.04±0.00 0.03 ±0.00

Percentage of Si in the hull 65.9±2.7 78.5±0.9 80.6±1.6 77.6±1.9 88.1±2.0 89.5±2.2 86.4 ±0.6