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IV. Silica in Relation to Plant Growth

IV. Silica in Relation to Plant Growth

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130



L. H . P. JONES AND K. A. HANDRECK



that silicon does not yet deserve a place among the essential elements

for plants.

Although the above statement should also apply for rice there are

rather convincing reports (Ishibashi and Kawano, 1957; Iwata and Baba,

1962; Mitsui and Takatoh, 1963; Okamoto, 1957; Okuda and Takahashi,

196la,b,c, 1964; Yoshida, 1965; Yoshida et al., 1959) showing that when

this plant is grown in culture solutions “minus” silicon, the yields of dry

matter, particuIarly grain, are lower than those from solutions with added

silicon. Several explanations for this effect are discussed below under

separate headings along with other beneficial effects of silicon on plant

growth.



B. INTERACTIONS

WITH HEAVY

METALS

Barley shows a narrow range of manganese tolerance between deficiency and toxicity levels in the external solution, and Williams and

Vlamis (1957a) have shown that characteristic symptoms of toxicity

appear when this plant is grown in standard Hoagland culture solutions

containing 0.5 ppm. manganese. The symptomatic brown spots which

occur on the older leaves have higher concentrations of manganese than

the surrounding tissues (Williams and Vlamis, 195713). When silicon

(10 ppm.) was added to the culture solution the toxicity symptoms were

alleviated and yields were increased threefold. The main known effect

of the silicon was to distribute the manganese more evenly through the

leaves. Very recently, Vlamis and Williams (1967) extended their studies

to several Gramineae, namely barley, wheat, oats, rye, ryegrass, and rice,

and varied the concentration of manganese in the culture solutions over

the range 0 to 5 ppm. Toxicity symptoms appeared at the high manganese levels, and the addition of silicon again prevented the appearance of these symptoms. The silicon treatment produced increased yields

of barley, wheat, oats, and rye at all levels of manganese but did not

significantly affect the yields of rice and ryegrass. In all species the

concentration of manganese was lowered slightly, an effect which could

be explained for the most part through dilution in the increased dry

matter.

Okuda and Takahashi (1962a, 1964) have also studied the interaction

between silicon and manganese in rice and barley, but their culture

solutions contained manganese in concentrations ranging from 0.4 to

140 ppm. Silicon added at the rate of 100 ppm. SiO, alleviated the

manganese toxicity and increased yields, but any other conclusion is

impossible because toxicity did not appear in the “minus” silicon cultures

until the manganese reached concentrations reported as 60 ppm. for rice

and 20 ppm. for barley.



SILICA



131



It may be concluded that silicon sometimes has a beneficial effect

through alleviating manganese toxicity. The effect of the silicon is to

alter the distribution of manganese in the leaf tissues, thereby preventing

it from collecting into localized areas which become necrotic. However,

the nature of the interaction between divalent manganese and silica,

whether as silicic acid or solid silica, is still obscure and calls for further

investigation.

Interactions between silicon and iron have been investigated by

Okuda and Takahashi (1962a,b, lW),whose interest in this topic stems

from the problem of iron toxicity in rice. In one series of culture solutions, where 1 to 140 ppm. iron was added as ferrous sulfate, silicon

alleviated the toxicity and increased yields in a manner resembling its

effect on manganese toxicity. Unfortunately, it is not possible to state at

what concentration ferrous iron became toxic because the extent of

autoxidation is unknown.

In more definitive studies Okuda and Takahashi (1962b, 1964) have

shown that the silica content of the rice plant affects the uptake of iron.

Their approach was to grow rice in culture solutions with varying concentrations of silicon and then to use these plants for studies of iron

uptake over short periods of time from dilute solutions of ferrous iron

(not exceeding 2.5 ppm.). Two main effects were observed. First, as the

content of silica in the tops increased over the range 0.2 to 7.0 percent

SiO,, decreasing amounts of iron were absorbed. Second, the rate of

oxidation of ferrous iron in the external solution was greater with plants

which contained silica. The excised tops of such plants also produced a

slightly greater rate of oxidation of ferrous iron in the external solution,

but the oxidizing power of excised roots did not vary with the silica

content. This work indicates that iron uptake by rice is inversely related

to the oxidizing power of the root which is, in turn, increased by increasing the silica content of the tops. Evidently the silica improves the oxygen

supply to the root, and Ponnamperuma (1964) has suggested that it does

this by increasing the volume and rigidity of the gas channels in the

shoot and root.



C. INTERACMONS

WITH PHOSPHORUS

There has long been interest in possible interactions between silica

(or silicates) and phosphates in soils, and although this topic was

recently reviewed by Taylor (1961) it calls for further comment here.

Among the best-known experiments are those at Rothamsted Experimental Station (Russell, 1961, pp. 44-45) where the addition of soluble

silicates has increased the growth of cereals, particularly at low levels of

phosphate supply. These positive results are not easy to interpret. They



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L. H. P. JONES AND K. A. HANDRECK



cannot be due to anionic exchange between silicate in solution and

adsorbed phosphate, as has often been implied, since silicate ion cannot

exist in important amounts in neutral or acid soils, The results could be

due to an increase in alkalinity, which is well known to liberate phosphate from its union with iron and aluminum. If however, this were

found not to explain all the results, we have the following possible

theories for the effect of silicic acid in higher concentration than normal.

First, it could lower the activity of aluminum ion in solution and SO

prevent it from precipitating phosphate. While little is known of the

mechanism or speed of this reaction, it remains a possibility. Second, a

theory that has often been suggested is that silicic acid competes against

phosphate for a place on the surface of hydrated sesquioxides. While

one can imagine a long-term effect in which gibbsite is silicified into

kaolinite, thereby lowering its affinity for phosphate, one cannot imagine

a short-term competition between silicic acid and phosphate ion for

adsorption on a sesquioxide surface, for the simple reason that they are

attracted to different kinds of sites. Silicic acid, being an acid, is attracted

via a hydrogen bond to an oxygen atom bridging two metal atoms (see

Section 11, A ) , while phosphate, being a base, is attracted to the metal

atoms. It is not surprising that the early hopes that silica might be a

partial substitute for phosphate as a fertilizer have not received any

support.



D. RESISTANCE

TO FUNGI

AND INSECTS

Silica has frequently been implicated as a factor influencing the

degree of susceptibility of cereals to fungal attack. The resistance of rice

to several diseases, namely brown spot (Helminthosporium oryzae),

stem rot (Leptosphaeria salvinii Catt.), and blast disease (Piricularia

oryzae Cav.) is stated to increase where the silica content of the plant,

particularly the leaves, has been raised by applications of siliceous slags

to the soil (e.g., Izawa and Kume, 1961; Kuo et al., 1963; Ota et ul.,

1957; Yoshida et al., 1 9 6 2 ~ ) .The greater proportion of the work on

fungal attack is centered on the problem of blast disease and the most

definitive studies are those of Volk et al. (1958), who avoided the use of

such complex materials as siliceous slags. They grew rice in culture solutions with silicon added at levels ranging from 3 to 130 ppm. SiO, and

determined the susceptibility of individual leaves to Piricularia oryzuzae

after inoculation under controlled conditions. In a recently emerged leaf,

resistance was directly related to the silica content of the dry matter and,

in turn, to the level of silica in the culture solution. With increasing age

the resistance of the leaf increased and became virtually complete, irrespective of the level of silica in the culture solution. Presumably



SILICA



133



resistance depends on the silica content of the leaf up to a certain level;

once this level is reached resistance is maximal, and it cannot be further

increased by additional silica. This generalization appears to explain why

the young plant shows a gradient of increasing resistance from the upper

toward the lower leaves and also why this gradient disappears in older

plants in which resistance is maximal (Kahn and Libby, 1958).

Resistance of various other cereals to powdery mildew (Erysiphe

graminis D.C. ) has been found to increase following applications of

amorphous silica to the growth medium. This increased resistance was

directly related to the silica content of the leaves (Germar, 1934; GrosseBrauckmann, 1957, 1958; Wagner, 1944).

There is also some evidence of a relationship between the silica content of the plant and its resistance to certain insect pests. Thus, resistance

of wheat to Hessian fly [Mayetioh destructor (Say)] (Miller et al., 1960;

Refai et al., 1955) and of rice to stem borer [Chilo suppressalis (Wlk.)]

(Ota et al., 1957; Sasamoto, 1958) has been found to increase with

increasing concentration of silica in the plant.

The way in which silica in plants increases resistance to fungal and

insect attack has not been elucidated. It is known, however, that most

parasitic fungi, including the blast fungus, penetrate their hosts by boring

through the epidermal cell walls (Butler and Jones, 1949; Yoshii, 1934).

The solid silica which is associated with these walls may therefore constitute a mechanical barrier to penetration either by fungal hyphae or by

the mandibles of insect larvae. Indeed, Sasamoto (1958) has reported

that the mandibles of larvae of the rice stem borer are damaged when

the concentration of silica in rice is high. Silica may protect the plant in

another way. Its association with the cell wall constituents is likely to

make these less accessible to the enzymatic degradation which accompanies the penetration of cell walls by fungal hyphae.



E. MISCELLANEOUS

BENEFICIAL

EFFECTS



1. Leaf Disposition

It is often stated that the leaves of rice show a drooping habit when

their silica content is low and that they are more erect when the supply

of silica is high (e.g., Mitsui and Takatoh, 1963; Okamoto, 1957; Okuda

and Takahashi, 1961b; Yoshida et al., 1959). The effect of silica on this

and related aspects of pIant growth has been investigated in some detail

by Iwata and Baba (1962). Rice was grown in culture solutions under

conditions of competition so as to simulate a crop; the solutions contained

50 and 200 ppm. SO,. The yield of dry matter per plant, the leaf area

index, and the absorption of light were all greater at the higher level of



134



L. H. P. JONES AND K. A. HANDRECK



silica supply; also, the leaves were less curved and more vertically

disposed. Iwata and Baba (1962) found, in addition, that the ratio of

leaf photosynthesis rate to leaf respiration rate was greater in the high

silica treatment but the net (leaf) photosynthesis rate was not increased.

The primary effect of silica on dry matter yields in these experiments is

difficult to specify.



2. Tranyiration

Two groups (Okuda and Takahashi, 1964; Yoshida et al., 1959) have

suggested that silica in the rice plant has an effect on transpiration. The

more comprehensive experiments are those of Okuda and Takahashi

(1964), who grew rice in culture solutions containing additions of silica

ranging from 0 to 100 ppm. SiO, and measured the transpiration rate at

intervals during a 2-month growth period, With increasing silica supply

there were consistent decreases in the transpiration rate. The magnitude

of this effect is shown in one instance where the transpiration rate was

decreased from 5.1 to 3.6 ml./g. fresh weight/24 hours for culture solutions containing 0 to 100 ppm. SO2,respectively. The way in which silica

might affect transpiration is obscure, but the suggestion (Yoshida, 1965;

Yoshida et al., 1962d) that the silica in epidermal cell walls enhances the

efficiency of the cuticle deserves further investigation.

3. Seed Retention

Loss of seed from the ripening inflorescence (seed shattering) is a

common characteristic of grasses and complicates the harvesting of seed

in some species. The canary grasses (Phalaris species) have poor seed

retention and the factors determining retention within five strains of

P h ~ l a ~tuberusu

is

L. have been examined by McWilliam (1963). A high

silica content of the glumes was found to be one of several characteristics

associated with high retention. It seems likely that solid silica would

contribute to the stiffening of the glumes, a process which McWilliam

(1963) considers of some importance in improving retention.



4. Lodging in Cereals

Lodging of cereals is a source of considerable loss in harvesting and

therefore constitutes a subject of continuing interest and investigation.

It is generally accepted that high levels of nitrogen and unlimiting soil

water give rise to rapid growth and the formation of long, weak, lower

internodes which are liable to bend. However, there has been considerable controversy about the specific plant characters which determine

resistance of the culms to bending. The earliest workers (e.g., Liebig,



135



SILICA



1840) considered that lack of silica was responsible for poor culm

strength. This possibility has been discounted because variation in culm

strength can be largely explained by variations in certain anatomical

features. In comprehensive investigations Mulder ( 1954) has shown that

the long, lower internodes giving rise to lodging sometimes have a small

diameter and thin culm walls. More generally, the area of lignified tissue

in the sclerenchyma zone, the thickness of lignified tissues and, particularly, the thickness of sclerenchyma cell walls are reduced under conditions favoring lodging, particularly high nitrogen supply.

The effect of nitrogen on the degree of lignification resembles its

effect on the concentration of silica in cereals (see Section 11, D, 1 ) .

Since there is evidence that the cell walls of sclerenchyma in the culm

are thickened with silica, as well as with lignin (Jones et aZ., 1%3), one

cannot rule out the possibility that variations in the degree of thickening

by silica would also contribute to variations in culm strength. This possibility seems to deserve further investigation, especially in view of the

results of some recent studies by F. van der Paauw (private communication). He observed that when oats and rye were grown on two soils, the

stems of plants on one soil bent at an earlier stage and lodging occurred

more frequently than on the other soil. A complete chemical analysis of

the growing stems showed a marked contrast between their content of

silica; those stems which lodged contained 0.19 percent SiO, wh,oreas

those from plants on the other soil contained 0.71 percent SiO,. Similar

effects have been noted by Jones and Handreck (unpublished) with oats

grown in two soils with 7 and 67 ppm. SiO, in solution. The internodes

from mature plants on these soils contained 0.03 and 1.12 percent SiO,,

respectively. The leaves of plants at the lower supply of silica were rather

soft, and their stems bent at an earlier stage than those of plants with the

higher supply.

V.



Silica in the Ruminant Animal



A.



QUANTITIES



INGESTED



The grazing ruminant inevitably ingests silica as a constituent of

pasture plants. The daily intake will vary with the animal and the silica

content of the plants which comprise the pasture. Some indication of the

way in which the pasture affects the daily intake of silica is seen in data

on the output of silica by sheep when grazing pastures of barley grass

(Hordeurn hystrix Roth.) (Nottle and Armstrong, 1966). At three different stages of growth these pastures contained 1.70, 2.81, and 3.65 percent

SO,, and the corresponding daily amounts of silica excreted (feces and

urine) were 6.2, 14.7, and 20.6g. SiOz. If it is assumed that a sheep



136



L. H. P. JONES AND K. A. HANDRECK



ingests 1 kg. dry matter per day we can conclude that an intake of 4Og.

SiO, represents an approximate upper limit because pasture grasses seem

to contain a maximum of about 4 percent SiO,. At the other extreme one

might assume the lower limit in feeds consisting of legumes to be 0.2

percent SiO, (Baker et al., 1961b), when a sheep’s daily intake would

be about 2g. SiO,.

An overwhelming proportion of the ingested silica would be in the

solid form, but when the plant is in the early stages of growth a small

proportion is in solution as monosilicic acid. Although there are no data

for the proportion of silica which is ingested in this form one can calculate this for a sheep on the assumptions that there are 4 kg. water associated with each 1 kg. dry matter and that this water contains 120 ppm.

SiO, in solution. If we use these figures the daily intake of monosilicic

acid will be 480 mg. SiO,; this would represent only 2.4 percent of the

total intake where the feed contained 2 percent SiO, on a dry matter

basis. The proportion of monosilicic acid decreases with increasing age

of the plant and approaches zero in mature plants; it would also approach

zero in dried plants whatever their stage of growth at harvesting.

B. FATEAND EFFECTSOF SOLD SILICA

The ingested solid silica is of interest because it may be useful as an

indigestible reference material for studying the fate of the digestible

constituents of plant feeds and also because of its physical effects on the

animal.



1. Excretion in the Feces

The solid silica of dry plant feeds has been followed through the

sheep, and it has been established (Jones and Handreck, 1965a) that the

ingested silica can be completely recovered in the feces and urine. All but

a small proportion of the total silica excreted was in the feces (Table

VII). Since the recovery in the feces was constant and practically complete, it appears that silica has all the qualities needed for an internal

reference for determining the fate of digestible constituents of feeds.

Pujszo et al. (1959) have come to a similar conclusion.

Despite this conclusion some other workers (Druce and Wilcox, 1949;

Gallup and Kuhlman, 1931, 1936; Gallup et al., 1945; Knott et al., 1936;

Wildt, 1877), who have considered silica as a reference material, have

reported that its recovery in the feces is too variable. This variability was

beyond the a 2 percent which might be explained as due to contamination with siliceous dusts or to the urinary excretion of silica. Some of the

variability beyond this range can be explained by gross contamination,

but much of it was probably due to the fact that the method of silica



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IV. Silica in Relation to Plant Growth

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