Role of silicon in salt tolerance of wheat (Triticum aestivum L.)

Plant Science, 85 (1992) 43-50

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Elsevier Scientific Publishers Ii'eland Ltd.

Role of silicon in salt tolerance of wheat (Triticum aestivum L.) R a f i q A h m a d , S y e d H a s a n Z a h e e r a n d S h o a i b Ismail Department of Botany, University of Karachi, Karachi-75270 (Pakistan) (Received October 30th, 1991; revision received May 29th, 1992; accepted May 29th, 1992) The study was undertaken to determine the possible role of silicon (Si(OH)4) in salt tolerance of wheat (Triticum aestivum) at germination and vegetative/reproductive growth stages. Germination percentage and growth decreased with increasing NaC1 concentration in the absence of silicon. However, addition of silicon caused significant recovery from salt stress. Dry weight of the shoot increased significantly after silicon addition at 0.6% salinity, whereas, dry weight of the root was unaffected. Chlorophyll content remained unaffected by the addition of silicon at both salinities. Silicon significantly reduced the Na + content in flag leaves and roots, under saline conditions. Concentrations of silicon in roots increased with increasing salinity and silicon levels.

Key words: chlorophyll; dry matter; reproductive growth; salinity; silicon; sodium chloride; Triticum aestivum; vegetative growth

Introduction

Accumulation of salts associated with organic granular structures in the cytoplasm of some halophytes is considered as one of the adaptive mechanisms of salt tolerance [1]. Analysis of these salt-impregnated organic structures isolated from halophytic plants shows the presence of 3% SiO2 in addition to other salts, which raises the question, whether silicon plays any role in building salt tolerance? Silicon is considered as an essential element for members of Poaceae [2,3] and Cyperaceae families, but not for dicotyledonous plants [3]. Its role in providing rigidity, plant protection against pathogens and distribution of cellular trace elements is well documented [4,5]. Selective accumulation of silicon by two halophytic species Halocnemum strobilaceum and Juncus rigidus containing 5.4% and 2.1% silicon, respectively [6] grown in the Qattara depression of Egypt, further stresses its role in salt tolerance of plants. Experiments were carried out to determine the possible role of silicon in salt tolerance of wheat plants grown at different salinities. .Correspondence to: Rafiq Ahmad, Department of Botany, University of Karachi, Karachi-75270, Pakistan.

Materials and Methods

L Germination studies Seeds of wheat (Triticum aestivum L. var. Pak81) were surface sterilized with 0.1% HgCI and placed on thin sponge sheets in plastic pots containing Hoagland's solution. In another set, Hoagland's solution was supplemented with 20 ppm of silicon with or without 0.3 and 0.6% NaCI. Silicon was used in the form of water glass (sodium silicate). There were three replicates of each treatment. The pots were kept in a completely randomised design in a controlled environmental chamber maintained at 10 h photoperiod with 10 000 lux liRht intensity and 60% RH at 24 and 20 ± I°C day and night temperatures, respectively. Germination of seeds was recorded at 24-h intervals for 1 week. Emergence of coleoptile was recorded from the fifth day onwards. II. Growth studies Seeds of wheat (T. aestivum L. var Pak-81) were sown in a plastic tray containing 2-4 mm diameter plastic beads moistened with half-strength Hoagland's solution. One-week-old seedlings of uniform size were transplanted in 15-1 plastic con-

0168-9452/92/$05.00 © 1992 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland

44 tainers containing 10 kg plastic beads with 4 replicate plants per container. The treatments included Hoagland's solution with or without 20 p p m silicon and 0.3 or 0.6% NaC1. Each treatment was replicated three times. The plants were constantly aerated and kept in a controlled environment chamber as described earlier. Plants were harvested at maturity (135 days, ripe grain stage) and divided into roots and shoots. The shoots were further separated into vegetative (stem and leaves) and reproductive (infloresence containing

mature grains) parts. The roots were washed with deionized water to remove adsorbed minerals prior to mineral analyses.

111. Estimation of cations and chlorophyll Cations were extracted by acid digestion of dried plant material [7] and estimated using a Jarrel Ash AA-782-A atomic absorption spectrophotometer. Chlorophyll was extracted and estimated according to the method of Maclachlan and Zalik [8].

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Fig, 1, Effectof NaCI salinity on the germination and leaf emergence of wheat with and without silicon amendment. (A) Control (half strength Hoagland's solution). (B) Control + 20 ppm Si. (C) Control + 0.3% NaCI. (D) Control + 0.3% NaCI + 20 ppm Si. (E) Control + 0.6% NaCI (F) Control + 0.6% NaCI + 20 ppm Si.

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Results Germination The addition of 20 ppm silicon reduced the effect of salinity on germination of wheat seeds and leaf emergence (Fig. 1). Pooled estimates of variance showed that variation in treatments and days caused significant differences, but their interactions were non-significant (Table I). Emergence of first leaf from the coleoptile showed a significant response on the 5th day (120 h), whereas, on the 8th day (192 h), no significant differences were observed in treatments with or without silicon (Fig. 1). Pooled ANOVA showed a significant difference (P < 0.001) between treatments, days and their interaction. Growth studies An increase in salinity caused a gradual reduction in tiller initiation and dry matter production of the shoot. The addition of 20 ppm silicon increased the number of tillers both under saline and non-saline conditions. There was a significant increase in shoot dry weight at 0 and 0.6% NaCI. Dry weight of roots did not show any significant difference in both salinity and silicon treatments (Fig. 2).

Table I. Pooled ANOVA for germination and emergence percentage of wheat plants grown in culture solution.

Source

Sum of squares

D.F.

Mean squares

F ratio

P

4.710 < 0.001 13.493 < 0.001 1.128 n.s.

Germination Treatments (T) Days (D) T x D Error Total

30O9.259 3448.148 1440.741 4600.000 12 498.148

5 2 l0 36 53

601.852 1724.074 144.074 127.778

25 32 16 14 89

5 3 15 48 71

5108.88 10892.59 1108.14 298.61

The mineral composition of roots and flag leaves (Table II) showed an increase in concentration of Na ÷ and Si ions in plants grown under non-silicon saline conditions as compared to control (non-silicon, non-saline). Their translocation towards the shoot was always higher than the amount retained in the roots. Plants grown under salt stress showed marked reduction in net Na ÷ uptake in response to silicon treatment. However, plants raised in control with 20 ppm Si medium did not show much difference in net sodium uptake when compared with control, though its upward translocation was significantly reduced. A significant promotion in Si content of the roots was observed in plants supplied with silicon under both saline and non-saline conditions. In the plants grown in silicon amended saline culture, translocation of Si was considerably reduced to that observed in plants grown in silicon free saline nutrient media. Pooled analysis of variance between uptake of Na ÷ and Si ions against treatments showed significant effect of silicon on the Na + uptake both in leaves and roots (Table III). Both salinity and silicon treatments had no significant effect on the chlorophyll fractions and their ratios. An increase of salinity in the rooting medium resulted in a significant decrease in ear production, number of seeds/ear and grain weight/plant (Fig. 3). It is interesting to note that addition of silicon in the presence of NaC1 reduced the inhibitory effects of salts, since ~ 20% increase in ear production and number of seeds/ear was observed. No significant differences were observed in weight of seeds in all the treatments. Addition of 20 ppm silicon increased the total yield of plants (grain weight/plant) by 37.31 and 22.86% respectively, at 0.3 and 0.6% NaC1 as compared to silicon-free salinity levels. Discussion

Emergence Treatments (T) Days (D) T x D Error Total

544.44 677.778 622.22 333.33 177.77

17.10 36.47 3.71

<0.001 <0.001 <0.001

Like other glycophytes, wheat plants have also shown significant toxic responses to salt stress at germination, vegetative and reproductive growth. Salt sensitivity of wheat plants is well documented [9,10]. Increasing concentrations of NaCI generally results in a decrease in germination percentage

46

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B

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30 20 10 0 A

B

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Effect of NaCl salinity on the vegetative growth of wheat at harvest with and without silicon amendment. A - F as Fig. I.

Table II.

Effect of Si amendment on the Na + and Si content in vegetative parts of wheat (Pak-Sl) grown in Hoagland's solution

(H.S.). Treatments

Control (half strength-H.S.) H.S. + 2 0 ppm Si H.S. + 0.3% NaCl H.S. + 0.3% NaCl + 20 ppm Si H.S. + 0.6% NaCl H.S. + 0.6% NaCl + 20 ppm Si,

Flag leaves

Roots

Na + (mmol 1-1)

Si (mmol 1-1)

Na + (mmol 1-1)

Si (mmol 1-I)

2.31 0.64 7,45 4.73 13.69 5.58

1.023 1.583 1.763 1.023 1.980 0,913

1,10 3,10 5.97 2.59 10.80 9.92

0.120 0.423 0.200 0,303 0.342 0.707

q+ + + ± +

0.23 0.33 0,43 1.18 0.10 0.49

a+ ± aaa-

0.012 0.031 0.037 0.160 0.260 0.390

+ ± ± ± + +

0.05 0,44 0.53 0.15 1,37 0.40

+ ± + ± + +

0.015 0.017 0.023 0.019 0.018 0.029

47 Table HI.

Pooled ANOVA for Na + vs. Si interaction in wheat plants grown in culture solution.

Plant part

Source

Sum of squares

D.F.

Mean squares

F ratio

P

Flag leaves

Treatments (T) Cations (C) T × C Error Total

177.284 170.345 140.971 12.342 500.943

5 1 5 24 35

35.457 170.345 28.194 0.514

68.948 331.250 54.826

P < 0.001 P < 0.001 P < 0.001

Roots

Treatments (T) Cations (C) T x C Error Total

129.721 246.202 115.113 0.417 491.453

5 1 5 24 35

25.944 246.202 28.023 0.017

1491.463 14 153.500 1323.505

P < 0.001 P < 0.001 P < 0.001

and delay in emergence of 1st leaf. Salinity affects the seed germination either, (a) osmotically and/or (b) by inducing toxicity due to presence of excessive Na + ions. Halophytic plants manage to overcome these toxic effects through some biochemical and physiological adaptations.

Haloxylon salicornicum (Moq.) Bunge ex Boiss and Juncus maritimus Lamk, are capable of germinating up to salinity of 500-600 mM NaCI (unpublished data) [11], while some moderately salt tolerant glycophytic crops (cotton and alfalfa) have been reported to germinate up to 200 mM

6

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0.6%

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0.0%

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Effect of NaCI salinity on the reproductive growth of wheat with (1~i) and without ([21) silicon amendment.

0.6%

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NaC1 [12,13]. In comparison, wheat is reported to be more sensitive at germination than barley plants [14]. The presence of coexisting ions often alleviates the specific ion effects. Small concentrations of Ca 2+ and Si ions are known to reduce Na ÷ toxicity [15,16]. In the present investigation, significant differences (P < 0.001) between treatments and days of germination and nonsignificant relationship between their interaction, indicate that the supplemented silicon facilitates better osmotic and ionic conditions with time. Delay in emergence of primary leaf due to salinity was significantly checked by the presence of silicon in saline nutrient media. This ameliorative effect of silicon may be due to its hydrophylic nature by protecting the plants from physiological drought. Inclusion of silicon under salinity partially alleviated the salinity induced reduction in vegetative growth by an increase in the number of tillers and dry biomass. It is known that silicon stimulates the growth and yield of a number of monocot and dicot plant species under normal growth conditions [3,17,18]. The mechanisms by which silicon affects osmotic imbalance and sodium toxicity under saline conditions are not yet known. Silicon pre-treatment is reported to increase the symplastic water volume of Loblolly Pine seedlings grown under water stress [19]. Promotion in dry biomass production can also be described as a function of increased water influx, which apart from preventing physiological drought may help to maintain a balanced and efficient absorption and translocation of mineral elements required for energy generation [20,21]. It may be worthwhile to mention here that marine diatoms exhibit specific demand of Na + ions for active silicon influx [22]. If the same mechanism is operative in terrestrial plants, it may help to explain the differences in salinity levels in response to silicon. The salt concentration of 0.3% seems to be insufficient to favour the active transport of silicon. These results are in agreement with that observed for Prosopis juliflora [16], where added silicon increased the whole plant dry weight of the plants receiving 260 mM NaC1, while at low salinities the response was negligible. With increase in salinity of the rooting medium, absorption of Na + significantly increased, but the

uptake and translocation of Na ÷ decreased in plants when silicon was added. Addition of silicon in NaCI treatments resulted in an enhanced uptake of silicon in roots. Differential absorption of silicon as compared to other cations in 175 plant species ranging from bryophytes to angiosperms have been reported [3]. There appears a possibility of an interaction between freely available sodium and silicon ions, forming some sort of complex thus reducing their transportation to the aerial parts of the plants. Low amounts of silicon in the flag leaves at 0.6% NaC1 with silicon, indicate the binding of soluble silicon with Na ÷ in root and retarding its upward translocation. Presence of low sodium concentration in these treatments also favours the possibility of this hypothesis. Similar results of low calcium levels in the straw of rice plants grown with silica in the presence of calcium salts have been reported as compared to the absence of calcium [231. Similar observations have been reported for earhead, leaf blades and stalks in wheat and rice [24]. A rapid and increased transportation of both the ions to the shoot can be related to an increase in mass flow transport of available free silicon across the roots. In contrast, restricted passive transport and increased transportation of ions have been reported in Lolium perenne, a perennial grass [25] and soybean. Most of the monocots are known to accumulate a significant portion of silicon in their shoot [3,26]. The higher concentration of silicon has been reported to be precipitated in older leaves, which is mostly associated with the cell wall and gives rigidity to the shoot [27]. Appearance of silicon or sodium in the control plants raised in silicon or sodium free culture could be attributed to their contamination in nutrient salts, water and air. Best purified water (Milli-Q-water) still contains about 0.5 ppb (2 x 10-8 M) silicon [28]. Higher plants transpire about 300-600 ml of water for production of 1 g dry matter. This means that even without any silicon amendment, 1 g of organic plant material will contain 0.36-0.72 /zg SiO2. Furthermore, silicon amendment made through sodium silicate may be a carrier of Na ÷ in very low concentration. Experimental evidences obtained so far are

49

insufficient to suggest any specific mechanism regarding Na+-Si interaction in higher plants. However, it appears that silicon present in traces is unable to form hydrophilic gel to check sodium as well as its own translocation. When concentration of silicon increases in the root, excessive sodium is bound in hydrophilic, salicious gel, hence both silicon and sodium are unable to be released in xylem for upward translocation. The remaining unbinded sodium and silicon ions still manage to remain free for upward translocation. There could be a definite ratio of sodium and silicon necessary for hydrophilic binding, whereas, remaining ions are available for upward transport. Hence silicon or sodium, if present in very low concentration in root, is translocated upward without interference. Increase in grain yield at both salinities after silicon amendment indicates an improvement in the translocation of minerals and metabolites necessary for seed setting. The role of silicon as a beneficial mineral nutrient for reproductive growth of plants is well documented [3,29]. A decrease in grain weight per plant and nonsignificant differences in weight of 100 seeds could suggest that grain filling is not affected. The reduction may also be due to adverse effects on gametogenesis and fertilization, or loss in viability of pollen grains as reported earlier [30]. Silicon is known to enhance pollen fertility of some crop plants [29,31] thereby overcoming the adverse effects of salinity on grain yield.

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Acknowledegments This research was supported by a grant from Pakistan Atomic Energy Commission. Thanks are due to Dr. A.R. Azmi, Director, Atomic Energy Agricultural Research Centre, Tandojam, for his keen interest and encouragement to carry out this research. References

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