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Radiation-induced degradation of sodium alginate and its plant growth promotion effect
⁎Address: National Center for Radiation Research and Technology (NCRRT), P.O. Box 29, Nasr City, Cairo, Egypt. Tel.: +20 10063110314. hatem_lotfy@yahoo.com (H.L. Abd El-Mohdy)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Alginate was irradiated as a solid with 60Co gamma rays in the dose range of 20–100 kGy to investigate the effect of radiation on alginates. One of the principle factors for reducing the cost is achieving the degradation at low irradiation doses which occurs with addition of chemical initiator to NaAlg during irradiation process that leads to a synergistic effect, which remarkably increases the degradation efficiency of alginate. The factors affecting the degradation process such as irradiation dose and potassium per-sulfate (KPS) addition were studied. The average molecular weight of the irradiated alginate was investigated in detail by using several complementary techniques such as chromatography and viscometry. The lowest molecular weight of alginate resulted at 100 kGy and added KPS, whereas the highest one at 20 kGy in absence of KPS. Characterization of the oligoalginates obtained by radiation degradation was performed by FT-IR and UV–vis spectroscopy, XRD and TGA. The effect of water-soluble radiation-induced alginate fractions on the growth promotion of Faba bean plant was studied. The highest plant growth and seed yield compared with control occurred for plants sprayed with low molecular weight NaAlg fractions (treated with 100 kGy and added KPS).
Keywords
Irradiation
Degradation
Sodium alginate
Growth promotion
Faba bean plant
1 Introduction
Alginate is a natural polysaccharide with a large available quantity in nature. It has widespread applications in the food and drink, pharmaceutical and bioengineering industries (Gacesa, 1988). Alginate is the sodium salt of alginic acid, the structural component of the algal tissue of Phaeophyceae, a class of brown seaweeds (Craigie et al.,1984; Avella et al., 2000). It is a copolymer composed of b-d-mannuronate (M) and a-l-guluronate (G) residues organized into blocks of homo-polymeric segments of MM or GG and alternating sequences of M and G. While the M-block segments develop in linear and flexible structures, the G-block residues give rise to fold and rigid structures and are responsible for a pronounced stiffness in the molecular chains (Fig. 1). Modification by crosslinking, grafting and degradation of alginates is expected to widen their application. The alginate oligomer fractions with the degree of polymerization from tetramer to hexamer have been reported to show a special growth-promotion effect for plant (Iwasaki and Mastsubara, 2000). Degradation of alginates can easily be carried out by chemical, enzymatic hydrolysis or by using radiation.
NaAlg chain sequence.
Radiation is a very convenient tool for the modification of polymer materials through degradation, grafting and crosslinking. Recently, radiation effects on polysaccharides such as chitosan, sodium alginate, carrageenan, cellulose, pectin have been investigated to enhance their use for recycling these bioresources and reducing the environmental pollution. These polysaccharides were degraded by radiation and the biological activities such as anti-microbial activity, promotion of plant growth, suppression of heavy metal stress, phytoalexins induction, etc. were induced. Polysaccharides and their derivatives exposed to ionizing radiation had been long recognized as degradable type of polymers (Huang et al., 2007). Jaroslaw et al. applied three radiation degradation methods: ultrasonic, ultraviolet and gamma irradiation to sodium alginate and chitosan in aqueous solutions, the changes in molecular weight were monitored by GPC measurements (Jaroslaw et al., 2005). It has been found that from the energetic point of view the most effective method for polymers is gamma radiation method. The significant advantages of gamma radiation degradation are the final biological sterilization of irradiated materials that can be easily used for manufacturing biomedical products (Rosiak et al., 1995; Clough, 2001) and the ability to promote changes reproducibly and quantitatively, without the introduction of chemical reagents, and without special needs to control for temperature, environment and additives (Charlesby, 1977). Degradation by radiation processing of polysaccharides has gained much attention due to its technological effectiveness in producing low molecular weight oligomers (Luan et al., 2003) which have concrete application not only in the biomedical field but also in agriculture, as a plant growth promoter.
Applying ionizing radiation to degrade natural bioactive agents and then using them as growth promoting substances is a novel emerging technology to exploit the full genetic potential of crops in terms of growth, yield, and quality. Polysaccharides, such as sodium alginate, have been used as wonderful growth promoting substances in their depolymerized form regarding various plants (Nagasawa et al., 2003). The sodium alginate irradiated by gamma rays, has growth promoting activities like other plant growth promoters and acts as bio-fertilizer (Mollah et al., 2009). Depolymerized sodium alginate showed various biological effects on plants including enhanced seed germination, shoot elongation, and root growth (Yonemoto et al., 1993; Natsume et al., 1994; Hu et al., 2004).
In the present work, degraded alginate is prepared by exposure of alginate as a dry powder to γ-rays from Co-60 radiation and added KPS initiator. The effects of oligomers obtained from radiation depolymerized alginates on the growth-promotion of some Faba bean plants are examined.
2 Experimental
2.1 Materials
Sodium alginate, of high molecular weight was supplied from Nice lab, India. Potassium per-sulfate (98%) was purchased by British Drug House (BDH) Limited, England. Faba bean seeds were supplied from Advanced Group, Export & Trading, for the production of seeds and agricultural crops, Egypt.
2.2 Preparation of degraded alginate
Degraded NaAlg was prepared by exposing it in the powder form to γ-irradiation 60Co source at different doses ranged from 20 to 100 kGy in the presence of KPS as chemical initiator at dose rate, 6.6 kGy/h.
2.3 Determination the molecular weight of degraded alginate
The weight-average molecular weights of the degradable alginates were determined by two methods:
2.3.1 Viscosity measurement
The weight average molecular weight of polymers was calculated with determination of viscosity of polymeric materials on the basis of the Mark–Houwink equation (Ghosh et al., 2009):
With known values for α and K it is easy to take the values from an intrinsic viscosity experiment [η] and calculate average molecular weight (M). The molecular weight of sodium alginate can be determined by measuring the intrinsic viscosity of polymer in 0.01 M NaCl solution taking K = 8.1 × 10−3 ml/g and α = 0.92 (Lui et al., 2002).
2.3.2 Gel permeation chromatography (GPC) method
Gel permeation chromatography of un-irradiated and irradiated samples was performed on an 1100 Agilant instrument equipped with organic and aqueous GPC-SEC-SEC start up kits with a flow rate of 2 ml/min, maximum pressure 150 bar, minimum pressure 5 bar, injection volume 50 L and column thermostat temperature at 25 °C. The eluent was monitored by a refractive index detector of optical unit temperature 25 °C and peak width 0.1 min. polymer concentration was 0.1 wt %.
The molecular weights were determined from a calibration curve using polystyrene and polyethylene oxide standards for organic and aqueous systems, respectively. The molecular weight can be calculated from the following equation:
2.4 UV - vis spectroscopy
UV–vis spectroscopy of irradiated alginate solution was carried out at 25 °C using a Shimadzu spectrophotometer UV-265FW in the range of 200–400 nm. The polymer concentration of aqueous solution used for the spectroscopy was 0.025% (w/v).
2.5 FT-IR spectroscopy
Analysis by infrared spectroscopy was carried out by using Mattson 1000, Unicam, Cambridge, England in the range from 400–4000 cm−1.
2.6 Thermal gravimetric analysis (TGA)
Perkin Elmer TGA system under Nitrogen atmosphere 10 ml/min was used. The temperature range was from ambient temperature to 600 °C at a flow rate of ≈10 ml/min.
2.7 X-ray diffraction (XRD)
X-ray diffraction patterns were obtained with a XRD-DI series, Shimadzu apparatus using nickel-filter and Cu–K target.
2.8 Plantation
2.8.1 Seed germination and emergence
The growth promotion effect on the germination and seedling growth of Faba bean plant was studied. The aqueous irradiated solutions of oligosaccharides were applied in the following mode: the Faba bean seeds were soaked for 24 hs in 70 ppm solutions of native alginate and oligo-alginates prepared at different irradiation doses with KPS. The un-treated plants were used as control. The treated and control seeds were then sown in a tray containing soil. All experiments were repeated 5 times and every time experimental sets were run in four replicates. The trays were kept at room temperature and irrigated with water every alternate day. At plantation of seeds, the germination% was daily exhibited for various replicates. After the emergence of the first leaf, the seedling height was measured from transition zone to the tip of the shoot. The average number of emergent plants at different time intervals was calculated.
2.8.2 Plant growth promotion
To investigate the effect of degraded alginate on the growth promotion of Faba bean plants, the field is divided into different separate lines and four replicates for each treatment. The solutions of native and oligoalginates were sprayed on the plants after plantation at ages 25, 55 and 75 days with 70 ppm concentration. The growth parameters of the Faba bean plants treated with native and degraded NaAlg such as averages of plant height, leaf width, total dry weight of plant and seed yield were detected compared with those planted in control lines.
3 Results and discussion
3.1 Effect of γ-radiation and KPS initiator on alginate
Low molecular weight naturally occurring polysaccharides like alginates, prepared by conventional methods, have been reported to possess novel features such as promotion of seed germination and shoot elongation of plants. As compared to the conventional techniques, like acid or base hydrolysis or enzymatic methods (Araújo et al., 2004), radiation processing offers a clean one step method for the formation of low molecular weight polysaccharides in aqueous solutions even at high concentrations. Radiation can induce degradation of natural polymers like alginate and the degradation of solid NaAlg requires high irradiation doses. In fact, from the economic point of view, the cost of these doses is high. Therefore, trials have been made to reduce the cost of degradation process of solid alginates by adding chemical initiator to reduce the dose required for degradation and controlling the irradiation conditions. It was reported that the radiation-induced degradation yields of polysaccharides vary widely depending on the molecular weight of the polymer (Sen et al., 2010; Jong-il et al., 2009), so in this study the molecular weight of degraded fractions of alginate was determined. Viscometry and gel permeation chromatography are two essential tools to identify the molecular weight of alginates that exposed to different irradiation doses.
3.1.1 Viscometric determination for oligoalginate molecular weight
The conditions under which irradiation occurs can significantly influence the properties of the final materials. Alterations in the molecular structures of the polymers appear as changes in the chemical or physical properties. Alginate belongs structurally to polysaccharides for which is known that the irradiation conditions can significantly influence the properties of the final materials (Davenas et al., 2002; Bartolotta et al., 2005). Effect of ionizing radiation on the chain length of sodium alginate as well as its molecular weight was viscometrically monitored. The changes in the viscosity average molecular weights of native NaAlg, radiation induced degraded ones and that mixed with 10% KPS initiator were shown in Fig. 2. As the irradiation dose increases, the viscosity average molecular weight of oligoalginate decreases, a further decrease in the viscosity average molecular weight occurred with adding KPS. It was observed that the addition of such initiator to alginate during the radiation treatment accelerates the degradation process. Fig. 3 shows the effect of KPS concentration on the degradation of alginate treated with gamma radiation at 100 kGy as a function of the change in its viscosity average molecular weight. The data showed that the viscosity average molecular weight of alginate oligomers decreased with increased KPS concentration as well as alginate degradation. These data confirmed that the addition of chemical initiator such as KPS to NaAlg during irradiation process leads to a synergistic effect, which accelerate the degradation of alginate (Abd El-Rehim et al., 2011).
Changes in the viscosity average molecular weight of alginate powder and alginate mixed with 10% KPS after exposing to γ-irradiation at the dose rate; 6.6 kGy/h.
Effect of KPS concentration on the degradation process of NaAlg at irradiation dose; 100 kGy and dose rate; 6.6 kGy/h.
3.1.2 Chromatographic determination for oligoalginate molecular weight
Alginate can be degraded by irradiation; the molecular weight change of alginate by radiation degradation is measured by gel permeation chromatography (GPC) (Hien et al., 2008). GPC pattern shows the depolymerization process as well as chain scission mainly occurred by irradiation of alginate. It is very well known that polysaccharides in dry form degrade when exposed to ionizing radiation; in this study, raw NaAlg was irradiated with gamma rays at various doses in order to prepare its degraded fractions. To investigate the effect of gamma rays on the molecular weight of NaAlg, their weight average molecular weight (Mw) and number average molecular weight (Mn) values were evaluated. Figs. 4 and 5 show GPC elution curves of alginates irradiated with doses of 0, 20, 60, 80 and 100 kGy and that mixed with 10 wt % KPS initiator as a function of retention time, respectively. The peak of the elution curve shifted toward longer retention time with increasing irradiation dose and further shift occurred with adding KPS. As KPS was mixed to alginate and increased irradiation dose to 100 kGy, the GPC chromatogram of NaAlg samples shifted to higher retention time, indicating that the molecular weight of the sample decreased with enhanced irradiation dose and adding KPS during irradiation process (Abd El-Rehim et al., 2011).
GPC elution curves of sodium alginates as a function of retention time, (a) native alginate, (b) irradiated at 20 kGy, (c) 60 kGy, (d) 80 kGy and (e) 100 kGy.
GPC elution curves of sodium alginates as a function of retention time, (a) native alginate, (b) irradiated at 20 kGy, (c) 60 kGy, (d) 80 kGy and (e) 100 kGy in the presence of 10% KPS.
Table 1 shows the changes in the number-average molecular weights for NaAlg measured by GPC as a function of the irradiation dose in absence and presence of 10 wt % KPS. It is observed that, as the irradiation dose increases the number-average molecular weight of NaAlg decreases. Also, the addition of KPS initiator to the NaAlg during the irradiation process as oxidizing agent accelerates the degradation process. Combining ionizing radiation and oxidizing agents accelerates the rate of polymer scission and reduces the dose required for sodium alginate degradation (Şen, 2011). Table 1.
| Irradiation dose (kGy) | The number average molecular weight of | ||
|---|---|---|---|
| Native NaAlg | Irradiated NaAlg | Irradiated NaAlg with 10 wt.% KPS | |
| 0 | 1.29 × 104 | – | – |
| 20 | 1.13 × 104 | 1.90 × 103 | |
| 60 | 1.98 × 103 | 3.20 × 102 | |
| 80 | 1.85 × 102 | 5.76 × 101 | |
| 1000 | 5.60 × 101 | 1.25 × 101 | |
3.2 UV - vis spectroscopy
Degradation process of native alginate and that irradiated with 20, 60 and 100 kGy with adding KPS was monitored by UV spectrometry; the results are shown in Fig. 6. UV spectra of degraded alginate solutions showed a new absorption band appeared at approximate 265 nm compared with native NaAlg. The peak intensity increased with increasing the absorbed dose and presence of KPS. Thus, the formation of new peak and changes of peak intensity could be assigned to the formation of carbonyl groups formed after the main chain scission of alginate and hydrogen abstraction followed by the ring opening in the radiation-induced degradation process (Nagasawa et al., 2003).
UV spectra of aqueous solutions of alginate irradiated with different doses with adding KPS.
3.3 FT-IR spectroscopy
Fig. 7 shows FTIR spectra of un-irradiated sodium alginate (a), irradiated ones (b and c) and that mixed with KPS with irradiation (d and e). The native NaAlg showed the polysaccharide structure characteristic absorption bands at 1095 and 1037 cm−1 for C–O stretching and at 3440 cm−1 for hydroxyl group (Sartori et al., 1997). The asymmetric and symmetric stretching of carboxylate vibrations appeared at 1619 and 1420 cm−1, respectively (Leal et al., 2008), (Fig. 5a). Peak at 1619 cm−1 for alginate was taken as the reference peaks due to the fact that carboxyl groups do not change after degradation. The spectrum of irradiated NaAlg exhibited most of the characteristic adsorption peaks of native alginate but with some differences (Fig. 5b–e). For instance, the bands at 1619 cm−1 for carboxylate groups, at 3440 cm−1 for OH groups and at 1095 cm−1 and 1037 cm−1 for C–O stretching became broader and shift to another wave numbers. The spectra indicated the formation of new C⚌O and OH groups suggesting that ionizing radiation and KPS treatment under extreme conditions lead to the scission of glycosidic bonds with the change of the structure of reducing end residue. This is manifested as an increase in the ratio of OH group peak and broad C⚌O peak to the reference peaks. Simultaneous decreasing of the peak ratio of C–O–C group to the reference should be perceived.
FTIR spectra of alginate taken before and after irradiation, (a) native alginate, (b) irradiated at 40 kGy, (c) 100 kGy, (d) 40 kGy with KPS and (e) 100 kGy with KPS.
Based on the results of UV and FT-IR spectral analyses of the tested irradiated alginates in this study and several schemes for the radiation degradation of polysaccharides which have been proposed by several authors, e.g., chitosan (Zainol et al., 2009), pectin (Zegota, 1999), carrageenans (Abad et al., 2009) and alginate (Abd El-Rehim et al., 2011), a possible mechanism for the radiation degradation of solid alginate can then be as follows:
3.4 X-ray diffraction (XRD) studies
XRD study was made to illustrate the structural changes on NaAlg treated with γ-radiation and initiator. Fig. 8 shows the X-ray diffraction patterns of (a) native NaAlg and that exposed to irradiation doses of (b) 20 kGy, (c) 20 kGy with 10 wt % KPS, (d) 100 kGy and (e) 100 kGy with 10 wt % KPS. Native and treated alginate exhibited a characteristic peak at 2θ = 23 but there is a reduction in its value and the intensity of this peak with increasing irradiation dose and further decrease occurred with adding KPS initiator. This means that irradiation process caused degradation and destruction in NaAlg structure. The degradation was probably due to the direct effect of radiation and the indirect effect due to oxidation process. Irradiation of polysaccharides leads to breakdown of the ordered system of intermolecular hydrogen bonds. Consequently, the rigidity of chains is influenced by intra-molecular hydrogen bonding and the degree of crystallinity of the material decreases. It was assumed that the degradation first took place preferentially in the amorphous region and then proceeded very moderately from the edge to the inside of the crystalline at higher doses (Mitomo et al., 1994).
XRD spectra of (a) native NaAlg and that exposed to irradiation doses of (b) 20 kGy, (c) 20 kGy with 10 wt.% KPS, (d) 100 kGy and (e) 100 kGy with 10 wt.% KPS.
3.5 Thermal gravimetric analysis (TGA)
Table 2 shows the TGA curves and the weight loss percent of native and irradiated NaAlg at different irradiation doses in the presence of 10 wt % KPS. As seen from the results, the weight loss took place in two stages. The first one starts below 120 °C, it assigned to loss of water molecules that interact with OH and –COO− polar groups in alginate chain by hydrogen bonding, since a considerable amount of water is released at temperatures below 150 °C. The second stage starts at 220 °C and reaches a maximum at 400 °C corresponds to the decomposition (thermal and oxidative) of NaAlg. It was found that there is a significant change in the thermal stability of NaAlg irradiated at different doses and that contained KPS compared with native one whereas no significant difference occurred among each other (Abd El-Rehim et al., 2011).
| Irradiated NaAlg (kGy) | Weight loss (%) at | |||
|---|---|---|---|---|
| 25–120 °C | 120–220 °C | 220–300 °C | 300–400 °C | |
| Native NaAlg | 5.6 | 10.2 | 22.0 | 35.0 |
| 20 | 8.2 | 16.0 | 36.8 | 45.0 |
| 20 + KPS | 9.3 | 17.4 | 37.6 | 48.8 |
| 60 | 10.6 | 18.6 | 45.8 | 50.9 |
| 60 + KPS | 11.4 | 19.2 | 46.7 | 54.8 |
| 100 | 14.2 | 23.0 | 52.0 | 62.4 |
| 100 + KPS | 15.1 | 25.0 | 54.3 | 64.3 |
3.6 Using of radiation processed alginate as plant growth promoter
As mentioned earlier, radiation processed polysaccharides are now being investigated as plant growth promoters by a number of researchers (Mollah et al., 2009; Abd El-Rehim et al., 2011). These studies have mainly been carried out in hydroponic or spraying systems for some specific type of crops only; hence, more studies are desired to establish the optimum conditions for use of such materials as plant growth promoters. With this in view, the growth promotion activity of radiation depolymerized alginate has been investigated for Faba bean plants. The growth promotion activity of Faba bean plants was tested with establishes various growth parameters such as seed germination and emergence, average of plant height, leaf width, total dry weight and seed yield. Seed germination and emergence were predicted by soaking the plant seeds in oligosaccharide solution before planting whereas, the other growth parameters were performed in spraying system at various growth stages up to crop production. Oligoalginate solutions which prepared at irradiation dose ranging from 20 to 100 kGy with added KPS were used to study the optimum oligoalginate molecular weight as well as irradiation dose for the growth promotion of Faba bean plants.
The germination% and emergence rate of tested plants were predicted by soaking the plant seeds in native and irradiated alginate solutions before planting. The results showed that the germination% and emergence rate of seeds improved with soaking it in oligoalginate solutions, especially which have low molecular weights, compared with control, Table 3. The highest seed germination was 97% for seeds soaked in alginate irradiated with 100 kGy with KPS whereas, the control has 89%. The results showed that the emergence rate of Faba bean seeds increases with reducing the molecular weights of soaked oligoalginate solutions. The time needed for 100% seed emergence was reduced from 13 to 6 days with varying irradiation dose of soaked oligoalginate solution ranged from 20–100 kGy and added KPS compared with control. The soaked oligoalginate solution of the lowest molecular weight which irradiated with 100 kGy and KPS has the highest seed emergence. The plants treated with native alginate showed the poorest results for germination% and emergence rate. As a result, survival of seedlings is improved, especially in arid and semi-arid environments, and emergence rates are enhanced. It is concluded that the seed treatments as well as soil application are two effective ways for stimulating plant growth and radiation processed alginate exhibiting relatively better plant growth activity.
| Radiation treatment (kGy) | Germination (%) | Emergence (%) at time/day | |||||||
|---|---|---|---|---|---|---|---|---|---|
| 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | ||
| Control | 89 | 0 | 0 | 10 | 30 | 45 | 60 | 95 | 100 |
| Native NaAlg | 87 | 0 | 0 | 0 | 20 | 50 | 60 | 85 | 100 |
| 20 | 92 | 0 | 0 | 20 | 30 | 60 | 90 | 100 | |
| 20 + KPS | 92 | 0 | 10 | 20 | 60 | 80 | 100 | ||
| 60 | 94 | 0 | 20 | 30 | 65 | 90 | 100 | ||
| 60 + KPS | 95 | 0 | 20 | 35 | 70 | 100 | |||
| 100 | 95 | 0 | 25 | 50 | 80 | 100 | |||
| 100 + KPS | 97 | 10 | 40 | 75 | 100 | ||||
Effect of radiation-induced degraded alginates on average of plant height, leaf width, total dry weight and seed yield of Faba bean plants was investigated; data presented in Table 4 indicated that all plant parameters improved with oligoalginate treatments compared with control at various plant stages. The plant growth enhanced plants treated with oligoalginates have lower molecular weight (alginate exposed to high irradiation dose and KPS) whereas, the plants treated with native alginate showed the poorest results for most of the studied parameters, Table 4. The different molecular weights of NaAlg investigated here enhance the growth and productive yield of Faba bean plant. The average growth of the over-ground plant parts was about 5.5–17.4%, 3.1–14.6% and 3.7–13.2% more than that in control at 40, 70 and 90 days, whereas, for leaf width it increased with about 0–19%, 0–22% and 0–20% more than that in control at 40, 70 and 90 days, respectively. Also, the total dry weight increased approximately by 6.5–20%, 2.7–12.4% and 4–11% more than that in control at 40, 70 and 90 days, respectively. The treatment of Faba bean plant by oligosaccharides has positive effect on seed yields; it was about 5–20% higher than that in control with reducing oligoalginate molecular weight. This can be explained as follows: as the plants sprayed with irradiated sodium alginate, this is effective in increasing the total alkaloid content compared to the control plants. The increase in alkaloid content owing to application of oligoalginate might be ascribed to the expected increase in the leaf nitrogen content that might have promoted the plant growth (Idrees et al., 2011). It is clear that the Faba bean plants treated with the lowest molecular weight NaAlg that irradiated at 100 kGy in the presence of 10 wt % KPS have the best plant growth rate and productive yield (Chmielewski et al., 2007).
| Radiation treatment (kGy) | Average of | Seeds yield/plant (g) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Plant height (cm) at age/days | Leaf width (cm) at age/days | Total dry weight (g) at age/days | ||||||||
| 40 | 70 | 90 | 40 | 70 | 90 | 40 | 70 | 90 | ||
| Control | 40.3 | 73.7 | 85.3 | 1.6 | 2.7 | 3.0 | 30.6 | 52.4 | 68.3 | 50.6 |
| Native NaAlg | 39.2 | 72.9 | 84.2 | 1.6 | 2.6 | 2.9 | 29.0 | 50.0 | 67.2 | 49.2 |
| 20 | 42.5 | 76.0 | 88.5 | 1.6 | 2.7 | 3.1 | 32.6 | 53.8 | 68.9 | 53.2 |
| 20 + KPS | 43.0 | 77.9 | 90.8 | 1.7 | 2.8 | 3.1 | 33.2 | 54.7 | 70.1 | 54.7 |
| 60 | 43.6 | 78.2 | 92.7 | 1.75 | 3.0 | 3.2 | 34.8 | 55.9 | 71.2 | 55.1 |
| 60 + KPS | 44.8 | 79.1 | 94.5 | 1.8 | 3.1 | 3.2 | 35.9 | 56.7 | 72.7 | 57.2 |
| 100 | 45.9 | 82.2 | 95.9 | 1.85 | 3.2 | 3.4 | 36.0 | 57.0 | 74.3 | 58.5 |
| 100 + KPS | 47.3 | 84.5 | 97.0 | 1.9 | 3.3 | 3.6 | 36.8 | 58.9 | 75.9 | 60.8 |
The data studied here agree with a lot of studies which investigated the plant growth promotion of radiation processed polysaccharides in a variety of crops under different environmental conditions. The results of these studies have clearly shown that radiation-induced degraded polysaccharides even at very low concentrations of a few tens of ppm are very effective for use as plant growth promoter. Hien et al. reported that the radiation degraded alginate in concentrations 20–50 ppm promotes the growth of rice seedlings, in concentration 100 ppm causes increasing of peanut shooting approximately by 60% compared to control (Hien et al., 2008). Kume et al. mentioned that degraded alginate can increase tea, carrot, or cabbage productivity by 15–40% (Kume et al., 2002).
4 Conclusions
The degradation process of alginate was carried out by γ-irradiation in the presence of KPS initiator as a synergistic effect to reduce the irradiation dose as well as the cost required to degradation. The molecular weight of alginate was reduced significantly by increasing irradiation dose and further reduction occurred with adding KPS to NaAlg during radiation. FTIR and UV spectrometry measurements confirmed that the radiation-induced chain degradation of NaAlg proceeds by scission of the glycosidic bonds and formation of unsaturated double bonds without significant change in chemical structure. The crystal structure of NaAlg destructed with enhancing degradation as well as irradiation dose and KPS addition. There is a significant change occurred in the thermal stability between native alginate and irradiated ones, whereas a non-significant difference showed among irradiated NaAlg in compared with each other. The growth promotion of Faba bean plants as well as seedling growth was studied, the subsequent growth of the seedling and germination were improved by soaking in oligosaccharide solutions. The germination% and emergence rate of Faba bean seeds increased with reducing the molecular weights of soaked oligoalginate solutions. The time needed for 100% seed emergence was reduced from 13 to 6 days with varying irradiation dose of soaked oligoalginate solution ranged from 20–100 kGy and added KPS compared with control. The water-soluble fractions separated from radiation-induced degraded alginate that sprayed on the Faba bean plants improved the average of plant height, leaf width, total dry weight and seed yield. The highest plant growth and seed yield occurred for plants sprayed with low molecular weight NaAlg which treated with 100 kGy and added KPS compared to control. The results suggest that alginate-derived oligosaccharide by using radiation has a positive effect not only on plant growth but also on the productive yield of Faba bean plant, which suggested its possible use in agriculture purposes as growth promoters.
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