Cellulose Anionic Hydrogels Based on Cellulose Nanofibers As

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Cellulose anionic hydrogels based on cellulose nanofibers as natural stimulants for seed germination and seedling growth Hao Zhang, Minmin Yang, Qian Luan, Hu Tang, Fenghong Huang, Xia Xiang, Chen Yang, and Yuping Bao J. Agric. Food Chem., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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Journal of Agricultural and Food Chemistry

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Cellulose anionic hydrogels based on cellulose nanofibers as natural stimulants for seed germination and seedling growth

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Hao Zhang#, †, Minmin Yang#, ‡, Qian Luan†, Hu Tang*, †, Fenghong Huang*, †, Xia

6

Xiang†, Chen Yang†, Yuping Bao†

7 8

†

9

Chemistry and Nutrition, Ministry of Agriculture Key Laboratory of Oil Crops

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Oil Crops Research Institute, Chinese Academy of Agricultural Sciences, Wuhan

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430062, China

Department of Product Processing and Nutriology, Hubei Key Laboratory of Lipid

‡

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Key Laboratory of Biology and Genetic Improvement of Oil Crops, Ministry

13

of Agriculture, Sesame Genetic Improvement Laboratory,Oil Crops Research Institute,

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Chinese Academy of Agricultural Sciences, Wuhan 430062, China

15 16 17 18 19 20

*

Corresponding author: E-mail:[email protected] (Dr. Hu Tang); Tel: +86 027 86711615; Fax: +86 027

86815916

ACS Paragon Plus Environment


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Abstract Cellulose

anionic

hydrogels

were

successfully

prepared

by

dissolving

23

TEMPO-oxidized cellulose nanofibers in NaOH/urea aqueous solution and

24

crosslinked with epichlorohydrin. The hydrogels exhibited microporous structure and

25

high hydrophilicity, which contribute to the excellent water absorption property. The

26

growth indexes including the germination rate, root length, shoot length, fresh weight,

27

and dry weight of the seedlings were investigated. The results showed that cellulose

28

anionic hydrogels with suitable carboxylate contents as plant growth regulators could

29

be beneficial for seed germination and growth. Moreover, they presented preferable

30

antibacterial activity during the breeding and growth of the sesame seed breeding.

31

Thus the cellulose anionic hydrogels with suitable carboxylate contents could be

32

applied as soilless culture mediums for plant growth. This research provided a simple

33

and effective method for the fabrication of cellulose anionic hydrogel and evaluated

34

its application in agriculture.

35 36

Keywords: Cellulose nanofiber; cellulose anionic hydrogel; soilless culture; plant

37

growth regulator; antibacterial

38 39 40 41 42


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Introduction

44

Most of the crops are aerobic organisms depending upon a steady supply of oxygen

45

from environment, especially the ancient oil crop sesame which widely planted in

46

many countries, and it meets a high-end requirement for high quality oil with the

47

abundant and component balanced content of linoleic and oleic acid.1-3 However,

48

sesame is very sensitive to waterlogging. Waterlogging is the first stress factor which

49

seriously affecting the organs built of sesame at early blooming, thereby affecting the

50

sesame growth and the productivity.4-7 Waterlogging will inhibit the oxygen supply to

51

the roots thus limiting root respiration, leading to a serious decline in energy status of

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root cells endangering important metabolic processes of sesame. Thence, one way of

53

solving the problem involves the application of a superabsorbent, which could absorb

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and retain large number of water while providing an environment for seed

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germination and growth.8

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Hydrogels, attractive functional materials with excellent hydrophilic, biodegradable

57

and biocompatible properties, have been extensively researched in food, agriculture,

58

industry, biomedical field.9-12 Recently, several researches have pointed out that

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hydrogels could act as seed growing mediums and preserve water and nutrients for

60

seed growth.13, 14 The hydrogels can reduce the stress of waterlogging periods for

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plant seed by improving the reserved water availability, and more than 95 % of stored

62

water store in hydrogels could gradually release to the root system. Thus, hydrogels

63

could be applied as medium for the germination of plant seed instead of soil or

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agar.15-17



65

Cellulose is the most abundant organic resource in nature which has potential to be

66

an attractive alternative for many materials due to it is nontoxic, biodegradable,

67

chemically stable and highly hydrophilic.18-23 Hydrogels based on cellulose nanofibers

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could be fabricated from chemical modification of cellulose, cellulose-polymer

69

compositing and cellulose derivatives. Chemical modifications of cellulose could be

70

exclusively modified on the hydroxyl groups through oxidation, esterification and

71

etherification reactions.24-26 In present work, we attempted to design an attractive

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ultra-hydrophilic cellulose plant hydrogel based on cellulose nanofibers and applied

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for sesame seed germination and growth. Cellulose is a safe, biodegradable, and

74

hydrophilic natural polymer, and the prepared hydrogels with suitable carboxylate

75

groups applied as plant growth regulators could facilitate seed germination and

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growth, and might be a promising candidate for soilless culture in agricultural

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application. 22, 27-29

78 79

Materials and methods

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Materials. Commercial never-dried softwood sulphite pulp was provided by

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Nordic Paper. The pulp contained ca. 86% cellulose (degree of polymerization, 1200),

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13.8% hemicellulose, and traces of residual lignin (0.7%). 2,2,6,6,-tetramethyl-1-

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piperidinyloxy radical (TEMPO), hydrochloric acid, sodium hydroxide, sodium

84

bromide (NaBr), sodium hypochlorite (NaClO) were purchased from Sigma-Aldrich

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and used without further purification.

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Preparation of cellulose nanofibers. Cellulose nanofibers (CNF) were prepared


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through a TEMPO-mediated oxidation reaction as previously reported by Isogai. et

88

al.30 As presented in Scheme 1a, wood pulp fibers were suspended at a concentration

89

of 1 wt% in water with the addition of TEMPO (16 mg/g cellulose) and NaBr (100

90

mg/g cellulose). NaClO (7.5 mmol/g cellulose) was added dropwise to the suspension

91

with vigorous stirring. The pH of the suspension was maintained at a constant value

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of 10 with the addition of 0.1 M NaOH solution until all NaClO was consumed. The

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resulting fibers were then thoroughly washed by using deionized water through

94

filtration until the filtrate solution was neutral, and the CNF was obtained. The

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carboxylate contents of CNF were measured by using a conductometric titration

96

method and calculated as 1.5 mmol/g.31 By changing the added amount of NaClO (3.5,

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5.0, 6.5 and 7.5 mmol/g cellulose), CNF with various carboxylate contents of 0.7, 1.0,

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1.3 and 1.5 mmol/g were obtained. The CNF were never dried and kept in water

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suspension with a solid content of 1.0 wt%.31

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Preparation of cellulose anionic hydrogels. Preparation of cellulose anionic

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hydrogels is presented in Scheme 1b, a mixture of NaOH/urea/water (7:12:81 by

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weight) was pre-cooled to -12.5 oC. 6 g CNF with different carboxylate contents was

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immediately added into the NaOH/urea mixtures and stirred vigorously for 5 min to

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obtain a 6% cellulose solution.23 Then cross-linker epichlorohydrin (ECH) was added

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(5 mL/100 g cellulose solution), stirred at room temperature for 20 min, and

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centrifuged at 3000 rpm for 5 min. The resulting mixtures were heated at 60 oC for 30

107

min to transform into hydrogels. Finally, the hydrogels were washed with distilled

108

water until the residual NaOH, urea and extra ECH were removed. The hydrogels



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coded as CH, CH07 and CH15, according to the carboxylate contents of 0, 0.7 and 1.5

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mmol/g.

111

Characterization. The carboxylate contents of CNF were determined by

112

conductometric titration.31 The morphology of the CNF samples were examined by

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transmission electron microscopy (TEM). A drop of the dilute water dispersion of

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CNF was deposited on a carbon-coated grid and treated with 1% uranyl acetate

115

negative stain. The specimens were observed using the Hitachi Model HT7700

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transmission electron microscope operated in high-resolution mode at 100 kV.

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Fourier transform infrared spectroscopy (FTIR) was conducted on a Perkin-Elmer

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Spectrum 2000 FTIR. The test specimens were prepared by the KBr-disk method.

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Thermal gravimetric analysis (TGA) was carried out by using a thermogravimetric

120

analyzer (Perkin-Elmer Co., USA) at a heating rate of 15 oC min-1 from 30 to 800 oC

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under nitrogen atmospheres. The dried samples were cut into powders and dried in

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vacuum oven at 60 °C for 24 h before FTIR and TGA measurements.

123

The micro porous structure of the hydrogels was observed by using a scanning

124

electron microscope (SEM, Hitachi, S-570, Japan). The swollen samples were frozen

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directly in liquid nitrogen, freeze-dried under vacuum, and sputtered with gold prior

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to the SEM observation.

127

The gravimetric method was employed to measure the swelling ratios of the

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hydrogels in distilled water at 37 oC. The equilibrium swelling ratio (ESR) was

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calculated as:

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ESR = Ws/Wd


(1)

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131

where Ws is the weight of the swollen hydrogel after reaching swelling equilibrium,

132

and Wd is the weight of the dried hydrogel.

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Seed germination. The seed materials used in this study were black sesame

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cultivar (zhongzhi NO.9, 2n=26) from the National Oil Crops Medium-term

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Genebank of China (Wuhan, China). Germination experiments were performed in

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green house (65 % relative humidity) and incubated 27 ± 2 oC for 16h light culture

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and 8h dark culture and random designed with 3 replications and 9 seeds for each

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replicate. Seeds grown in the summer (from June to September in 2015) were

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collected, dried and stored in desiccant preserved. A seed was considered germinated

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when the tip of the radicle (1-2 mm) had grown out of the seed.32 Germination

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percentage was recorded daily for 7 days, then the seedlings were harvested, and root

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length, shoot length, fresh weight and dry weight were evaluated.33 Soil, CH, CH07,

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CH15, agar hydrogels with agar concentration of 0.3 wt% (AG03) and 0.6 wt%

144

(AG06) were used as culture mediums. The black sesame cultivar was sowed on the

145

surface of the culture mediums.

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Fungal growth experiments. Pathogen was obtained from the National Oil Crops

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Medium-term Genebank of China (Wuhan, China), and identified as Colletotrichum

148

gloeosporioides (C109) and Fusarium solani (83). The pathogen was cultured under

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dark condition in Potato Saccharose Agar (PSA) medium for two days at 30 oC. Then

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punched at the edge of colony and transferred to fresh PSA medium and cultured in

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the same condition for another two days, and repeated 3 times. Subsequent transferred

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the activated C109 and 83 into petri dishes with sterile AG03, AG06, Soil and CH07,



153

cultured for 3 days at 30 oC and measured the colony diameters.

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Statistics of swelling ratios of the hydrogels, seed germination and colony

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diameters were analyzed using SAS system V8.0 by performing repeated measure

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analysis of variance (ANOVA). Statistical significance was determined by Duncan’s

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new multiple range test (P < 0.05), and values not sharing a common superscript (a, b,

158

c, d and e) differ significantly.

159 160

Results and discussion

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Figure 1 displayed the photographs of a suspension of Cellulose fibers (CF) (0.1%)

162

and dispersions of CNF (0.1%) of different carboxylate contents treated with an

163

ultrasonic homogenizer. It was clearly observed that the CF precipitated to the bottom

164

of the bottle. However, CNF with various carboxylate contents ranging from 0.7 to 1.5

165

mmol/g was homogeneously dispersed, and the transparency of the dispersions was

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improved with increasing carboxylate contents of CNF.

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The dispersions of CNF with lower carboxylate contents displayed a low

168

transparency were owing to the reason that these CNFs will form lateral aggregates in

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water, as shown in Figure 2a and b in TEM images of CNF07 and CNF15. CNF07

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formed lateral aggregates, whereas CNF15 was mostly transformed to uniform

171

cellulose nanofibers, and it was the small width that contributes to the transparency of

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the dispersion.34

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Figure S1 (Supporting Information) shows the FTIR of CF, CNF07 and CNF15. In

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comparison with CF, an absorption band at 1731 cm-1 appeared in the spectra curve of


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CNF07 and CNF15, and this was owing to the C=O stretching of the protonated

176

carboxyl groups.35, 36 And it was obviously exhibited that the intensity of CNF15 is

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stronger than CNF07 due to the increased contents of carboxyl groups. These results

178

confirmed the successful preparation of CNF with various carboxylate contents.37

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Figure S2a and b showed the TG and DTG curves of CF, CNF07 and CNF15. It was

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shown that CF began to degrade at about 300 °C under N2 atmosphere, however,

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degradation started at about 200 °C for CNF07 and CNF15. The thermal degradation

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of CNF07 and CNF15 included two independent pyrolysis processes, owing to the

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thermal degradation of TEMPO-oxidized cellulose on the surface and pristine

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cellulose in the core of CNFs, respectively.24, 38 The results indicated that CNF07 and

185

CNF15 possessed decreased thermal stability because of the formation of carboxylate

186

groups on cellulose nanofiber surfaces during the TEMPO-mediated oxidation

187

process.39

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The porous structure of hydrogels is presented in Figure 3, where SEM images of

189

CH, CH07 and CH15 are shown. It was clearly observed that the hydrogels exhibited

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homogeneous porous structure, and this could be beneficial for soilless culture, for the

191

reason that a lot of water molecules could be easily diffused and stored in the porous

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hydrogels, resulting in higher swelling behavior.40 The average pore diameters of

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hydrogels were found to increase from 40 to 60 µm from CH to CH15, owing to the

194

increasing carboxylate contents in hydrogels which would contribute to the formation

195

of pores, and it was obvious that the pore walls folded with higher carboxylate

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contents in hydrogels because of high hydrophilicity of the carboxyl group would



197

absorb large number of water and enlarge the pore sizes.

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Figure 4a shows the FTIR spectra of CH, CH07 and CH15, the absorption band at

199

3440 cm-1 detected was ascribing to the stretching vibration of hydroxyl groups on the

200

cellulose backbone.41 In comparison to CH and CH07, the –OH stretching vibration

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bands of CH15 were shifted to a lower wavenumber, indicating that a stronger

202

hydrogen bond occurred when carboxylate content increased. Moreover, the peak at

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1649 cm-1 for CH is attributed to water molecule, however, it shifted to 1634 cm-1 for

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CH07 and 1620 cm-1 for CH15 are assigned to the antisymmetric COO- stretching

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vibration of the salified carboxyl group.35, 37

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The swelling behavior was studied to evaluate the water absorption property of the

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hydrogels, and the equilibrium swelling ratios (ESR) of CH, CH07 and CH15 in

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distilled water at 37 oC were exhibited in Figure 4b. It was shown that ESR of CH,

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CH07 and CH15 were 80 ± 8 %, 174 ± 2 % and 309 ± 6 %, respectively, which

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displayed an increase of ESR with the increase of the carboxyl groups in hydrogels.

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The results indicated that the hydrophilicity of carboxyl group would be favorable for

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water absorption of hydrogels and it would be helpful for the application of these

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hydrogels in plant growth.13

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The starting temperature of the thermal degradation of CH was almost the same as

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CF, whereas the initiate temperature of thermal degradation of CH07 and CH15

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shifted toward higher temperatures (~220 oC), which was ~20 oC higher than CNF07

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and CNF15 as shown in Figure 4c. And the DTG curve of CH15 consist of one peak

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near 320 oC, for CNF15, it consists of two peaks. These results indicated that only one


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kind of homogeneous substance existed in CH15 due to the crosslinking process.42

220

To investigate whether the synthesized hydrogels can affect germination and

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development of seedlings, sesame seeds were sowed on the surface of CH07 and

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photographed on 1, 3, 4 and 5 days, respectively. As shown in Figure 5a-d, the sesame

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seeds spread uniformly on the surface of CH07, the seed germinated within 3 days,

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and followed the appearance of plumules in 4 days, and complete emergence of all

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seedlings occurred after 5 days.

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Most seeds need sufficient water to be moistened but not to be soaked, therefore,

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suitable water is of great importance for germination and seedling growth.43 As shown

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in Figure 6a, cellulose anionic hydrogel with suitable carboxylate groups could be an

229

appropriate candidate for soilless culture because of the proper absorption of water

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through the presence of carboxyl groups could stimulates germination and seedling

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growth. We investigated the seeds germination on various culture mediums, as shown

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in Figure 6b, the germination percentage in soil, CH, CH07, CH15, AG03 and AG06

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with time increasing was recorded.44 The germination rate of CH07 was the highest of

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all on the first day, and it increased to 100% within four days. However, the seeds in

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CH haven’t germinated completely until the sixth day, and seeds in CH15 germinated

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from 42% to 75% with no further increase. Seeds in CH07 germinated earlier by

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comparison to the others. The highly effective germination of CH07 resulting from

238

the homogeneous macro-porous structure of the CH and the proper carboxylate

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contents, which retaining large amount of water.

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We further studied the influence of various culture mediums on seedlings growth



241

and development.45 The root length, shoot length, fresh weight and dry weight of the

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seedlings were shown in Figure 6c and d. Seedling germinated and grown on CH07

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displayed longer root and heavier fresh weight compared with those on CH, CH15,

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AG03 and AG06, but presented similar lengths of root and fresh weight compared

245

with soil seedlings. These results indicate that cellulose hydrogel with suitable

246

carboxylate contents could be beneficial for seed germination and growth.

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Figure 7 shows a typical antimicrobial test result of AG03, AG06, Soil and CH07

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against Colletotrichum gloeosporioides (C109) and Fusarium solani (83) as

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determined by evaluating fugal growth through the measurement of colony

250

diameter.46 It was clearly displayed in Figure 7a and b that the fugal growth was

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closely affected by the culture medium. Experimental data were obtained by

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calculating the average of colony diameter on the surface of eight different plates, as

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shown in Figure 7c, the colony diameter of CH07 against either C109 or 83 was

254

almost the same as on the surface of Soil. The cellulose anionic hydrogel presented

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preferable antibacterial activity during the breeding and growth of the sesame seed

256

breeding.

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In summary, we have prepared CNF with various carboxylate contents through a

258

TEMPO-mediated oxidation reaction. Then a simple and effective method for the

259

fabrication of cellulose anionic hydrogels from CNF dissolved in NaOH/urea aqueous

260

solution was reported for the first time. The prepared cellulose anionic hydrogels

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exhibited an increasing water absorption property with the increase of carboxylate

262

contents of CNF. The seed germination experiments confirmed that the cellulose


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anionic hydrogels with suitable carboxylate contents could act as a plant growth

264

regulator and promote the germination and growth of seed. Fungal growth

265

experiments indicated that the cellulose anionic hydrogels exhibited antimicrobial

266

activity as well as soil. These results proved that cellulose anionic hydrogels with

267

suitable carboxylate contents can be an attractive seed germination material for

268

agricultural application.

269 270 271 272

Supporting information FTIR spectra of CF, CNF07 and CNF15 (Figure S1), TG and DTG curves of CF, CNF07 and CNF15 (Figure S2).

273 274 275

Acknowledgments

276

This work was supported by the Agricultural Science and Technology Innovation

277

Project of Chinese Academy of Agricultural Sciences (CAAS-ASTIP-2013-OCRI)

278

and the Director Fund of Oil Crops Research Institute (1610172016006).

279 280 281 282 283 284



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Scheme caption

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Scheme 1. Schematic illustration for the preparation of cellulose nanofibers (a) and

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cellulose anionic hydrogels (b).

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Figure captions

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Figure 1. Photographs of dispersion states of CF and CNF with different carboxylate

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contents.

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Figure 2.TEM images of dispersions of CNF07 (a) and CNF15 (b).

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Figure 3. SEM images of CH (a), CH07 (b) and CH15 (c).

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Figure 4. FTIR spectra (a), Equilibrium swelling ratios (b), TG (c) and DTG (d)

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curves of CH, CH07 and CH15.

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Figure 5. Photographs of sesame seeds sowed on the surface of CH07 after 1 (a), 3

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(b), 4 (c) and 5 (d) days.

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Figure 6. The mechanism of seed germination and seedling growth (a), the

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germination percentage of sesame seeds (b), comparison of the length of roots and

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shoots (c) and comparison of the weight of seeding at fresh and dry states (d) in soil,

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CH, CH07, CH15, AG3 and AG6.

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Figure 7. Colony appearance of C109 (a) and 83 (b), and colony diameter (c) grown

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on AG03, AG06, Soil and CH07 after 3 days of incubation.

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Scheme 1.

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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Table of content

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Cellulose anionic hydrogels were successfully prepared from dissolving

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Tempo-oxidized cellulose nanofibers in NaOH/urea aqueous solution and then

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crosslinked with epichlorohydrin. The prepared cellulose anionic hydrogels exhibited

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an increasing water absorption property with the increase of carboxylate contents of

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CNF. The seed germination experiments confirmed that the cellulose anionic

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hydrogels with suitable carboxylate contents could act as a plant growth regulator and

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promote the germination and growth of seed. Fungal growth experiments indicated

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that the cellulose anionic hydrogels exhibited antimicrobial activity as well as soil.

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These results proved that cellulose anionic hydrogels with suitable carboxylate

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contents can be an attractive seed germination material for agricultural application.

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Cellulose Anionic Hydrogels Based on Cellulose Nanofibers As

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