k-Oligocarrageenan Promoting Growth of Hybrid Maize: Influence of Molecular Weight

k-Oligocarrageenan (OC) is an effective biostimulator and a protector against disease infections for plants. However, the effect of OC molecular weight (MW) on plant growth is not fully understood. In this work, OCs with three different MWs (42, 17 and 4 kDa) was prepared by varying the degradation reaction time using ascorbic acid as a reagent. The product structure was confirmed by Fourier-transform infrared spectroscopy (FTIR) data. The growth promotion for maize (Zea mays L.) plants was investigated by foliar spray application of the prepared OCs. Field trials were carried out in two years, 2018 and 2019. The results showed that among treatments, OC with 4 kDa exhibited the best performance in both crop growth and grain yield parameters which indicated increases compared to the control in plant height (6.9–19.9%), length of ears (12.2%), diameter of ears (9.1%), fresh grain weight (17.8%), dry grain weight (20.0%) and grain yield (21.3%). Moreover, low MW OC augmented NP uptake in the plant growth while no effect on K uptake was observed. Therefore, OC with low MWs is potentially promising to apply as a promoter to enhance yield of crops.


Introduction
Plant growth is affected by various endogenous and exogenous factors such as temperature, light, enzymes and nutrient availability [1]. Biopromoters simulating plant growth are well known as biodegradable, biocompatible, biologically reactive and non-toxic molecules. They are particularly considered as chemical agents that enhance plant growth and crop yields [2]. Natural polysaccharides including alginate, chitosan and carrageenan are popular biopromoters possessing multidirectional effects in plant [3,4]. Furthermore, marine algae oligosaccharides could also simulate plant growth [5].
Carrageenan, the main cell-wall material in red algae or Rhodophyta, is a polysaccharide that contains repeating galactose units and 3,6-anhydrogalactose in both sulfated and non-sulfated types. There are three main classes, including κ-, ιand λ-carrageenans that differ in their degree of sulfation [6]. Among them, κ-carrageenan extracted from Kappaphycus alvarezii, is frequently used for wide applications in food science, pharmaceutical technology and agriculture [7].
Oligocarrageenans (OCs) are commonly prepared by acidic hydrolysis of carrageenans using strong acids (HCl/H 2 SO 4 ) [8], dilute HCl/LiCl [9,10] and gamma ray irrigation [11]. Their MWs Molecules 2020, 25, 3825 2 of 11 could be controlled by varying the acid concentration, hydrolysis time and temperature. Although acidic hydrolysis uses simple, low cost and large scale techniques, strong acid reagents can damage the environment and produce toxic compounds. Therefore, green technologies are required for the fabrication of OCs with low MWs which could simulate plant growth through effectively enhancing photosynthesis, cell cycle and basal metabolism [12]. According to a literature review, some biopromoters have been applied for simulating maize growth. For instant, biopromoters derived from alfalfa could increase maize biomass, possibly as a result of improved nitrogen metabolism [13,14] or biopromoters made from various plant extracts could increase maize seed germination, seedling growth, radicle extension, and yield [15,16]. In this work, we have fabricated OCs with different MWs by acid hydrolysis using ascorbic acid as a reagent and applied them as biopromoters to simulate maize plant growth. Parameters such as plant growth, grain yield and NPK uptake have been investigated during two years, 2018 and 2019.

Synthesis and Characterizations of Oligocarrageenan
The hydrolysis of κ-carrageenan in the presence of a natural acid (ascorbic acid) and H 2 O 2 (2%) as a catalyst refluxed at 95 • C produces a degradation into OCs with different MWs. An obvious linear decrease of MW against reaction time is observed ( Figure 1). The results agree well with previous reports [17,18]. The undegraded κ-carrageenan was determined to possess a MW of 670 kDa while the OCs obtained after the treatment with ascorbic acid for 60, 90 and 120 min had MW values of 42, 17 and 4 kDa, respectively. Sun et al. [19] have reported various methods for the degradation of carrageenan by using reactants including HCl or H 2 SO 4 or/and H 2 O 2 . Our experiments used ascorbic acid as a reactant, which not only reduces significantly the risk to the environment but also plays a role as a nutrient component for plant growth.
Molecules 2020, 25, x FOR PEER REVIEW 2 of 11 be controlled by varying the acid concentration, hydrolysis time and temperature. Although acidic hydrolysis uses simple, low cost and large scale techniques, strong acid reagents can damage the environment and produce toxic compounds. Therefore, green technologies are required for the fabrication of OCs with low MWs which could simulate plant growth through effectively enhancing photosynthesis, cell cycle and basal metabolism [12]. According to a literature review, some biopromoters have been applied for simulating maize growth. For instant, biopromoters derived from alfalfa could increase maize biomass, possibly as a result of improved nitrogen metabolism [13,14] or biopromoters made from various plant extracts could increase maize seed germination, seedling growth, radicle extension, and yield [15,16]. In this work, we have fabricated OCs with different MWs by acid hydrolysis using ascorbic acid as a reagent and applied them as biopromoters to simulate maize plant growth. Parameters such as plant growth, grain yield and NPK uptake have been investigated during two years, 2018 and 2019.

Synthesis and Characterizations of Oligocarrageenan
The hydrolysis of κ-carrageenan in the presence of a natural acid (ascorbic acid) and H2O2 (2%) as a catalyst refluxed at 95 °C produces a degradation into OCs with different MWs. An obvious linear decrease of MW against reaction time is observed ( Figure 1). The results agree well with previous reports [17,18]. The undegraded κ-carrageenan was determined to possess a MW of 670 kDa while the OCs obtained after the treatment with ascorbic acid for 60, 90 and 120 min had MW values of 42, 17 and 4 kDa, respectively. Sun et al. [19] have reported various methods for the degradation of carrageenan by using reactants including HCl or H2SO4 or/and H2O2. Our experiments used ascorbic acid as a reactant, which not only reduces significantly the risk to the environment but also plays a role as a nutrient component for plant growth.   This result confirms no changes occurred in the structures of the OCs during hydrolysis, even for the OC with the lowest MW.
Molecules 2020, 25, x FOR PEER REVIEW 3 of 11 [20]. This result confirms no changes occurred in the structures of the OCs during hydrolysis, even for the OC with the lowest MW.

Growth Characters of Crop
Plant have evolved via various complex metabolic pathways to capture energy and form the metabolites required for their growth. Oligocarrageenan and carrageenans can promote the growth of crops by modulating different biochemical processes [21]. Many studies have shown that OCs are effective promoters for the growth of various crops. The different kinds of OCs such as κ, λ, ι-OCs have been shown to augment growth characteristics (e.g., plant height, leaf biomass, cell division and chlorophyll content) in tobacco by improving carbon fixation and nitrogen assimilation [12]. Saucedo et al. [22] found that treatment of Pinus radiata plants with OCs improved their growth parameters by inducing the accumulation of C, N and S. Gonzalez et al. [23] have reported that carrageenans induced increases of height and trunk diameter in Eucalyptus globulus plants. Among carrageenans, κ-carrageenan was found to be the most efficient crop promoter. However, the influence of oligocarrgenan MWs on the growth of maize plants as well as their nutrient uptake has not been investigated so far. In the present work, three samples of degraded OCs with different MWs (4, 17 and 42 kDa) were utilized to modulate the growth of hybrid maize during field experiments conducted over two years (2018 and 2019). The principle plant growth parameters (plant height, ear length, ear diameter) and crop yields (grain mass and grain yield) were explored. To study their effect on nutrient uptake ability, the contents of principle elements (N, P, K) in the main parts of the plants were evaluated by laboratory analysis.
The heights of maize plants were measured at 25, 40 and 60 days post-sowing in 2018 and 2019 as shown in Figure 3. The data of both years showed that the maize treated with OCs revealed an increase in plant height compared to the control plants and the OC with the lowest MW (4 kDa) induced the best crop growth. For instance, Figure 3A shows that the heights of control plants were 48.84, 159.56 and 176.58 cm at 25, 40 and 60 days post-sowing, respectively, while the heights of maize treated with 42 kDa OC were 175.70 cm and 194.58 cm, which corresponds to an increase of 10.1% and 10.2% at 40 and 60 days, respectively. On the other hand, the heights of plants treated with 4 kDa were 52.20, 184.06 and 211.69 cm at 25, 40 and 60 days, respectively, for an increase of 6.9%, 15.4% and 19.9% compared to the control plants and 8.2%, 4.8% and 8.8% compared to samples treated with 42 kDa MW OC. Results for 4 kDa OC were significantly different from each other, while crops treated with 42 kDa and 17 kDa OCs showed no significant differences in any of the parameters. Similar results were also obtained in 2019. This indicated that the OC small fraction might easily be absorbed by the plant and the existence of an absorption limitation for high MW OCs. However, the

Growth Characters of Crop
Plant have evolved via various complex metabolic pathways to capture energy and form the metabolites required for their growth. Oligocarrageenan and carrageenans can promote the growth of crops by modulating different biochemical processes [21]. Many studies have shown that OCs are effective promoters for the growth of various crops. The different kinds of OCs such as κ, λ, ι-OCs have been shown to augment growth characteristics (e.g., plant height, leaf biomass, cell division and chlorophyll content) in tobacco by improving carbon fixation and nitrogen assimilation [12]. Saucedo et al. [22] found that treatment of Pinus radiata plants with OCs improved their growth parameters by inducing the accumulation of C, N and S. Gonzalez et al. [23] have reported that carrageenans induced increases of height and trunk diameter in Eucalyptus globulus plants. Among carrageenans, κ-carrageenan was found to be the most efficient crop promoter. However, the influence of oligocarrgenan MWs on the growth of maize plants as well as their nutrient uptake has not been investigated so far. In the present work, three samples of degraded OCs with different MWs (4, 17 and 42 kDa) were utilized to modulate the growth of hybrid maize during field experiments conducted over two years (2018 and 2019). The principle plant growth parameters (plant height, ear length, ear diameter) and crop yields (grain mass and grain yield) were explored. To study their effect on nutrient uptake ability, the contents of principle elements (N, P, K) in the main parts of the plants were evaluated by laboratory analysis.
The heights of maize plants were measured at 25, 40 and 60 days post-sowing in 2018 and 2019 as shown in Figure 3. The data of both years showed that the maize treated with OCs revealed an increase in plant height compared to the control plants and the OC with the lowest MW (4 kDa) induced the best crop growth. For instance, Figure 3A shows that the heights of control plants were 48.84, 159.56 and 176.58 cm at 25, 40 and 60 days post-sowing, respectively, while the heights of maize treated with 42 kDa OC were 175.70 cm and 194.58 cm, which corresponds to an increase of 10.1% and 10.2% at 40 and 60 days, respectively. On the other hand, the heights of plants treated with 4 kDa were 52.20, 184.06 and 211.69 cm at 25, 40 and 60 days, respectively, for an increase of 6.9%, 15.4% and 19.9% compared to the control plants and 8.2%, 4.8% and 8.8% compared to samples treated with 42 kDa MW OC. Results for 4 kDa OC were significantly different from each other, while crops treated with 42 kDa and 17 kDa OCs showed no significant differences in any of the parameters. Similar results were also obtained in 2019. This indicated that the OC small fraction might easily be absorbed by the plant and the existence of an absorption limitation for high MW OCs. However, the increase in the plant height 25 days after sowing was not significantly different between samples, in agreement with a previous experiment for oligochitosan. This can be due to the fact foliar application insignificantly affected intercellular CO 2 concentrations in the photosynthesis of maize during the short time of growth [24].
Molecules 2020, 25, x FOR PEER REVIEW 4 of 11 increase in the plant height 25 days after sowing was not significantly different between samples, in agreement with a previous experiment for oligochitosan. This can be due to the fact foliar application insignificantly affected intercellular CO2 concentrations in the photosynthesis of maize during the short time of growth [24]. Abad et al. [25] showed that application of fractionated radiation-modified OC solutions onto pechay plants induced plant growth in the decreasing order of 1 kDa > 3 kDa > 5 kDa. Application of other oligosaccharides such as oligochitosan have been known to enhance plant growth. Nge et al. [26] have reported a significant improvement in orchid growth when oligochitosans with low MWs were applied to orchid plant meristematic tissue. The results found that 1 kDa and 10 kDa oligochitosans were four times more effective compared to 100 kDa oligochitosan. Dzung et al. [27] applied oligochitosans with MWs in the range of 2.5-7.8 kDa to chili plants (Capsicum frutescens). The results showed that shoot weights increased in the order of applications 7.8 < 5.0 < 2.5 kDa.
The effect of OC MWs on the growth parameters of maize ears is presented in Figure 4. It shows that the crops treated with OCs revealed an increase in length and diameter of ears and the data was slightly different between the two years. Experiments performed in 2018 showed that the mean length of ears increased in plants treated with OC 4 kDa by 17.1% compared to the control plant whereas this parameter was not different in plants treated with OCs 17 and 42 kDa compared to the control plant. Similar results also observed for the mean diameter of ears in the same year. Data collected in 2019 showed that the increases in mean length of ears were found to be 7.3%, 5.6% and 12.2% and the increases in mean diameter of ears were 5.5%, 3.2% and 9.1% when plants were treated with 42, 17 and 4 kDa OCs, respectively. Although different data are obtained between years which can be due to difference in mean rainfalls between 2018 and 2019, the highest increase in all parameters was clearly observed for crops treated with 4 kDa compared to the others. As a result, we can conclude that OC with a low MW induced an obvious enhancement in the growth of hybrid maize plants. The observed increase in growth of the plant treated with κ-carrageenan is in agreement with previous reports [12,28]. Umhaw et al. [29] applied binary mixtures of fertilizer (120-28-58 NPK) and κ-carrageenan to maize plant in The Philippines. Their data showed that the maximum increase in ear length is 9.0%, slightly lower than the result obtained from application of OC 4 kDa in the present work. Meanwhile, κ-carrageenan application also enhanced the growth characters of chickpea and maize plants [30]. Abad et al. [25] showed that application of fractionated radiation-modified OC solutions onto pechay plants induced plant growth in the decreasing order of 1 kDa > 3 kDa > 5 kDa. Application of other oligosaccharides such as oligochitosan have been known to enhance plant growth. Nge et al. [26] have reported a significant improvement in orchid growth when oligochitosans with low MWs were applied to orchid plant meristematic tissue. The results found that 1 kDa and 10 kDa oligochitosans were four times more effective compared to 100 kDa oligochitosan. Dzung et al. [27] applied oligochitosans with MWs in the range of 2.5-7.8 kDa to chili plants (Capsicum frutescens). The results showed that shoot weights increased in the order of applications 7.8 < 5.0 < 2.5 kDa.
The effect of OC MWs on the growth parameters of maize ears is presented in Figure 4. It shows that the crops treated with OCs revealed an increase in length and diameter of ears and the data was slightly different between the two years. Experiments performed in 2018 showed that the mean length of ears increased in plants treated with OC 4 kDa by 17.1% compared to the control plant whereas this parameter was not different in plants treated with OCs 17 and 42 kDa compared to the control plant. Similar results also observed for the mean diameter of ears in the same year. Data collected in 2019 showed that the increases in mean length of ears were found to be 7.3%, 5.6% and 12.2% and the increases in mean diameter of ears were 5.5%, 3.2% and 9.1% when plants were treated with 42, 17 and 4 kDa OCs, respectively. Although different data are obtained between years which can be due to difference in mean rainfalls between 2018 and 2019, the highest increase in all parameters was clearly observed for crops treated with 4 kDa compared to the others. As a result, we can conclude that OC with a low MW induced an obvious enhancement in the growth of hybrid maize plants. The observed increase in growth of the plant treated with κ-carrageenan is in agreement with previous reports [12,28]. Umhaw et al. [29] applied binary mixtures of fertilizer (120-28-58 NPK) and κ-carrageenan to maize plant in The Philippines. Their data showed that the maximum increase in ear length is 9.0%, slightly lower than the result obtained from application of OC 4 kDa in the present work. Meanwhile, κ-carrageenan application also enhanced the growth characters of chickpea and maize plants [30].        in grain characters of plants treated with oligosaccharides possessing low MW can be related to more effective activation of enzymes such as nitrate reductase and glutamine synthetase which can yield high levels of protein in the treated plants [31,32]. Therefore, the determination of nutrient contents in the different parts of plant is particularly necessary to evaluate the enzyme activities and the metabolism in the plant.

Yield Parameters of Grain
Molecules 2020, 25, x FOR PEER REVIEW 6 of 11 number of grains per row and number of grains per ear were considerably increased compared to the control plants and the maximum values were recorded in plants treated with OC 4 kDa. The increase in grain characters of plants treated with oligosaccharides possessing low MW can be related to more effective activation of enzymes such as nitrate reductase and glutamine synthetase which can yield high levels of protein in the treated plants [31,32]. Therefore, the determination of nutrient contents in the different parts of plant is particularly necessary to evaluate the enzyme activities and the metabolism in the plant.

Nutrient Uptake
Vigorous growth of maize under application of OCs can be related to an improved nutrient uptake ability in the plants. Figure 7 shows the content of NPK in various parts of maize analyzed in both years. An unambiguous increase of nitrogen and phosphorous contents in plants treated with OC 4 kDa was observed, while the effect of MW on potassium uptake was not clear in the growth of maize. For instant, the application of OC 4kDa in 2018 ( Figure 7A,C,E) evoked nitrogen increases of 27.7%, 18.4% and 10.3% compared to the control plants as determined in leaves, trunk and grain, respectively, while the increase in phosphorus content was found to be 15.38%, 27.7% and 11.0%, respectively. However, the increase in potassium content in plant organs was not significant compared to the control plants, except for content in leaves of plants treated with 4 kDa OC. Similar results were observed in 2019. This was consistent with other reports that found the NP uptake closely related to tissue growth, yet K plays a major role in enzyme activation and maintaining cell osmotic potentials but is not incorporated into plant tissues [33].

Nutrient Uptake
Vigorous growth of maize under application of OCs can be related to an improved nutrient uptake ability in the plants. Figure 7 shows the content of NPK in various parts of maize analyzed in both years. An unambiguous increase of nitrogen and phosphorous contents in plants treated with OC 4 kDa was observed, while the effect of MW on potassium uptake was not clear in the growth of maize. For instant, the application of OC 4kDa in 2018 ( Figure 7A,C,E) evoked nitrogen increases of 27.7%, 18.4% and 10.3% compared to the control plants as determined in leaves, trunk and grain, respectively, while the increase in phosphorus content was found to be 15.38%, 27.7% and 11.0%, respectively. However, the increase in potassium content in plant organs was not significant compared to the control plants, except for content in leaves of plants treated with 4 kDa OC. Similar results were observed in 2019. This was consistent with other reports that found the NP uptake closely related to tissue growth, yet K plays a major role in enzyme activation and maintaining cell osmotic potentials but is not incorporated into plant tissues [33].
Ning et al. [34] have also reported that K was necessary for vegetative growth pre-silking while NP content was important during the maturity period because grain development was mainly a process of carbohydrate deposition in maize. Therefore, it is clear that OCs having low MW can augment the NP uptake to grow plant tissues via a carbohydrate deposition process. Ning et al. [34] have also reported that K was necessary for vegetative growth pre-silking while NP content was important during the maturity period because grain development was mainly a process of carbohydrate deposition in maize. Therefore, it is clear that OCs having low MW can augment the NP uptake to grow plant tissues via a carbohydrate deposition process.

Degradation of Carrageenan
Refined k-carrageenan extracted from the red seaweed Kappaphycus alvarezii was supplied by Sonhai Carrageenan Factory in Ninh Thuan province, Vietnam. OCs with various average MWs were prepared by hydrolysis of the k-carrageenan using ascorbic acid (0.4 M) and catalytic H2O2 (2%) at 90 °C for 60, 90 or 120 min.

Characterization of Oligocarrageenans
The average MWs were determined by the gel permeation chromatography (GPC) technique with an Agilent GPC-Addon Rev. B.01.01l. To study the possibly functional groups present in the OC chains, all samples were analyzed by Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra were recorded on a Tensor 27 instrument (Bruker, Karlsruhe, Germany) over a wavelength number range of 500-4000 cm −1 .

Degradation of Carrageenan
Refined k-carrageenan extracted from the red seaweed Kappaphycus alvarezii was supplied by Sonhai Carrageenan Factory in Ninh Thuan province, Vietnam. OCs with various average MWs were prepared by hydrolysis of the k-carrageenan using ascorbic acid (0.4 M) and catalytic H 2 O 2 (2%) at 90 • C for 60, 90 or 120 min.

Characterization of Oligocarrageenans
The average MWs were determined by the gel permeation chromatography (GPC) technique with an Agilent GPC-Addon Rev. B.01.01l. To study the possibly functional groups present in the OC chains, all samples were analyzed by Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra were recorded on a Tensor 27 instrument (Bruker, Karlsruhe, Germany) over a wavelength number range of 500-4000 cm −1 .
The field experiments were conducted under a randomized complete block design with four treatments and three repetitions as shown in Figure 8. An area of land (14 m × 23 m) was selected and divided into three equal blocks named Block A, Block B and Block C. Each block was further divided into four plots with a distance between two plots of one meter so that each plot covered 20 m 2 (4 m × 5 m). Four biostimulator treatments were randomly applied to the four plots in each block. Each plot containing 68 plants was divided into four rows so that there is 17 plants in each row. For the field trials, the OCs with different MWs (4-42 kDa) were used to evaluate growth and physiological activity of hybrid maize (Zea mays L.). The experiments were carried out in CuMgar, Daklak province, Vietnam (12°47′31′N and 108°04′35′E) at 500 m of height over the seawater level for two years (2018 and 2019). Basal soil was used for field trial and characters of experimental soil including nitrogen (0.15 ± 0.02%), phosphorous (0.192 ± 0.032%), potassium (0.098 ± 0.023), pH (5.08 ± 0.203), humidity (33.58 ± 4.029%), humus (3.33 ± 0.769%) and bacteria (3.56 × 10 5 ± 1.56 × 10 5 CFU mg −1 ) were analysized. Meteorological data for each month including air temperature and total rainfall in two years 2018 and 2019 collected by the Tay Nguyen regional hydrometeorological center and is listed in Table S1 (Supplementary Materials).
The field experiments were conducted under a randomized complete block design with four treatments and three repetitions as shown in Figure 8. An area of land (14 m × 23 m) was selected and divided into three equal blocks named Block A, Block B and Block C. Each block was further divided into four plots with a distance between two plots of one meter so that each plot covered 20 m 2 (4 m × 5 m). Four biostimulator treatments were randomly applied to the four plots in each block. Each plot containing 68 plants was divided into four rows so that there is 17   The sowing process was carried out by the dibbling method placing two sown seeds per hill later thinned to one plant per hill. NPK fertilizer was applied in the form of urea (46.3% nitrogen), calcium dihydrophosphate (15-17% phosphorous) and potassium chloride (50-60% potassium), respectively. The different fertilizer contents were irrigated at various periods during the plant growth process. OCs with different MWs (at the same concentration) were applied by foliar spray. Cattle manure (10 ton ha −1 ), nitrogen (40 kg ha −1 ) and phosphorous (80 kg ha −1 ) fertilizers were applied at 20 days pre-sowing. Spray of OCs (100 ppm), and irrigation of nitrogen (40 kg ha −1 ) and potassium (40 kg ha −1 ) fertizers were firstly performed when the maize had 4-5 leaves (~15 days post-sowing). Secondly, a spray of OCs (100 ppm), and irrigation of nitrogen (80 kg ha -1 ) and potassium (40 kg ha −1 ) fertilizer were applied when the maize had 7-9 leaves (~30 days post-sowing). Finally, a spray of OCs (100 ppm), and irrigation of nitrogen (80 kg ha −1 ) and potassium (40 kg ha −1 ) fertilizers were applied at 10-15 days pre-anthesis (~50 days post-sowing). Therefore, a total amount of nitrogen of 240 kg ha −1 , phophorus 80 kg ha −1 and potassium 120 kg ha −1 was applied to ensure the physiological stages The sowing process was carried out by the dibbling method placing two sown seeds per hill later thinned to one plant per hill. NPK fertilizer was applied in the form of urea (46.3% nitrogen), calcium dihydrophosphate (15-17% phosphorous) and potassium chloride (50-60% potassium), respectively. The different fertilizer contents were irrigated at various periods during the plant growth process. OCs with different MWs (at the same concentration) were applied by foliar spray. Cattle manure (10 ton ha −1 ), nitrogen (40 kg ha −1 ) and phosphorous (80 kg ha −1 ) fertilizers were applied at 20 days pre-sowing. Spray of OCs (100 ppm), and irrigation of nitrogen (40 kg ha −1 ) and potassium (40 kg ha −1 ) fertizers were firstly performed when the maize had 4-5 leaves (~15 days post-sowing). Secondly, a spray of OCs (100 ppm), and irrigation of nitrogen (80 kg ha -1 ) and potassium (40 kg ha −1 ) fertilizer were applied when the maize had 7-9 leaves (~30 days post-sowing). Finally, a spray of OCs (100 ppm), and irrigation of nitrogen (80 kg ha −1 ) and potassium (40 kg ha −1 ) fertilizers were applied at 10-15 days pre-anthesis (~50 days post-sowing). Therefore, a total amount of nitrogen of 240 kg ha −1 , phophorus 80 kg ha −1 and potassium 120 kg ha −1 was applied to ensure the physiological stages of the crops. The maize was kept free of weeds to avoid weed-crop competition. The crop was manually harvested on December 2018 and December 2019.
The crop traits and grain yields were evaluated via the national standard guidance (QCVN 01-56:2011/BNNPTNT) as follows: (1) plant height of 30 random crops, which planted in two central rows of the plots, were measured as the distance from ground level to the lowest branch of corn tassel at 25, 40 and 60 days post-sowing; (2) length and diameter of the ears were averagely determined from 30 ears (the lowest ear for each plant was used); (3) number of grains per ear was determined by counting the total number of grains in 30 ears and then dividing by the number of the ears; (4) number of grain rows per ear in 30 ears was identified as the average number of grain rows in an ear; (5) number of grains per grain row in 30 ears was calculated by the number of grains for only a mean row; (6) grain yield (kg ha −1 ) were calculated by Equation (1): where m 1 is fresh corn mass of two central rows in each plot; m 2 is dry grain mass of 30 plants (humidity < 14%); m 0 is fresh corn mass of 30 plants; S 0 is land area of two central rows (7 m 2 ). Analysis of variance and mean comparisons of the collected data were performed Duncan's Multiple Range Test (DMRT) software package.

Laboratory Analysis
The NPK uptakes in different parts of the crop including leaves, trunk and grains were estimated by multiplying of dry biomass weight. Fifteen random plants (100 days old) in each plot were selected for analysis. The samples were dried in air at 75 • C for 72 h and then grounded into a fine powder. The powder was digested in a solution of H 2 SO 4 (98%) containing H 2 O 2 (30%). The nitrogen content was estimated by the Kjeldahl method while phosphorus and potassium content were analysed by a spectrophotometric method, and atomic absorption spectrometry (AAS), respectively.

Conclusions
Foliar spraying of OCs on maize plants promotes growth and enhances grain yield. The results supported the notions that application of OC with low MW (4 kDa) induced significant increases in plant growth and the grain yield parameters of maize plants. Furthermore, the present work showed that OC with low MW can augment NP uptake but not K uptake during maize plant growth. This result provides particularly important information for famers in development of green agriculture and also promotes investigation of the effect of application of OCs on other crops in the future.  Table S1: Meteorological data for each month in Daklak province, Vietnam collected in two years 2018 and 2019 collected, Table S2: Influence of molecular weight of oligocarrageenan on maize plant height and maize ear in two years, Table S3: Influence of molecular weight of oligocarrageenan on parameters of ear growth and crop yield, Table S4: Influence of molecular weight of oligocarrageenan on NPK uptakes (kg ha −1 ) by maize plant in two years.

Conflicts of Interest:
The authors declare no conflict of interest.