The Growth of Pak Choy (Brassica rapa L.) on the microalgae (Spirulina platensis) biomass-based nutrient solution

Microalgae (Spirulina platensis), as an organic nutrition source, may play an important role in stimulating the nutritional needs of Pak Choy (Brassica rapa) in a hydroponic system. This study aimed to use S. platensis dry biomass as a nutrient source of Pak Choy grown hydroponically compared to commercial hydroponic nutrients (ABmix) as a control in screenhouse conditions. The experiment was arranged in a factorial design, the first factor was maceration time (0, 1, and 2 weeks), and the second was the concentration of S. platensis solution (0.1, 0.5, 1.0 g/l, and ABmix). The experimental design was arranged in a completely randomized design with three replications. The observed variables were plant height, total leaf, fresh weight of root, shoots, total weight, and chlorophyll content. The results showed that there was no interaction in all observed variables. The maceration treatments did not affect the plant growth, except for plant weight (23.6 g) and leaf width (0.8 cm). However, S. platensis concentration treatments affected all observed variables significantly (p<0.01). On the other hand, all treatments showed tip burns three to four weeks after planting. It concluded that organic substances from S. platensis dry biomass have a potential use as hydroponics nutrient solutions. However, further study needs to be conducted to obtain the optimum treatments (concentration and maceration time) to produce healthier vegetables.


Introduction
Microalgae often grow in hydroponic systems, especially in nutrition.Generally, microalgae will influence the quality of water, such as dissolved oxygen (DO) and pH, and sometimes compete with the primary plant for nutrition uptake [1].However, on some occasions, microalgae improve the buffering of the nutrition for preventing the effect of sanitizing chemical like bleach treatment.So, it is essential to maintain level of microalgal to keep in low level in hydroponic systems [2].The other beneficial of microalgae, the photosynthesis of microalgae produced oxygen in the root zone of plant can prevent anaerobic condition and inhibit the sulfide injury in plant with susceptible of sulphate reduction character, while the high content of organic compound which can be used as the source of nutrition for plant growth.
Organic fertilizer solutions allow food production in hydroponic systems [3].Production of hydroponics vegetable can be upgraded through microalgae together, which can be economically and eco-friendly sustainable production environment [4].Several studies to improve plant growth using 1230 (2023) 012205 IOP Publishing doi:10.1088/1755-1315/1230/1/012205 2 microalgae have been done.The effectiveness of Chlorella vulgaris microalgae in mitigating drought stress in broccoli plants was studied by [5] and [6].They used a phycocyanin-rich Spirulina extract (PRSE) source of nutrition in lettuce to reducing time from seeding to harvest and improving quality, such as taste, texture, color and the other.According to [7] found that Chlorella vulgaris increased vitamin C content in tomato fruit and reduced nutrition levels.Furthermore, a preliminary study by [8] in vertical farming microalgae derived biostimulants possibly have an essential role in economic and environment friendly for sustainability.
The use of microalgae such as Spirulina in the hydroponic system is expected to be an organic fertilizer for plants.This Cyanobacterium species are a source of protein, vitamins, minerals, and polyunsaturated fatty acids (gamma-linolenic acid, phycocyanin, and ω-3 and ω-6 polyunsaturated fatty acids.The minerals content such as Ca, Mg, N, P, K, Fe, Zn, [9] in S. platensis liquid extracts were used to enhance germination of seed, growth of seedling, and other growth parameters of saltstressed Triticum aestivum [10].Spirulina platensis extract was also use to improve the common bean production in pollutes soil [11].Furthermore, a study by [12] found that the application of spraying with S. platensis extract can increase the growth, photosynthetic ability, and yield of Lupinus luteus.
Applying two species of green algae, Chlamydomonas reinhardtii and Chlorella sorokiniana, in maize can increase the number of secondary roots, so plants are more tolerant of drought and nitrogen deficiency.Moreover, microalgae cultivation for biostimulant production can be considered a bio-based process.In general, microalgae can be used as source of plants nutrient in hydroponics systems.The aim of the research is to used microalgae as source organic nutrition in hydroponic system with some degree of concentration.

Experimental Details
This study was conducted in the screenhouse of the Research Center for Horticulture and Plantation.Research and Innovation Agency (BRIN), Cibinong, Bogor, Indonesia.The experiment started in March to May 2022.

Materials
Biomass powder of Spirulina platensis (microalgae with 3-5 % water content) was obtained from the Research Center for Applied Microbiology, Research and Innovation Agency (BRIN).Macro and micronutrient contents of S. platensis were analyzed at the Integrated Laboratory of Bioproduct (iLab) of BRIN (Table 1.).
The seed of Pak Choy (Brassica rapa var.Nauli) from East-West Seed with 99% purity and 85% viability was obtained from a local agricultural shop in Bogor.The seeds were germinated on Rockwool with 1.5 cm square for one week prior to transferring to the wick hydroponics system.

Hydroponic culture of Pak Choy
The vigorous one-week seedlings were transferred to hydroponics with a wick system of 42x 31,5x15 cm box container, consisting of 3 x 4 plant holes.Each plastic container was filled with water, and the nutrient with the final total volume was 5 liters.The pH was maintained at 5-7.KOH (10% w/v) and H3PO4 (10% w/v) were used to pH adjustment.The wick hydroponic system was aerated continuously during the experiment.Total dissolved solid (TDS) and Electronic Conductivity (EC) was measured in the first weeks after planting.The observed variable is plant high (measured from the basal of the plant to the highest part of the plant), and leaf number (counting the number of leaves perfectly open) was recorded weekly until six weeks.The other characters were acquired at the end of observation (6th week).Those were plant weight (weighed the whole plant), shoot weight (weighed the shoots), root weight (weighed the root of the plant), leaf length (measured from the basal of the leaf to the tips of the leaf), leaf wide (measured taken the center of the leaf) and chlorophyll content (counted used chlorophyll meter CCM-200 plus Opti science).

Experimental design and data analysis
The experiment consisted of two factors: There were microalgal concentration and time of maceration.The first factor is microalgal consisting of 0.1, 0.5,1 g/L and ABMix as the control standard nutrient.The second factor was maceration.Maceration is done by dissolving a certain amount of microalgae biomass (dry based powder S. plantesis).S. platensis powder was dissolved in one liter with concentration for each treatment 2.1, 10.5 and 21 g.The mixing of biomass was carried out at different times; 0, 1, and 2 weeks.Maceration aims so that the nutritional elements contained in the microalgae biomass are dissolved into the water.The experiment was arranged as a factorial in Completely randomized design (CRD) of twelve treatments with three replications.
Each replicate consisted of one container containing 12 plants with culture medium of seven-liter medium.Plant growth measurements were carried out on the same six plants in each tray.The acquired data were analyzed using the F test for differences and will continue using the Tukey test for a 5% error if present.

Plant characteristic
Table 2 showed that none of plant growth characters were significantly different within maceration time, except for plant weight and leaf width.However, those characters were significantly different (p<0.01)within the nutrient treatment.On the contrary no significant differences in their interaction.Although there is no interaction between maceration time and plant characteristics, the effect of level maceration time and concentration of microalga does not depend upon the level of the other factor [13]. ns= not significant.*= significant for α 5%.**= significant for α 1%

Plant growth
Some of the growth characteristics are plant height and leaf number.These characters were affected by the environment, especially the nutrition of the medium used to cultivate.In the hydroponic system, nutrition content is crucial when water is the medium and nutrients are fed directly to the roots.One known media is ABmix consisting of macro and microelements for growing the plant.The treatment of ABmix provided higher plant height (figure 1) and leaf number (figure 2) in Pak Choy from three to six weeks after planting than other treatment which prepared from microalgae with different concentration and maceration time.The chemical content in AB mix nutrients such as nitrate (NO3) and ammonium (NH4) are useful for plant growth but in the long term it can accumulate in vegetable plants it affects to human health [14].Therefore, the use of organic nutrition sources as a replacement or supplement to chemical nutrition is an attractive solution for hydroponic planting.The growth of lettuce in organic hydroponic nutrition is slower than inorganic conventional hydroponic (ABmix) [15].The ready use of ions in inorganic nutrition that root plants absorb is the main difference when organic nutrition is in molecules rather than ions.Some of the Pak Choy crops were shown tips burn in microalgal nutrition (64,7%), but ABmix did not show the same (figure 3).The tip burns symptoms on the tips of Pak Choy are suspected of the deficiency of concentration Ca in the tips.The tips burn in some green leafy vegetables is due to the negative correlation of Calcium in the nutrition solution [16].It is widely known the effect of Cadeficiency-related disorders, such as blossom-end rot in tomatoes, bitter pit in in apple fruit, and tip burn in leafy vegetables.The deterioration of the membranes cell caused loss fluid cell by leakage and caused loss of turgor, is correlate to localized Ca-deficiency in the related tissue [17].The cell that occurs this situation showed the collapse membrane, likely resulted from a weak cell wall structure [18].The Ca content in microalgae is far lower than the ABmix nutrition; almost 160 parts of the ABmix content are suspected of tips burning in Pack Choy.

The nutrition effect
All the Pak Choy growth parameters were significantly different from the control (ABmix solution), but no one microalgal treatment was better or the same as the control (table 3).Inorganic nutrition supplies more ions and is faster than organic, so the growth of inorganic-based solutions is better than organic ones [15].The S. platensis solution concentration of 1 mg/l and 0.5 g/l resulted in relatively higher growth parameters such as plant height (15.9 and 16 cm), plant weight (9.4 g and 6.7 g), shoots weight (7.8 and 5.3 g), and leaf length (9.6 and 9.3 cm) from the other concentrations.Meanwhile, the other characteristics were not different between microalgal concentration (leaf number, root weight, and chlorophyll content).

Effect of maceration time
The best plant weight (21.3 g) was obtained by a maceration time of 0 weeks (table 4).It was not significantly different with a maceration time of 1 week, and significantly different with a maceration time of 2 weeks.The best leaf width (3.6 cm) was obtained by the maceration time of 2 weeks which was not significantly different from those one week.The most effective maceration time was two weeks for leaf width.The maceration technique breaks down the bigger molecules into smaller molecules or ions by the chemical reaction (oxidation-reduction).According to the data of total dissolved solids (TDS) and electrical conductivity (EC) on the first weeks of measurement (table 5), the higher EC value is after one-week maceration in TDS and EC (65.7 to 138.0 ppm and 128.4 to 217.7 µS/cm).That indicated increasing the ions or the solute in the water.It can be guessed that oxidation-reduction is due to oxygen in the water.
Increasing TDS in water refers to dissolved organic matters and inorganic salts, including sodium, potassium, calcium, magnesium, chloride, bicarbonates, and sulfates [9] affected the plant weight and leaf width.The solutes consist of ions and molecules, while plants can absorb the ion through the root.
The nutrient in a hydroponic system is one of the important elements of biomass production and plant growth.The increase plant weight can be expected due to the microalgae's macro and micronutrient content being dissolved, which is higher in one week maceration time than two weeks.

Conclusion
The used of microalgae as organic nutrition in hydroponics have the potential value to improve the alternate nutrition medium.Organic nutrition in hydroponic was used to study using microalgae in some concentration, the result showed non one of treatment show the same as standard ABmix (inorganic nutrition).But this study showed the potential use of organic source nutrition.

Figure 1 .Figure 2 .Figure 3 .
Figure 1.Plant height of Pak Choy after six weeks planted on different treatment of microalgae concentrations and maceration time Means with different letter is significantly different by Tukey's test at P ≤ 0.05

Table 2 .
Analysis of variance showed the means a square value of treatments and its interaction of some Pak Choy characters in S. platensis as a hydroponic solution

Table 3 .
Effect of S. platensis concentration as a hydroponic medium in some Pak Choy character

Table 4 .
Effect of maceration time for microalgae preparation hydroponic nutrition in some Pak Choy characters Means with a different letter is significantly different by Tukey's test at P ≤ 0.05

Table 5 .
Electrolyte conductivity of microalgal concentration after one week of planting