Mycorrhiza Enhanced Protein and Lipid Contents of Potatoes Grown on Inceptisol with Addition of Organic Matter

Kata kunci: Inceptisols, mikoriza, bahan organik, kentang Enhancement of productivity of potato plants grown on poor-P soil such as Inceptisols due to application of arbuscular mycorrhizal fungi (AMF) has been acknowledged. However, whether the AMF inoculation can improve the quality of potato tubers still needs further study. This study was conducted to evaluate the effectiveness of AMF in enhancing potato quality and determine whether the addition of compost and biochar to soil can support the AMF biofertilizer in enhancing the nutrient content in potato tubers. A greenhouse experiment was set up in a factorial design consisting of organic matter types (compost and compost plus biochar) and inoculation of arbuscular mycorrhiza fungi (without and with AMF consisted of Glomus sp. and Gigaspora sp.). The results showed that there was no interaction effect between organic matter and AMF on the quality of potato tubers, however, the individual factor especially AMF increased the contents of protein and lipid in the potato tubers. Application of biochar together with compost also increased the lipid content of potato tubers. The finding showed that AMF inoculation on potato seedlings grown on poor P-soil increased the quality of potato tubers.


INTRODUCTION
Potato (Solanum tuberosum L.) is one of the most important food crops in the world, which is rich in essential minerals providing both biological and nutritional values to human beings (Ekin 2011). Potato demands high level of soil nutrients for its tuber production due to relatively poor developed and shallow rooting system (Joshi 1997). The tremendous use of chemicals in agricultural production has deposited toxic chemicals in water and food, especially in fresh vegetables, including potatoes. As a result, there is a high demand for chemical-free foods (El-Sayed et al. 2014).
One of the solutions to reduce the use of chemicals in agriculture is by using biofertilizers contained of living microorganisms which have the ability to mobilize nutrients in soil from unavailable to available forms through biological processes (Ismail et al. 2014). Application of biofertilizers to soil allows the microorganisms to colonize the rhizosphere together with the plants and thus increase the supply or availability of nutrients to the host plants (Vessey 2003) as well as increase the soil fertility and crop production in sustainable farming (Wu et al. 2004, AL-Zabee andAL-Maliki 2019).
Among the soil microbial groups, the arbuscular mycorrhizal fungi (AMF) are able to establish symbiotic associations with the roots of most terrestrial vascular plants (Schüssler et al. 2001). The main role of AMF is able to increase the uptake of nutrients by host plants since the fungal hyphae act as a complementary root system, increasing the absorption area and allowing acquisition of nutrients with low mobility in soil or those are located far from the roots (Clark and Zeto 2000). The abilities of AMF to colonize their host plants and facilitate the transport of nutrients from the soil to the plant make these fungi interesting as an object of research in order to improve crop yield and soil quality (Smith and Read 2008). Enhancement of productivity of potatoes grown on P-poor soil such as Inceptisols due to application of AMF has been reported (Nurbaity et al. 2016). However, whether this biofertilizer can improve the quality of potatoes still needs further study.
The application of organic materials such as composts and biochars are able to improve soil physical and chemical characteristics enabling marginal soils to become productive soils (Hunt et al. 2010, Mahajan et al. 2019. Compost is usually enriched with different kinds of nutrients like N, P, S and micronutrients in order to not only speed up the decomposition process, but also to improve the quality of composts in terms of nutrient concentrations (Mahajan et al. 2019). Biochar is the carbon-rich charcoal derived from heating biomass in little or no oxygen conditions (Singh et al. 2010). The use of biochar as a soil amendment will improve soil properties with longer lasting effects than other organic materials such as compost, manure or crop residues (Youseef et al. 2017, Nurbaity andAnggraeni 2019). It has also been reported that biochar affects nutrients and microbial activity in soil (Momayezi et al. 2015).
The aim of this study was to evaluate the effectiveness of AMF in enhancing potato quality and to determine whether the application of compost and biochar on Inceptisol can support the AMF biofertilizer in enhancing the nutrient content of the potatoes.

Experimental Design
The experiment was conducted in the Faculty of Agriculture, Universitas Padjadjaran, West Java, locating at 766 meters above sea level. The experiment was arranged in a Randomized Block Design with two factors, i.e. (1) types of organic matter (compost or compost plus biochar) and (2) AMF inoculations (without or with AMF), and three replications.

Soil Samples and Treatments
The soil samples were air dried and sieved (4 mm). The properties of Inceptisol taken from Jatinangor, West Java is presented in Table 1. The soils were then mixed with organic materials, either compost from cow manure (the proportion of soil:compost was 2:1) or compost plus rice-husk biochar (the ratio of soil: compost: biochar was 2:1:1), and potted 10 kg each according to the treatments ppm as shown in the experimental design. The physical characteristics of the soil that has been added with organic matter according to the treatments as growing media is presented in Table 2.

Inoculation of Arbuscular Mycorrhizal Fungi
Arbuscular mycorrhizal fungi used in this experiment were Glomus sp. and Gigaspora sp. Twenty five grams of mycorrhizal inoculum (granular zeolite carrier contained about 50 spores of Glomus sp. and Gigaspora sp. per gram) were incorporated in each pot at the day of planting. The mycorrhizal inoculum was placed in the planting hole at the depth of 10 cm from the surface, followed by certified potato seed-tubers of Medians cultivar, and subsequently covered with a layer of soil in the pots containing 10 kg of soil. Seedlings were grown for 12 weeks under greenhouse condition with natural light interception, and watered every day to maintain 80% of soil water holding capacity.

Analysis of Potato Tubers
The potato tubers were analysed to determine the water content (oven method), ash content (dryashing method), protein content (Kjeldhal method), lipid content (Soxhlet method) and carbohydrates (by difference).

Data Analysis
The data from the treatments were compared using the F-test at 5% significance level followed by the Duncan's Multiple Range Test at 5% significance level.

RESULTS AND DISCUSSION
The results of the analysis of variance showed that no significant difference on the potato tuber moisture contents among the treatments of organic matter and mycorrhiza (Table 3). Both biochar and compost applications resulted in similar effects on this paramater. Chan et al. (2008) indicated that biochar and compost applications improved soil physical properties such as bulk density, total porosity, drainage pores, water content, and permeability. Jeffery et al. (2011) showed that compared to the control treatment, the biochar application resulted in the greatest effect on acidic (14%) and neutral pH soils (13%) and on soils with a coarse (10%) or medium texture (13%). The results suggested the three main benefits of biochar addition to the soil include a liming effect (increased pH) and improved water holding capacity and nutrient availability. Rice husk biochar could be used as a substitution for lime materials to increase the pH of acidic soils. A lot of works have shown that biochar is able to improve soil properties, including soil aggregation, soil water holding capacity and soil strength and to increase soil organism population and activity (Rondon et al. 2007, Nurbaity et al.2013. Furthermore, the treatments also showed similar effects on ash contents of potato tubers (Table 2). Clearly both compost and biochar resulted in the same ash contents of potato tubers. Biochar caused a significant increase in microbial efficiency as a measure of units of CO 2 released per microbial   (Kolb et al. 2009). Yamamoto et al. (2006) further indicated that the increase of amount and colonization rate of arbuscular mycorrhizal fungi was observed on maize roots after biochar application. A more rapid cycling of nutrients in soil organic matter and microbial biomass as well as better colonization of roots by arbuscular mycorrhizal fungi will improve water holding capacity, nutrient availability and crop yields. The interaction between organic matter and AMF biofertilizer showed no significant differences (p<0.05) on the protein, lipid and carbohydrate contents of potato tubers. However, the contents of protein and lipid of potato tubers in the AMF biofertilizer treatment is higher than in the control treatment (without AMF biofertilizer). Furthermore, the lipid content of potato tubers applied with compost + biochar is higher compared to that in the compost treatment (Table 4).
Higher protein content after application of AMF biofertilizer is supported by the findings of Utomo et al. (2011) which indicated that the availability of N in soil increased after AMF biofertilizer application. Further elaboration explained that protein is an essential compound in food, in which its concentration in plants is highly dependent on N availability in soil at sowing time, N released during the growing seasons through mineralization of soil organic matter and N applied as organic or inorganic fertilizer. The increase in the protein content in the potato tubers due to AMF presence seems often to be observed, since such protein increments due to AMF presence in roots have been reported regularly in previous studies (Miransari 2010). Adavi and Tadayoun (2014) showed that mycorrhizal plants were more successful than non-mycorrhizal plants in transportation of assimilates from source to sink parts and produced high tuber protein.
Inoculation of AMF biofertilizer significantly increased lipid content of potato tubers. In arbuscular mycorrhizal fungi, vesicles serve as carbon storage compartments for the fungi and are rich in lipids. The development of vesicles depends on the environmental conditions such as high or low P availability in soil (Smith and Read 2008). Both the increase of N and P uptake are influenced by AMF, thus contributing to the increase of lipid content in potato tubers. The results of the current study showed that the quality of potatoes in term of protein and lipid contents are significantly higher in the mycorrhizal plants compared to non-mycorrhizal plants.

CONCLUSIONS
There was no interaction effects between organic matter and mycorrhiza on the quality of potato tubers, however, the individual factor especially application of mycorrhiza increased the content of protein and lipid in potato tubers. Biochar application together with compost also increased the lipid content in potato tubers.