Inoculation and Soil Texture Effects on Yield and Yield Components of Mungbean

Mungbean [Vigna radiata (L.) Wilczek] is a short-duration and relatively drought-tolerant crop grown predominantly in the tropics. This grain legume can improve soil fertility through biological nitrogen (N) fixation. To assess the effects of Bradyrhizobium (group I) inoculation on yield and yield attributes of mungbean, a greenhouse study was conducted during Fall 2016 with two mungbean cultivars (‘Berken’ and ‘OK2000’), two inoculum treatments (inoculated and uninoculated), and two soil textures (loamy sand and silt loam). Pots were laid out in a completely randomized design and treatment combinations were replicated seven times. The main effects of cultivar and soil texture significantly (P ≤ 0.05) affected mungbean seed weight and plant residue mass. Seed yield (13%), plant residue (22%), and protein content (6%) of OK2000 were significantly higher than Berken cultivar. A 31% seed yield and 40% plant residue increase were recorded on silt loam soil compared to loamy sand soil. Significant increase in plant height (18%) and number of pods per plant (21%) were also recorded when mungbean plants were grown on silt loam compared to loamy sand soil. Bradyrhizobium inoculation significantly increased the number of pods per plant, the number of seeds per plant, and seed yield. [Cultivar × inoculation] and [cultivar × soil texture] interactions had significant (P ≤ 0.05) effects on number of seeds per pods and plant height, respectively. Understanding the agronomic practices and soil physical properties that may limit mungbean production could help in optimizing its establishment and growth in non-traditional growing areas.


Introduction
Mungbean [Vigna radiata (L.) Wilczek], also known as mung, moong, green gram, golden gram, chickasaw pea, oregon pea, and chop suey bean, is a pulse species widely grown in tropical and sub-tropical regions of the world (Nair et al., 2013).It is an ancient crop believed to be a native of India and is extensively cultivated in South, East, and Southeast Asia, as well as East Africa (Kim, Nair, J. Lee, & S.-H. Lee, 2015;Sharma, Priya, Bindumadhava, Nair, & Nayyar, 2016).Mungbean is an important food and livestock feed legume crop in these regions and is consumed for its protein-rich seeds (Dahiya et al., 2015;Foyer et al., 2016).Mungbean, a short growth duration legume (55-110 days), fixes atmospheric N through a symbiotic association with rhizobia living in its root nodules (Peoples & Herridge, 1990;Razzaque, Haque, Karim, & Solaiman, 2016).However, under unfavorable conditions such as very acidic and heavy-textured soils, or very hot, dry conditions, Rhizobium survival and crop establishment can be constrained, thereby limiting root nodulation and yield (Kirchhof & So, 1995;Rahmianna, Adisarwanto, Kirchhof, & So, 2000).Therefore, seed inoculation with a strain of Rhizobium, appropriate to the cultivar and soil texture is recommended for improving growth and yield of mungbean (Hussain et al., 2014;Sharma & Khanna, 2010).Anjum, Ahmed, and Rauf (2006) reported that Rhizobium inoculation of mungbean seeds increased the number of pods per plant, number of seeds per plant, seed weight and seed yield over the control.Rhizobium inoculation also increased plant height, number of nodules per plant, number of pods per plant, number of seeds per pod and weight of seeds per plant, and N and phosphorous (P) accumalation compared to the control (Hussain et al., 2014).Molla, Solaiman, Jahiruddin, Mridha, and Khanam (2011) observed that inoculation of seed with Rhizobium produced a higher number of nodules per plant, plant height, number of pod per plant and seed yield.Travlos and Karamanos (2006) compared the responses of marama bean [Tylosema esculentum (Burch.) A.
Schreib.], a perennial tropical grain legume of southern Africa on four different soil textures.They reported significant restriction in marama bean growth under clay and clay loam soils compared to sandy clay loam, and sandy soils.Thomson, Siddique, Barr, and Wilson (1997) evaluated the growth and seed yields of six commercially grown and eight potential new grain legumes species on a sandy clay loam and silty clay loam.They observed that faba bean (Vicia faba) cv.Fiord and field pea (Pisum sativum L.) cv.Dundale had the highest grain yield compared to other legume species on fine-textured, neutral to alkaline soils.
Most previous studies have shown that the use of a short-duration legume crop such as mungbean could be useful for producing nutrient-rich foods under various agronomic practices in regions facing food insecurity issues.However, few studies have investigated the yield performance of mungban cultivars as affected by Bradyrhizobium inoculants under various soil conditions.The present study evaluated the combined effects of Bradyrhizobium (group I) inoculation, cultivar, and soil texture on yield and yield components of mungbean under greenhouse conditions.

Soil Description
A Eunola loamy sand and a Guernsey silt loam soil from 0-15 cm depth were collected from Virginia Tech's Tidewater Agricultural Research and Extension Center in Suffolk, VA (36°39′50.9″Nlatitude, 76°44′01.0″Wlongitude) and Kentland Agricultural Research Farm in Blacksburg, VA (37°11′47.3″Nlatitude, 80°34′49.7″Wlongitude), respectively.The site in Suffolk had been under a corn (Zea mays L.) and cotton (Gossypium hirsutum L.) rotation while the one in Blacksburg was subjected to a continuous corn system.The presence of naturally occurring symbiotic bacteria in these samples was not evaluated, but due to the lack of legumes in either of these rotations, the presence of high levels of native bacteria was deemed unlikely.The field capacity of the soil samples was determined as described by Gupta and Larson (1979).Soil pH and plant available Ca, Mg, P, and K and were determined as described by Maguire and Heckendorn (2011).Soil pH was determined using a 1:1 (vol/vol) soil-water mix.Phosphorus and potassium were determined via extraction with Mehlich 1 at a 1:1 solution:soil ratio, (Mehlich, 1976) filtered (Whatman #2), and analyzed via inductively coupled plasma atomic emission spectroscopy (ICP) (SPECTRO ARCOS, Kleve, Germany).Total C and N were analyzed by dry combustion using a vario MAX CNS Element Analyzer (Elementar Analysensysteme GmbH, Hanau, Germany).Prior to determining the concentrations of ammonium (NH 4 + ) and nitrate (NO 3 -) using Lachat QuikChem AE flow-injection autoanalyzer and ion chromatography, the soil samples were prepared as described by Bremner and Keeney (1966).The physical and chemical properties of the two soil types are presented in Table 1.

Experimental Design and Management
A greenhouse study was conducted at a Virginia Tech greenhouse in Blacksburg (Virginia, USA) to examine the yield responses of mungbean cultivars to Bradyrhizobium inoculation and soil type.The experiment consisted of two mungbean cultivars, 'Berken' and 'OK2000', two inoculum treatments (inoculated and uninoculated conditions), and two soil textures (loamy sand and silt loam).The treatments were a factorial combination of three factors and the experiment was laid out as a completely randomized design with seven replications (Table 2).Berken is a medium-large seeded cultivar while OK2000 is a large-seeded cultivar with good lodging and shattering resistance.Berken is widely grown and has 15% lower seed weight than OK2000 with similar seed yields.Experimental pots (19 cm tall, 19 cm outside diameter, and 3785 cm 3 volume) were lined with polythene bags to avoid loss of water and filled with 4 kg of soil.Three seeds were sown (immediately after inoculation if applicable) per pot by hand at 3-4 cm depth on December 9, 2016 and grown at 80% field capacity.After seedling emergence, density was thinned to one plant per pot.Plant height was measured with a ruler at harvest maturity (March 14, 2017) by stretching out the plant to the tip.At full maturity, mungbean pods were hand harvested twice and number of pods per plant and number of seeds per pod were counted.Seed yield was calculated as dry matter of seeds from each plant at harvest and seed yield per plant was computed.For plant dry matter determination, aboveground plant material was collected after seed harvest and oven dried at 50 o C to constant weight.Seed protein content was estimated using near infrared spectroscopy.
Analysis of variance (ANOVA) was used to test the effects of cultivar, Bradyrhizobium inoculation, and soil texture on plant parameters, seed yield and yield components using JMP Pro version 13.0.0statistical software (SAS Institute Inc., Carey, NC).Differences among means were considered significant at the α = 0.05 level of probability and were separated using Fisher's protected LSD.Most interactions were not significantly different for yield and yield components of mungbean, therefore most main effects were subjected to mean separation tests.

Plant Height at Maturity
The comparison of loamy sand with silt loam revealed that the average plant height (28.7 cm) of plants grown in silt loam soil was 18% greater than average height of plants grown in loamy sand soil (Figure 1).These results are in agreement with Singh et al. (2011) who reported differences on mungbean growth when grown in different soil textures.They observed greater plant height on a sandy loam soil compared to a loamy sand soil and attributed this to higher organic carbon and moisture content in the sandy loam.Results obtained by Ntukamazina et al. (2017) revealed that differences in mungbean growth under different soils may be due to variations in moisture retention of soils.Increased height of plants grown in silt loam soils can likely be explained by better tilth compared to the sandy loam soil.Previous findings have demonstrated that different soil structures can result in differences in particle composition, chemical properties, mechanical impedance and bulk density which affect crop growth and development (Cook, Gupta, Woodhead, & Larson, 1995;Travlos & Karamanos, 2006).The cultivar and inoculation interaction was statistically significant for plant height (Table 3).Height of inoculated OK2000 plants was greater than uninoculated OK2000 plants (30.0 vs. 26.8cm, respectively) while inoculated and uninoculated Berken plants did not differ from each other (mean = 23.9cm) (Figure 2).Similar results were reported by Bhuiyan, Mian, and Islam ( 2008) who observed greater plant height for inoculated BARI Mung-2 cultivar due to increased nodulation resulting in enhanced vegetative growth.
Figure 1 Mungbean seeds were mixed at a rate of 10 g inoculant kg -1 seed with Bradyrhizobium inoculum (group I) purchased from Hancock Farm & Seed Co., Inc. (Dade City, FL, USA) to allow thorough coating and sown in the pots.The seed inoculant contained less than 1% of active ingredients by weight of Bradyrhizobium sp.(Vigna), Bradyrhizobium japonicum, Rhizobium leguminosarum biovar phaseoli, Rhizobium leguminosarum biovar viceae and 99% of inactive ingredients.Mungbean cultivars were obtained from Oklahoma Foundation Seed Stocks (Stillwater, OK, USA).

Table 1 .
Chemical and physical characteristics of the Eunola soil and Guernsey soil

Table 2 .
Cultivars, inoculation treatments, and soil texture used in the pot experiment

Table 3 .
Analysis of variance of the effects of Bradyrhizobium inoculation, cultivar, and soil texture and their interactions on yield and yield components of mungbean (n = 7 per treatment)