Agrophysiological Performance of Mungbean Accessions ( Vigna radiata (L.) R. Wilczek) and Nitrogen Balance under Mungbean in Burkina Faso

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Introduction
Burkina Faso, like other countries of sub-Saharan Africa, is experiencing signifcant population growth. Tis strong population growth leads to an increase in food needs, thus inducing a prevalence rate of undernourishment estimated at 19.2% in Burkina Faso [1]. In addition, increasingly severe climatic hazards have contributed to weakening the food situation of peasant populations, thus compromising numerous development initiatives [2]. Moreover, the degradation of land fertility [3] and the low level of crop diversifcation in Burkina Faso limit agricultural production.
In view of this situation, it is not only necessary to diversify agricultural production but also and above all to identify crops that are resilient to difcult climatic conditions and that meet the nutritional needs of the population.
Te cultivation of seed legumes is recognised as one of the best and least expensive options for addressing malnutrition and particularly protein defciencies in sub-Saharan Africa [4]. Mungbean (Vigna radiata (L.) Wilczek) is an annual seed legume that is a major source of dietary protein and minerals for urban and rural populations as well as for livestock [5]. As a result, mungbean is a legume with high protein potential that can substitute animal protein, which is not easily accessible to the poor. Moreover, this legume has specifc properties with regard to the nitrogen cycle through its capacity to fx atmospheric nitrogen. To this end, the integration of mungbean (Vigna radiata (L.) Wilczek) into cropping systems could be an alternative towards achieving food and nutritional security in Burkina Faso and also for improving soil fertility through its contribution of nitrogen, which is an essential element generally lacking in soils. According to the authors in [6], V. radiata could contribute signifcantly to food and nutritional security and to generate substantial income for the households that produce it. Unfortunately, mungbean remains a very little cultivated species in Burkina Faso. Tus, this study was initiated with the objective of evaluating the physiological and agronomic parameters of mungbean accessions under the climatic conditions of the Sudano-Sahelian zone of Burkina Faso.

Experimental Site.
Te experiment was conducted under rainfed conditions at the experimental station of the Institut du Développement Rural (IDR) based in Gampèla, about 20 km east of the city of Ouagadougou, not far from the national road (RN) n°4 ( Figure 1).
Established in 1975, with an area of about 490 hectares, the Gampèla experimental station is located between parallels 12°24.613′ and 12°25.413′ north latitude and meridians 1°20.464 and 1°21.652 west longitude [8]. Te climate in the area of the station is of the Sudano-Sahelian type characterised by a rainy season from June to October with the maximum rainfall in August and a long dry season from November to May [9]. Te average rainfall varies between 700 mm and 900 mm.
Te soil of Gampèla is of low chemical fertility and predominantly sandy loam texture [10].

Plant Material.
Te plant material consists of 15 mungbean accessions from the World Vegetable Center collection in Mali. Tese accessions difer essentially in seed colour, which varies from light to dark green, in pod colour at maturity, and in the state of pubescence of the pods (Table 1).
Accessions have been assigned symbols from A1 to A15 to facilitate the processing of data with regards to long names.

Experimental Device.
Te experimental setup was a Fischer block with three replicates, oriented from East to West (Figure 2), covering a total area of 907.5 m 2 (75 m × 12.1 m). A total of forty-fve (45) elementary plots were used for the experiment, each replication comprising one elementary plot per accession, i.e., 15 elementary plots per replication. Te elementary plot had 7 lines of 2.7 cm each. Te spacing between the replicates was 2 m, and the distance between the elementary plots in a block was 1.5 m. Te rows were spaced at 0.6 m apart, and in each row, the pits were 0.3 m apart. Te seedlings were sown at one seed per cluster. Each accession was sown on three elementary plots comprising seven (07) lines each. Measurements were made on 5 plants in the 3 middle rows in each plot.

2.4.
Data Collection and Statistical Analysis. Measurements were made on growth, development, yield, and total soil nitrogen.
For growth, plant height (HP) in centimetres and diameter at stem collar (DSC) in millimetres were measured weekly from 21 st days after sowing until fruiting. Te length of the terminal leaf (LTL) in centimetres determined at fowering, the length of the petiole (LPt) in centimetres determined at the fourth node, the length of the peduncle (LPd) in centimetres measured at the ripening of the frst pod, and the number of branches per plant (NBrP) counted at the pod ripening stage were considered.
Te development cycle of the accessions was assessed through the number of days at the beginning of fowering (NDBFl), the number of days to 50% fowering (ND50Fl), the number of days to 50% pod maturity (ND50MatG), and the cycle length (CL) in days.
Yield components such as the number of pods per plant (NPoP), pod length (LPo) in centimetres, number of seeds per pod (NSePo), number of seeds per plant (NSeP) using a Tripette & Renaud seed counter, and 1000-seed weight (WTSe) using a 0.01 g precision balance were assessed.
Te seed yield (YSe) in kg/ha was estimated from the following formula: where kg: kilogram and ha: hectare. Te total nitrogen balance of the crop soil was determined on the basis of the diference between the nitrogen available in the soil at sowing and that available at harvest. To do this, three samples were taken at three levels along the diagonal in each replication using an auger before sowing and then mixed to obtain a composite sample. Te same operation was repeated at harvest for each elemental plot, and the crop soil samples from the same accession were mixed to obtain composite samples. Te total N content of the composite samples was measured according to the Kjeldhal method as described by [12]. Te total nitrogen balance was determined according to the following formula: where TNB: total nitrogen balance, QNS: quantity of nitrogen before sowing, and QNH: quantity of nitrogen at harvest. For data processing and analysis, the Microsoft Excel 2016 spreadsheet program was used to build the database.
Te analysis of variance (ANOVA) and the calculation of means and standard errors were carried out with the GenStat 2015 software. Te Newman-Keuls test at the 5% threshold was also used to compare the means and to produce the graphs. Te Newman-Keuls test was used to separate means 2 International Journal of Agronomy in order to determine signifcant diferences between the group means.
Te results were presented in the form of tables with average ± standard errors and in the form of box plots in order to best observe the variability of yields according to the accessions.

Plant Growth.
Te Newman-Keuls mean separation test from the analysis of variance revealed that the plant height at fowering (HP), diameter at stem collar (DSC), peduncle length (LPd), and number of branches per plant (NBrP)

Development Cycle of Accessions.
Te analysis of variance showed a signifcant diference (P < 0.001) between the accessions in the number of days at the beginning of fowering (NDBFl), the number of days to 50% fowering (ND50Fl), the number of days to 50% pod maturity (ND50MatPo), and the cycle length (CL). Indeed, the number of days at the beginning of fowering varied from 29.33 days to 41.33 days depending on the accessions with an average of 33.75 days (Table 3). Te number of days to 50% fowering and the number of days to 50% pod maturity varied, respectively, from 33.67 to 47.33 days and from 51.33 to 64.33 days depending on the accessions with an average of 39.38 days and 56.84 days, respectively. Accession A10 had a relatively short fowering start, days to 50% fowering, and days to 50% pod maturity, with about 29 days, 33 days, and 51 days, respectively. However, accession A7 had a longer fowering onset, days to 50% fowering, and days to 50% pod maturity than the other accessions at about 41 days, 47 days, and 64 days, respectively.
Te cycle length also varied between accessions from 82 days to 93 days, with an average of about 88 days. Tus, accessions A10 and A11 showed a relatively short cycle (82 days), while the long cycle (93 days) was observed on accession A7.

Yield and Its Components.
Te results showed that the seed yield of accession A3 was signifcantly higher (850.50 kg·ha −1 ) than that of the other accessions, followed by those of accessions A2 and A12 with 625.2 kg·ha −1 and 544.8 kg·ha −1 , respectively ( Figure 3). However, the distributions of values for accessions A3 and A12 showed higher ranges. Te accessions that expressed signifcantly low seed yields were A14, A13, and A11 with 151.2 kg·ha −1 , 240.7 kg·ha −1 , and 243.8 kg·ha −1 , respectively.

Relationships between Parameters.
Te Pearson correlation matrix at the 5% threshold (Table 5) revealed positive correlations between the number of branches and yieldrelated parameters, notably the number of seeds per plant  International Journal of Agronomy 5 (r � 0.62), the number of pods per plant (r � 0.59), and seed yield (r � 0.6). Te seed yield was itself positively correlated with the number of pods per plant (r � 0.96) and the number of seeds per plant (r � 0.96). Strong positive correlations were also noted between the number of days to start fowering and the number of days to 50% fowering (r � 0.90) and between the number of days to 50% pod maturity and the cycle length (r � 0.74).

Nitrogen Balance in the Soil after Cultivation. Te results
show that, at the end of the experiment, the total nitrogen balance in the cultivation soil of most accessions is positive. Indeed, initial nitrogen in the soil was 0.062 g·N·kg −1 of soil, but at the end of the experiment, we noted on average 0.326 g of nitrogen/kg of soil, i.e., an increase of 26.6% in the initial nitrogen in the soil. Te signifcance test of the means from the analysis of variance (Table 6)    International Journal of Agronomy diference (P < 0.0001) between the accessions' crop plots. Tus, on the A3 and A1 accessions, the amount of total nitrogen at harvest was higher than on the other accessions, with 0.94 g·N·kg −1 of soil and 0.83 g·N·kg −1 of soil, respectively. However, the N balance in the crop soils of accessions A11, A14, and A15 was negative.

Discussion
Te average seed yield per hectare varied between 151.20 and 850.50 kg depending on the accessions with an average of 407.16 kg/ha. Considering this average yield expressed by all the accessions, we note that it is largely below the average yield reported by [13] on mungbean, which was 721 kg/ha. Tis low yield could be explained by the fact that the cultivation plots of these accessions did not beneft from fertiliser applications. However, the same authors pointed out that mungbean yields are generally low, varying between 0.5 and 1.5 t/ha in most countries. Tus, each of the fve accessions studied (A2, A3, A7, A12, and A15 expressed a yield within this range. On the other hand, accession A3 expressed a higher yield (850.50 kg/ha). Te higher yield of this accession is attributable to its branching, number of pods per plant, and number of seeds per plant. Indeed, accession A3 produced taller plants with a larger diameter at fowering and a higher number of branches, pods per plant, and seeds per plant than the other accessions. Te expansion of the aerial organs of the plants of this accession would have induced good carbon nutrition and a production of an important quantity of  International Journal of Agronomy photoassimilates which would be exported towards the formation of pods and seeds which are metabolic sinks. Tus, according to [14], most of the useful yield comes from actual photosynthetic assimilation. However, the authors in [15] pointed out that the yield problem is not only a photosynthetic problem because in many cases, it is difcult to defne which of the source (photosynthesis) or the sink (storage capacity) played the determining role. Variability was observed between accessions for parameters related to the development cycle. Indeed, the number of days to 50% fowering varied between 33.67 and 47.33 days with an average of about 40 days for all accessions. However, the authors in [16] found that the number of days to 50% fowering varied between 29 and 48 days with an average of 34.46 on mungbean accessions evaluated in Burkina Faso. Furthermore, the cycle of the 15 accessions studied varied from 82 to 93 days with an average of 88 days.
Seed legumes have enormous agronomic potential, including the provision of nitrogen to the soil. Tus, the authors in [17] reported that mungbean is able to improve soil fertility and increase crop production. At the end of our experiment, the total nitrogen balance varied according to the mungbean accessions. Te work of [18] showed that, in cowpea, the capacity to fx nitrogen from the air varies according to the variety, fertilisation conditions, and climatic conditions. Te authors in [19] found a negative partial nitrogen balance under cowpea cultivation without fertilisation. However, in our case, the overall nitrogen balance under mungbean was positive with an average increase of 0.326 g·N/kg soil. If this balance was positive, this indicates the capacity of symbiotic fxation of atmospheric nitrogen (N 2 ) by mungbean. Te authors in [20] showed that variations in fxation between species depend on both the species-specifc N accumulation potential and climatic conditions. As our experiment was conducted without fertilisation, fxed nitrogen would have contributed to the mungbean's own nitrogen requirements and thus to the yield obtained because according to [21], the triggering of symbiotic fxation occurs when the level of mineral nitrogen availability becomes insufcient to meet the plant's nitrogen requirements. Furthermore, the authors in [22,23] reported that mungbean is mainly grown in rotation with cereals and can be in symbiosis with rhizobium, fxing up to 110 kg·N per hectare, which could help meet its own and the associated or following crop's N requirements.

Conclusion
Tis study increased our scientifc knowledge of mungbean.
To that goal, it demonstrated the variation between mungbean accessions in terms of development and yield, as well as nitrogen supply to the soil. Te fndings demonstrate accessions-to-accessions variation in growth characteristics, development cycle, and yield. Nevertheless, accession A3 showed better growth with a higher plant height at fowering, number of branches, and stem neck diameter than the other accessions. On the other hand, accessions A3, A2, and A12 showed better agronomic performance with, respectively, 850.50 kg·ha −1 , 625.2 kg·ha −1 , and 544.8 kg·ha −1 .
Te number of days to fowering, the number of days to 50% fowering, and the length of the cycle to harvest also varied between accessions, with an early fowering (29 days) observed in A10 and a relatively short number of days to 50% fowering in A10 and A2. In addition, the cycle at harvest ranged from 82 to 93 days with an average of 88 days.
Te total soil nitrogen balance at harvest was positive with an increase of 0.326 g·N·kg −1 of soil, and therefore, mungbean cultivation could be of great beneft in agricultural systems for its atmospheric nitrogen fxing capacity. Tis seed legume should be encouraged in the cropping systems in Burkina Faso not only as a diversifcation crop but also as an intercrop with cereals for its nitrogen supply to the soil through its symbiotic nitrogen fxation, and nitrogen being one of the macroelements likely to be depleted in the soil with the removal of crops.

Data Availability
Te data collected during the experimentation to write this scientifc article are available and can be made available to readers.

Conflicts of Interest
Te authors declare that there are no conficts of interest.