Nickel Fertilization as Plant’s Foes or Friends?- Evaluation by Yield Attributes of Mash [Vigna mungo (L.) Hepper] Genotypes

For a plant, soil is a source of heavy metals which acts as the sink of heavy metals resourced from geogenic and anthropogenic activities. The part of soil which is connected with plant roots termed as ‘rhizosphere’ is zone where the physical, chemical and biological characteristics are different from rest of soil zones (Seshadri et al., 2015).
       
With the passage of time, environment is getting polluted with heavy metals imposing a serious threat for human and even for whole ecosystem (Asad et al., 2019; Maleki et al., 2017). Finally, after entering in the food chain, these metals become the sources of toxicity for ecosystem functioning (Budijono et al., 2017; Ali and Khan, 2019). In addition to soil, air and water are also being contaminated by heavy metals which ultimately affect the various trophic levels of food chain (Aendo et al., 2020; Biswas et al., 2019). Nickel (Ni) is a heavy metal with non-biodegradable properties, posing threats of environmental pollution and damaging biosphere and human health worldwide (Das et al., 2008). Anthropogenic activities like metal smelting, municipal sludge, industrial effluents, fertilizers and pesticides are the main sources of Ni (Fabiano et al., 2015).
       
Nickel (Ni) was included in the list of essential nutrients to plants in 1983 (Eskew et al., 1983). Ni fertilization can promote fixation, metabolism and assimilation of Nitrogen (Tan et al., 2000; Malavolta and Moraes, 2007; Khoshgoftarmanesh et al., 2011; Hosseini and Khoshgoftarmanesh, 2013; Dalir and Khoshgoftarmanesh, 2015; Uruç Parlak, 2016; Kutman et al., 2013, 2014; González-Guerreroet_al2014; Macedo et al., 2016).
       
Blackgram (Mash bean) is one of the important pulse crops belongs to the Papilionaceae family (Mohanlal et al., 2021). The crop has genetic diversity for its growth and yield (Kumar et al., 2021; Priya et al., 2021). Legume plants are efficient in biological nitrogen fixation (BNF). During nitrogen fixation in root nodules, ammonia formed is converted into ureides (Collier and Tegeder, 2012). Ureides are converted to urea which then is metabolized by urease in which Ni plays important role (Zrenner et al., 2006). Also, when nitrogenase reduces Nitrogen to ammonia, molecular hydrogen is produced which is re-oxidized by the hydrogenase enzyme requiring Ni (González-Guerreroet_al2014).
 
In plants, excess Ni leads to inhibition of cell division, compromises plant growth, alters photosynthesis and plant water status, inhibits of Calvin cycle enzyme activities, represses nitrogen metabolism and generates oxidative stress, as well as blocks absorption of other essential nutrients metals (Sreekanth et al., 2013; Pietrini et al., 2015; Georgiadou, 2018).
       
Mash bean [Vigna mungo (L.) Hepper], is an important pulse crops. In Pakistan, it is has high nutritive and economic value. While considering the dependence of mash bean on BNF and presence of Ni in soils, this study was hypothesized to find the dose of Ni for toxicity and fertilization mash bean genotypes.

The experiment was devised to evaluate the effects of various concentrations of Nickel on number of legumes plant^-1, number of grains fruit^-1 and total yield plant^-1(g) of four mash bean [Vigna mungo (L.) Hepper]. The experiment was conducted in Botanic Garden of Bahauddin Zakariya University, Multan, Pakistan during 2020. Soil, free from contaminants and loamy in texture, was filled in pots which were lined with polyethylene sheet. Seeds of four mash genotypes i.e., MASH 80, MASH 88, MASH 97 and MASH ES-1 were sown. The genotypes have their origin in Ayub Agricultural Research Institute (AARI), Faisalabad (Pakistan) and National Agricultural Research Centre (NARC) Islamabad (Pakistan). Five seeds sterilized with 0.1% (V/V) HgCl₂, similar in size and weights were germinated in each pot and thinning was performed after germination to maintain three seedlings in each pot to supply uniform nutrition in each pot. Normal irrigation and pesticide application were practiced for healthy growth of plants. For imposing metal pollution in soil, Nickel chloride of Sigma Aldrich, Japan was added to develop 15.0, 30.0, 45.0, 60.0, 75.0 and 90.0 mg kg^-1 Nickel concentrations after twenty days of sowing. Pots without the addition of metals salts acted as control. Pots were placed with complete randomization of treatments and genotypes by design with four times replicated. Yield and its contributing factors were recorded at the maturity of crop (90 days age). Four plants per treatment in each replicate of all genotype were randomly selected and number of legumes plant^-1, number of grains fruit^-1 and total yield plant^-1 (g) was determined. The data collected were analyzed for analysis of variance using COSTAT computer package (CoHort Software, Berkeley, CA). Duncan’s new multiple range test at 5% level of probability (Duncan, 1955) was used to compare means. Significant F values were tested for mean differences by LSD tests at 0.05% significance level, by using MSTAT-C Computer Statistical Programme.

Number of legumes plant^-1
 
As revealed by Duncan’s Multiple Range test (Table 1), Nickel adversely affected the frequency of legume differentiation and development to an extent according to the concentration and induced a substantial reduction in number of legumes. Concentrations ranges from 45 to 90 mg kg^-1 have been found to alter legume number significantly while the lower level did not revealed statistically any clear cut differences from control. Documented data for mean performance, reflecting the role Nickel played  in checking legume setting, revealed that maximum (70.570%) effect was by 90 mg kg^-1 and minimum (0.178%) by 25 mg kg^-1 applied Nickel. This reduction accompanying the accelerated level of Nickel was expressed in all the genotypes reaching at the peak when 90 mg kg^-1 Nickel was applied. Although not statistically justified, but to a considerable extent of 5.290% increase in legume number was found when 15 mg kg^-1 Nickel was added to soil for plants of MASH ES-1. Among the genotypes, MASH 88 revealed maximum (9.559) and MASH 80 revealed minimum (7.821) values. MASH 97 differed statistically by a value of 7.761% less than MASH 80.
 

 
Number of grains fruit^-1 
 
Increasing amount of Nickel appeared to be responsible for gradual reduction in grain development (Table 2). This inhibitory effect of Nickel was statistically clear at concentrations from 45 to 90 mg kg^-1 while reduction in grain number lower than statistical approach was detected and documented by the effect of low concentration. All the genotypes responded in a similar fashion. Nickel concentration of 15 mg kg^-1, when supplied to plants of MASH ES-1, revealed an upset of 4.133% increase over the control plants. Among the genotypes, differences were of non significant extent and MASH 97 revealed maximum (5.864) and MASH 80 revealed minimum (5.649) values.
 

 
Total seed yield plant^-1 (g)
 
Nickel supplement in the soil medium had a negative linear relation with yield plants^-1 (Table 3). Detrimental effects of metal were statistically obvious by its concentrations from 45 to 90 mg kg^-1 while the difference from untreated plants were barely detectable by imposition of lower concentration. Nickel rendered the plants less productive at all levels of its application and the effect was in a concentration dependent manner. Maximum effect for reduction (86.241%) was by 60 mg kg^-1 and minimum (2.058%) by 15 mg kg^-1 Nickel. As regard individual genotypic response, maximum effect in all the genotypes was by 60 mg kg^-1 but in MASH 80 the same was conceived by 30 mg kg^-1. On applying 15 mg kg^-1 Nickel to plants of MASH ES-1, the observations were excluded from the ongoing trend and an increase of 9.892% over control in yield was recorded. Among the genotypes, MASH 88 revealed maximum (3.002) value being most productive and MASH 80 revealed minimum (2.496) as the least productive.
 

       
Increasing concentration of metal decreased yield attributes (Tables 1-3). Many reports revealed that Ni toxicity significantly decreases the seed numbers, seed weight and total seed yield per plant (Tripathy et al., 1981). Stress mediated by Ni causes reductions in flowers and fruits density (Balaguer et al., 2002). As a whole, reductions in total yield of plant can be ascribed to poor plant growth, development and reduced supply of nutrients to the reproductive organs (Ahmad et al., 2007).
       
During the growth of plant differentiation of flower and fruit is accompanied by a set of physiological changes in plant. These changes are controlled by a set of internal and external environmental factors including supply of nutrients. Any change in these factors can influence directly the growth and finally the reproductive phase of plant (Arun et al., 2005). Heavy metal stress creates an imbalance in micro and macronutrient availability to plant. Nutrients levels change may influence the development of floral buds, flowers and fruit (Hayati et al., 1995).
       
The reduction in growth and ultimately in yield might also be due to decreased photosynthesis as a consequence of reduction in photosynthetic pigments under metal stress (Fargasova, 2001; Jose et al., 2017) and sink limitations (Brun and Betts, 1984). Metal toxicity induced senescence of flower and pod may reduce number of viable pods and seeds (Sharma and Dubey, 2005).
       
Reduction of cytokinin contents by metal might be responsible for growth and finally the yield reduction by inhibition of cell division and cell elongation. This also may cause a decline in nitrate reductase activity (Bueno et al., 1994). This reduction in nitrate reductase activity might be also due to nutrients limitations (Andrews et al., 1999; Pilipovic et al., 2019).
       
Heavy metals accumulations in floral organs effectively alter plant reproductive potential of a floral organ like anther, pistil and nectarines. Heavy metal can negatively affect pollen viability, pollen senescence, pollen germination and pollen tube growth (Xun et al., 2017; Tuna et_al2002).
       
Whenever, in the experiment the absence of decline in yield was found, could be attributed to the fact that low level of metal may just be accumulated in roots than in the shoot and the effect is restricted to the root only (Selvam and Wong, 2008).

The studies revealed that the application of low doses of Nickel acted as fertilizer and high doses created toxicity.

Authors have no conflict of interest and there was no funding body for research and manuscript.

Ghulam Yasin designed and conducted experiment. Adeela Altaf and Ikram ul Haq participated in manuscript drafting. Aleem A. Khan and Shahzadi Saima participated in proof reading and correction of final version of manuscript.