Introduction

Wide-spread zinc (Zn) deficiency prevails in different soils and crops of the world (Alloway 2008). Shukla and Behera (2019) recently reported Zn deficiency in 36.5% agricultural soils of India. Zinc deficiency is mainly found in saline, sodic, calcareous, Vertisols, deeply weathered, sandy, and water logged soils (Alloway 2009). Prominent soil parameters like higher soil pH, low total Zn content, higher clay content and higher concentration of Ca2+, Mg2+, Na+, HCO3− and PO43− in soil solution adversely influence Zn availability to plants. Moreover, growing of high-yielding crop cultivars, and insufficient or no application of Zn fertilisers and / or organic manures also lead to Zn deficiency in different soil-crop situations (Fageria et al. 2002). This requires need-based Zn application for sustainable crop production (Behera et al. 2020a).

Soil Zn deficiency negatively influences crop growth, yield and Zn concentration in edible plant parts, as it is a vital component of several proteins regulating plant metabolism (Broadley et al. 2007; Singh et al. 2019). Many researchers reported adverse effect of low availability of soil Zn on grain and stover yield, harvest index and grain and tissue Zn concentration of pigeonpea (Cajanus cajan L. Millsp.) (Khudsar et al. 2008; Narayan and Kumar 2015), which is a protein rich legume crop cultivated and consumed in tropical as well as subtropical regions of the world. Proper management of Zn in soil–plant systems enhances crop yield and alters micronutrients (Zn, copper (Cu), iron (Fe) and manganese (Mn)) concentration in grain (Fageria et al. 2002; Manzeke et al. 2019). Alternation of Cu, Fe and Mn concentration in crop plants due to Zn supply is because of interaction between nutrients (Robson et al. 1983). Application of Zn improved yield and influenced micronutrients concentration and uptake in straw/stover and grain of wheat and maize crops (Behera and Singh 2009; Behera et al. 2015). Zinc application enhanced Zn concentration in grain and stover of maize grown on Vertisols of central India. However, Zn application did not influence Cu, Fe and Mn concentration in maize grain and stover but it increased total Cu and Fe uptake by maize. Moreover, crop genotypes exhibit variable responses to Zn application due to variations in rooting pattern, release of organic acids and uptake and translocation of Zn (Mathpal et al. 2015; Singh et al. 2020). Dube et al. (2001) reported variations in responses to Zn application by pigeonpea genotypes of Ustifluvent soils of western Uttar Pradesh, India. However, limited information regarding the responses of pigeonpea genotypes (in terms of grain and stover yield, micronutrient concentration and uptake) grown on Vertisols of India to Zn application is available.

Zinc deficiency prevails in 1/3rd of the global population (Welch and Graham 2002). It can be alleviated sustainably by providing Zn-enriched food (in terms of Zn enriched seed/ grain) from agriculture to the Zn-malnourished population. Agronomic practices such as Zn addition to soil and/or to leaves mitigate soil Zn deficiency and enhance plant growth, yield and micronutrient concentration and uptake (Rengel et al. 1999). Application of Zn to soil ranges from 1.0 to 35.0 kg Zn ha−1 depending upon soil types and nature of crops (Alloway 2008). In comparison with Zn application to soil, foliar or both soil and foliar application of Zn were found to be more effective for augmenting Zn concentration and uptake by seed/grain of the crops (Hussain et al. 2012; Li et al. 2017; Cakmak and Kutman 2018; Haider et al. 2020). The information pertaining to the effect of soil and both soil and foliar application of Zn on yield and micronutrients concentration and uptake by pigeonpea genotypes is limited. It was therefore hypothesized that pigeonpea genotypes grown on Vertisols respond variably to soil and both soil and foliar application of Zn. Keeping the above facts in view, the present investigation was carried out to study the effect of soil and both soil and foliar application of Zn on yield, micronutrients concentration and uptake by pigeonpea genotypes grown on Vertisols of India.

Materials and methods

Experiment location and material

A field experiment was carried out during kharif seasons of 2013–14 and 2014–15 using twenty commonly grown pigeonpea genotypes (namely, C11, ICPL 87119, AKT 8811, PKV Trombay, Hisar Manak, Hisar Paras, Hisar HO2-60, Pusa 9, BDN 2, JKM 7, Virsa Arhar 1, SKNP 05–05, GAUT 93–17, DT 23, AAUT 2007–04, GT 101, T 15–15, BSMR 853, GT 1 and AAUT 2007–10) at the farm of Indian Institute of Soil Science, Bhopal (located at 23° 18′ N latitude, 77° 24′ E longitude and 485 m above mean sea level). The initial grain Zn concentration (mean ± standard error of mean) of pigeonpea genotypes varied from 27.8 ± 0.58 to 40.0 ± 1.47 mg kg−1. The experimental location experiences semi-arid and tropical climate and it receives mean annual rainfall of 1005 mm. Major portion of rainfall is received during June to September months. The location experiences mean monthly minimum (in the month of January) and maximum (in the month of May) temperature of 10 and 40 °C, respectively. The soil of the experimental site belongs to hyperthermic Typic Haplustert (Soil Survey Staff 2014). It is deep black clayey in nature and having pH 7.67 (Jackson 1973), electrical conductivity 0.15 dS m−1 (Jackson 1973), organic carbon 0.41% (Walkley and Black 1934), available P 5.20 mg kg−1 (Olsen et al. 1954), exchangeable K 252 mg kg−1 (Hanway and Heidel 1952),exchangeable Ca 439 mg kg−1 (Jones 1998), exchangeable Mg 125 mg kg−1 (Jones 1998), CaCl2 extractable S 8.50 mg kg−1 (Williams and Steinbergs 1959), diethylene triamine penta acetic acid (DTPA) extractable Zn 0.32 mg kg−1 (Lindsay and Norvell 1978), DTPA extractable Fe 8.43 mg kg−1 (Lindsay and Norvell 1978), DTPA extractable Cu 1.07 mg kg−1 (Lindsay and Norvell 1978), and DTPA extractable Mn 11.8 mg kg−1 (Lindsay and Norvell 1978).

Experiment details

The experiment was carried out in a split plot design with Zn application as main plot treatment and pigeonpea genotypes as sub plot treatments. There were 3 levels of Zn treatments i.e., no Zn, soil Zn and soil + foliar Zn. In no Zn treatment, no Zn fertilizer was applied. Under soil Zn treatment, 20 kg Zn ha−1 (as ZnSO4.7H2O) was applied as basal soil application. Under soil + foliar Zn treatment, 20 kg Zn ha−1 (as ZnSO4.7H2O) was applied as basal soil application along with additional three foliar applications of 0.50% (w/v) ZnSO4.7H2O aqueous solution at flowering, pod formation and pod filling growth stages. The pigeonpea genotypes were cultivated in plots (each plot is having 6 × 5 m2 area), with row to row spacing of 0.80 m and plant to plant spacing of 0.20 m. The plot of each genotype was separated from the plot of another genotype by a distance of 1 m. The crops were provided with 40 kg of N (as urea), 80 kg of P2O5 (as diammonium phosphate), 30 kg of K2O (as muriate of potash), and 40 kg of S (as bentonite S). Hand weeding was carried out as and when required to keep the field weed-free.

Plant sampling and analysis

At maturity, grain, stover and total above ground biomass (grain + stover) yield for each plot were recorded. Grain yield was recorded after threshing and sun-drying of harvested pods. Harvest index was estimated as ratio of grain yield to total biomass yield and expressed in per cent. Grain and stover samples were washed with double-distilled water for three times and then oven-dried at 75 °C for 48 h for estimation of micronutrients (Zn, Cu, Fe and Mn) concentration. Dried grain and stover samples were ground in a stainless-steel mill. Powdered grain and stover samples were digested in a di-acid mixture (nitric acid and perchloric acid mixture in 3:1 ratio). The concentration of Zn, Cu, Fe and Mn in digested material was determined by atomic absorption spectrometer (Make and model: Varian AA 240 FS). The Zn, Cu, Fe and Mn uptake in pigeonpea grain and stover and total uptake in above ground biomass were estimated as described below.

$$\tt {\text{Zn}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\; = \;{\text{Grain}}\;{\text{yield}}\;\left( {t \, ha^{ - 1} } \right)\;\times\;{\text{grain}}\;{\text{Zn}}\;{\text{concentration}}\;\left( {{\text{mg kg}}^{ - 1} } \right)$$
(1)
$$\tt {\text{Zn}}\;{\text{uptake}}\;{\text{in}}\;{\text{stover}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\; = \;{\text{Stover}}\;{\text{yield}}\;\left( {t{\text{ ha}}^{ - 1} } \right)\;\times\;{\text{stover}}\;{\text{Zn}}\;{\text{concentration}}\;\left( {{\text{mg}}\;{\text{kg}}^{ - 1} } \right)$$
(2)
$$\tt {\text{Total}}\;{\text{Zn}}\;{\text{uptake}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\; = \;{\text{Zn}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\; + \;{\text{Zn}}\;{\text{uptake}}\;{\text{in}}\;{\text{stover}}\;\left( {g{\text{ ha}}^{ - 1} } \right)$$
(3)
$$\tt {\text{Cu}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; = \;\;{\text{Grain}}\;{\text{yield}}\;\;\left( {t{\text{ ha}}^{ - 1} } \right)\;\times\;{\text{grain}}\;{\text{Cu}}\;{\text{concentration}}\;\;\left( {{\text{mg kg}}^{ - 1} } \right)$$
(4)
$$\tt {\text{Cu}}\;{\text{uptake}}\;{\text{in}}\;{\text{stover}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; = \;\;{\text{Stover}}\;{\text{yield }}\left( {t{\text{ ha}}^{ - 1} } \right) \, \times\;{\text{stover}}\;{\text{Cu}}\;{\text{concentration}}\;\left( {{\text{mg kg}}^{ - 1} } \right)$$
(5)
$$\tt {\text{Total}}\;{\text{Cu}}\;{\text{uptake}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\; = \;{\text{Cu}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; + \;\;{\text{Cu}}\;\;{\text{uptake}}\;\;{\text{in}}\;{\text{stover}}\;\left( {g{\text{ ha}}^{ - 1} } \right)$$
(6)
$$\tt {\text{Fe}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; = \;\;{\text{Grain}}\;{\text{yield}}\;\left( {t{\text{ ha}}^{ - 1} } \right)\;\times\;{\text{grain}}\;{\text{Fe}}\;{\text{concentration }}\left( {{\text{mg kg}}^{ - 1} } \right)$$
(7)
$$\tt {\text{Fe}}\;{\text{uptake}}\;{\text{in}}\;{\text{stover}}\;\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; = \;\;{\text{Stover}}\;{\text{yield}}\;\left( {t{\text{ ha}}^{ - 1} } \right)\;\times\;{\text{stover}}\;{\text{Fe}}\;{\text{concentration}}\;\left( {{\text{mg kg}}^{ - 1} } \right)$$
(8)
$$\tt {\text{Total}}\;{\text{Fe}}\;{\text{uptake}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; = \;\;{\text{Fe}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; + \;\;{\text{ Fe}}\;{\text{uptake}}\;{\text{in}}\;{\text{stover}}\;\left( {g{\text{ ha}}^{ - 1} } \right)$$
(9)
$$\tt {\text{Mn}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; = \;\;{\text{Grain}}\;{\text{yield}}\;\;\left( {t{\text{ ha}}^{ - 1} } \right)\;\times\;{\text{grain}}\;{\text{Mn}}\;{\text{concentration}}\;\left( {mg \, kg^{ - 1} } \right)$$
(10)
$$\tt {\text{Mn}}\;{\text{uptake}}\;{\text{in}}\;{\text{stover}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\;\; = \;\;{\text{Stover}}\;{\text{yield}}\;\left( {t{\text{ ha}}^{ - 1} } \right)\;\times\;{\text{stover}}\;{\text{Mn}}\;{\text{concentration}}\;\left( {{\text{mg kg}}^{ - 1} } \right)$$
(11)
$$\tt {\text{Total}}\;{\text{Mn}}\;{\text{uptake}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\; = \;{\text{Mn}}\;{\text{uptake}}\;{\text{in}}\;{\text{grain}}\;\left( {g{\text{ ha}}^{ - 1} } \right)\; + \;{\text{Mn}}\;{\text{uptake}}\;{\text{in}}\;{\text{stover}}\;\left( {g{\text{ ha}}^{ - 1} } \right)$$
(12)

Statistical analysis

The dataset was analyzed using analysis of variance (ANOVA) (Cochran and Cox 1957), to evaluate the differences between treatment means. The standard errors of differences (SED) were worked out from ANOVA table using SAS 9.2 software package (SAS 2011). The least significant difference (LSD) (better than other post hoc methods) was used for comparisons, where F-probabilities were significant (p < 0.05). The year-wise analysis of data was carried out and then pooled for 2-years, as the year-wise variations in data were found non-significant.

Results

The studied parameters differed significantly with Zn treatments and between the pigeonpea genotypes in both years whereas the interaction between Zn treatment and genotype was not significant (Table 1). Since, there was no significant difference in grain, stover and total biomass yield, harvest index, Zn, Cu, Fe and Mn concentration in grain and stover and Zn, Cu, Fe and Mn uptake in grain and stover and total uptake by pigeonpea genotypes between the years, the datasets for the 2-years were pooled for the statistical analysis.

Table 1 Analyses of variance for grain yield (t ha−1), stover yield (t ha−1), total biomass yield (t ha−1), harvest index (%), micronutrients (Zn, Cu, Fe and Mn) concentrations (mg kg−1) in grain and stover and grain, stover and total uptake (g ha−1) of micronutrients of pigeonpea genotypes for two years separately and for pooled data. LSD denotes least significant difference, * denotes significant at p < 0.05, NS denotes not significant, and CV denotes coefficient of variation
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Grain, stover and total biomass yield and harvest index

Soil application of Zn led to significant increase in mean grain, stover and biomass yield of genotypes by 15.2, 12.7 and 13.4%, respectively, over no Zn application (Table 2, Fig. 1). Whereas, soil + foliar application of Zn enhanced mean grain, stover and biomass yield of genotypes by 24.0, 29.4 and 28.6%, respectively. Soil and soil + foliar application of Zn did not significantly increase or decrease mean value of harvest indices of the genotypes (Table 2). The genotypes under soil and soil + foliar application of Zn, differed among themselves pertaining to grain, stover, biomass yield and harvest index. Soil application of Zn enhanced grain yield of all genotypes but significantly higher % increase in yield was recorded by JKM 7 (29.3%), SKNP 05–05 (24.6%), GAUT 93–17 (20.5%) and AAUT 2007–10 (38.5%). The % increase in grain yield under soil + foliar application of Zn was significantly higher for C 11 (37.1%), AKT 8811 (31.6%), Hisar Manak (38.3%), Hisar Paras (37.0%), Hisar HO2-60 (35.7%), JKM 7 (45.0%) and AAUT 2007–10 (49.3%).

Table 2 Grain yield (t ha−1), stover yield (t ha−1), total biomass yield (t ha−1) and harvest index (%) of pigeonpea genotypes under different Zn treatments. SED denotes standard error of difference, LSD denotes least significant difference, * denotes significant at p < 0.05, and NS denotes not significant
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Fig. 1
figure 1

Mean values of grain yield (t ha−1), and stover yield (t ha−1) of pigeonpea genotypes under different Zn treatments. Error bars indicate standard error of mean. Two means having different letters differ significantly for each parameter

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Zinc concentration and uptake

Soil application of Zn did not lead to significant increase in mean grain Zn concentration of pigeonpea genotypes compared to no Zn application (Fig. 2). But grain Zn concentrations of different genotypes under soil application of Zn differed significantly (Fig. 3). Soil + foliar application Zn enhanced mean grain Zn concentration of genotypes significantly. The higher % increase in grain Zn concentration was recorded for AKT 8811 (7.00%), PKV Trombay (9.10%), Pusa 9 (15.0%), Virsa Arhar-1 (9.30%), DT 23 (16.6%) and AAUT 2007–10 (13.6%), under soil Zn application. Whereas, C 11(39.1%), ICPL 87119 (44.4%), PKV Trombay (72.2%), Hisar Manak (53.0%), Hisar HO2-60 (38.4%), GAUT 93–17 (36.9%) and DT 23 (42.5%) had higher % increase in grain Zn concentration under soil + foliar application of Zn. On the other hand, both soil and soil + foliar application of Zn significantly enhanced mean stover Zn concentration of genotypes (Fig. 2). Soil and soil + foliar application of Zn significantly enhanced mean grain, stover and total Zn uptake by pigeonpea genotypes compared to no Zn application (Fig. 3). The grain, stover and total Zn uptake by the genotypes, under soil and soil + foliar application of Zn, differed among themselves. Genotypes Pusa 9 (27.2%), JKM 7 (28.5%), Virsa Arhar 1 (28.8%), DT 23 (24.5%) and AAUT 2007–10 (57.4%) had significantly higher % increase in grain Zn concentration under soil application of Zn. Whereas, significantly higher % increase in grain Zn concentration was recorded for C 11(90.7%), PKV Trombay (113%), Hisar Manak (112%), Hisar HO2-60 (87.8%), JKM 7 (84.9%) and AAUT 2007–10 (97.9%) under soil + foliar application of Zn.

Fig. 2
figure 2

Mean values of grain Zn concentration (mg kg−1) and stover Zn concentration (mg kg−1) of pigeonpea genotypes under different Zn treatments. Error bars indicate standard error of mean. Two means having different letters differ significantly for each parameter

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Fig. 3
figure 3

Grain Zn concentration (mg kg−1), stover Zn concentration (mg kg−1), grain Zn uptake (g ha−1), stover Zn uptake (g ha−1), and total Zn uptake (g ha−1) of pigeonpea genotypes under different Zn treatments

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Copper concentration and uptake

Both soil and soil + foliar application of Zn significantly reduced mean grain Cu concentration of pigeonpea genotypes by 9.42 and 10.5%, respectively, and mean stover Cu concentration of genotypes by 23.3 and 32.2%, respectively, compared to no Zn application. The Cu concentration (in grain and stover) and Cu uptake (by grain, stover and total uptake) by the genotypes differed significantly among themselves under both soil and soil + foliar application of Zn (Fig. 4). There was non-significant decline in grain Cu concentration of C 11, AKT 8811, Hisar Manak, Virsa Arhar 1 and DT 23 under soil application of Zn. Under soil + foliar application of Zn, C 11, Hisar Paras, GAUT 93–17, GT 101 and GT 1 had non-significant reduction in grain Cu concentration. The genotypes ICPL 87119, BDN 2, Virsa Arhar 1 and AAUT 2007–10 had significantly higher grain Cu uptake under both soil and soil + foliar application of Zn.

Fig. 4
figure 4

Grain Cu concentration (mg kg−1), stover Cu concentration (mg kg−1), grain Cu uptake (g ha−1), stover Cu uptake (g ha−1), and total Cu uptake (g ha−1) of pigeonpea genotypes under different Zn treatments

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Iron concentration and uptake

Application of Zn (both soil and soil + foliar) did not significantly increase or decrease mean Fe concentration in grain and stover of pigeonpea genotypes compared to no Zn application. But the mean grain Fe uptake by the genotypes significantly increased under soil and soil + foliar application of Zn. Iron concentration (in grain and stover) and Fe uptake (by grain, stover and total uptake) by the genotypes differed under soil and soil + foliar application of Zn (Fig. 5). Significantly higher grain Fe uptake was recorded for ICPL 87119, BDN 2, JKM 7, Virsa Arhar 1 and AAUT 2007–10 under both soil and soil + foliar application of Zn.

Fig. 5
figure 5

Grain Fe concentration (mg kg−1), stover Fe concentration (mg kg−1), grain Fe uptake (g ha−1), stover Fe uptake (g ha−1), and total Fe uptake (g ha−1) of pigeonpea genotypes under different Zn treatments

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Manganese concentration and uptake

Soil and soil + foliar application of Zn did not significantly increase or decrease mean Mn concentration in grain and stover of pigeonpea genotypes compared to no Zn application. However, mean grain, stover and total Mn uptake by the genotypes significantly increased under soil and soil + foliar application of Zn. The Mn concentration (in grain and stover) and Mn uptake (by grain, stover and total uptake) by the genotypes under soil and soil + foliar application of Zn varied significantly (Fig. 6). Significantly higher grain Mn uptake was recorded for ICPL 87119, BDN 2, JKM 7, Virsa Arhar 1, SKNP 05–05, GAUT 93–17, DT 23 and AAUT 2007–10 under both soil and soil + foliar application of Zn.

Fig. 6
figure 6

Grain Mn concentration (mg kg−1), stover Mn concentration (mg kg−1), grain Mn uptake (g ha−1), stover Mn uptake (g ha−1), and total Mn uptake (g ha−1) of pigeonpea genotypes under different Zn treatments

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Relationship of grain and stover yield with micronutrients concentration in grain and stover

Pearson’s correlation coefficient analysis revealed positive and significant relation of grain yield with stover yield (r = 0.728 p < 0.01) and negative and significant relation of grain yield with Cu concentration in grain (r = −0.519 p < 0.01) and stover (r = −0.349 p < 0.01) of pigeonpea genotypes (Table 3). Stover yield was negatively and significantly correlated with grain (r = −0.499 p < 0.01) and stover (r = −0.353 p < 0.01) Cu concentration and grain Mn concentration (r = −0.478 p < 0.01). Grain Zn concentration was positively and significantly correlated with stover Zn concentration (r = 0.603 p < 0.01), grain Fe concentration (r = 0.469 p < 0.01), grain (r = 0.426 p < 0.01) and stover (r = 0.586 p < 0.01) Mn concentration and negatively and significantly correlated with stover Cu concentration (r = −0.342 p < 0.01). Stover Zn concentration was negatively and significantly correlated with stover Cu concentration (r = −0.391 p < 0.01) but it was positively and significantly correlated with grain (r = 0.305 p < 0.05) and stover (r = 0.286 p < 0.05) Fe concentration and stover Mn concentration (r = 0.404 p < 0.01). There was positive and significant relation (r = 0.438 p < 0.01) between grain and stover Cu concentration. Grain Fe concentration was positively and significantly correlated with grain (r = 0.421 p < 0.01) and stover (r = 0.409 p < 0.01) Mn concentration. Stover Mn concentration was positively and significantly correlated with stover Fe concentration (r = 0.258 p < 0.05) and grain Mn concentration (r = 0.434 p < 0.01).

Table 3 Relationship among the grain yield, stover yield, and Zn, Cu, Fe and Mn concentration (conc) in grain and stover of pigeonpea genotypes (n = 60)
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Discussion

Yield

Soil and soil + foliar application of Zn significantly increased mean grain, stover and total biomass yield of pigeonpea genotypes compared to no Zn application (Table 2, Fig. 1). Soil + foliar application of Zn resulted in significantly higher yield of the genotypes compared to soil application of Zn. Several researchers also recorded the response of pigeonpea to Zn application (Shukla and Raj 1980; Dube et al. 2001; Behera et al. 2020a). Zinc plays important roles in different enzymatic activities. It is involved in plant metabolism needed for proper growth and development, especially biomass production and translocation of photosynthates to grain (Marschner, 2012). Increase in grain, stover and total biomass yield in pigeonpea genotypes with soil Zn application is because of increased absorption of Zn by plant roots from soil solution. However, soil + foliar application of Zn leads to further increase in Zn absorption by the genotypes from plant leaves and stem in addition to plant roots. Soil application of Zn is recommended for enhancing yield of pigeonpea gown on Zn-deficient Vertisols. Soil + foliar application of Zn in pigeonpea is recommended for better Zn absorption and utilisation.

The differences in responses of the pigeonpea genotypes towards soil and soil + foliar application of Zn are attributed to the variations in Zn efficiency, and physiological and biochemical mechanisms responsible for efficiency of the genotypes (Graham et al. 1993; Takkar et al. 1993). Under soil application of Zn, genotypes ICPL 87119, BDN 2, JKM 7, Virsa Arhar 1, SKNP 05–05, GAUT 93–17, DT 23, BSMR 853 and AAUT 2007–10 had significantly higher grain yield (> 2.0 t ha−1) than the other genotypes. Among these, JKM 7, SKNP 05–05, GAUT 93–17 and AAUT 2007–10 were promising as they recorded higher per cent increase in grain yield under soil Zn application compared to no Zn application. Farmers need to grow these genotypes for higher grain yield under soil application of Zn. The genotypes C 11, ICPL 87119, AKT 8811, BDN 2, JKM 7, Virsa Arhar 1, SKNP 05–05, GAUT 93–17, DT 23, BSMR 853 and AAUT 2007–10 had significantly higher grain yield (> 2.0 t ha−1) compared to other genotypes under soil + foliar application of Zn. However, C 11, AKT 8811, JKM 7 and AAUT 2007–10 were promising as these genotypes had both significantly higher grain yield and per cent increase in grain yield under soil + foliar application of Zn. Therefore, it is recommended to choose the genotypes of pigeonpea for cultivation depending upon the available resources for Zn application.

Micronutrients concentration

Soil application of Zn did not enhance mean grain Zn concentration of pigeonpea genotypes compared to no Zn application (Fig. 2). But grain Zn concentration of the genotypes varied significantly under soil Zn application, and the genotypes AKT 8811, Hisar Manak, Hisar Paras, Hisar HO2-60, Virsa Arhar 1, DT 23 and T 15–15 recorded significantly higher grain Zn concentration compared to others (Fig. 3). Soil + foliar application of Zn increased mean grain Zn concentration of the genotypes by 10 to 72% with mean value of 33%. The genotypes C 11, ICPL 87119, PKV Trombay, Hisar Manak, Hisar HO2-60, GAUT 93–17, DT 23 and T 15–15 had higher grain Zn concentration and % increase in grain Zn concentration under soil + foliar application of Zn.

Our finding regarding the effectiveness of soil + foliar application of Zn for enhancing grain Zn concentration in pigeonpea genotypes is in line with the observations recorded by several other researchers in wheat (Yang et al. 2011; Saha et al. 2017; Zhao et al. 2020; Wang et al. 2020) and mungbean (Haider et al. 2020). Under foliar application, Zn is easily absorbed by leaves and translocated to developing grains through phloem (Curie et al. 2009). Soil Zn application did not enhance grain Zn concentration but increased grain yield. Therefore, soil + foliar application of Zn is recommended for obtaining higher grain yield as well as higher grain Zn concentration (Yilmaz et al. 1997; Ranjbar and Bahmaniar 2007). The variation in increase in grain Zn concentration in pigeonpea genotypes with soil + foliar application of Zn is ascribed to the differences in genetic ability of the genotypes to transfer Zn from stover portion to grain. Transfer coefficients (TC) (estimated as ratio of grain Zn concentration to stover Zn concentration) of the pigeonpea genotypes indicating transfer of Zn from stover to grain, also varied from 0.70 to 2.60 (data not provided). Genotypes C 11, Hisar Manak, Hisar HO2-60, BDN 2, JKM 7, Virsa Arhar 1, GAUT 93–17, DT 23, AAUT 2007–04, GT 101, T 15–15, and AAUT 2007–10 had higher TC value (than mean value of 1.57). Both soil and soil + foliar application of Zn enhanced stover Zn concentration of the genotypes. The % increase in stover Zn concentration was higher than the % increase in grain Zn concentration of pigeonpea genotypes with soil and soil + foliar application of Zn. This indicates lower mobility of Zn from plant parts (stover) to grain. This is probably due to higher number of negative charges in the apoplastic spaces limiting the translocation and moderate phloem mobility of Zn (Marschner, 2012; Fernández and Brown, 2013).

Soil and soil + foliar application of Zn led to decline in mean Cu concentration in grain and stover of pigeonpea genotypes compared to no application of Zn (Fig. 4). Moreover, grain and stover Zn concentrations were negatively and significantly correlated with stover Cu concentration (Table 3). This is in line with the findings of Imitaz et al. (2003) who recorded adverse effect of Zn application on Cu concentration in wheat plant tissue. This is attributed to the competition between Zn2+ and Cu2+ ions for the same site for absorption into plant roots. The genotypes AKT 8811, Hisar Manak, Virsa Arhar 1 and DT 23 had higher grain Cu concentration with non-significant decline in grain Cu concentration under soil application of Zn. Under soil + foliar application of Zn, C 11, Hisar Paras, GAUT 93–17, GT 101 and GT 1 had higher grain Cu concentration with non-significant decline in grain Cu concentration.

Soil and soil + foliar application of Zn did not influence mean Fe and Mn concentration of grain and stover of genotypes. The genotypes also did not differ in grain and stover Fe and Mn concentration with soil and soil + foliar application of Zn. However, there were positive and significant relations of grain Zn concentration with grain Fe and Mn concentration and of stover Zn concentration with stover Fe and Mn concentration. The contributions of grain Zn concentration towards grain Fe and Mn concentration were 22% (R2 = 0.2199) and 18% (R2 = 0.1811), respectively (Fig. 7). But the contributions stover Zn concentration towards stover Fe and Mn concentration were 8% (R2 = 0.082) and 16% (R2 = 0.1634), respectively. Pahlavan-Rad and Pessarakli (2009) and Wang et al. (2015) also recorded increased Fe concentration in wheat grain due to foliar application of Zn. This is attributed to the better transportation and utilization of Fe and Mn at increased level of Zn application.

Fig. 7
figure 7

Relationships of grain Zn concentration with grain Fe and Mn concentration and stover Zn concentration with stover Fe and Mn concentration

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Micronutrients uptake

Soil and soil + foliar application of Zn increased mean grain Zn, Fe and Mn uptake by pigeonpea genotypes compared to no Zn application (Fig. 8). Soil + foliar application of Zn had higher mean grain Zn, Fe and Mn uptake compared to soil Zn application. The grain Zn, Cu, Fe and Mn uptake by the genotypes were different under different treatments. This is attributed to the differences among the genotypes for adaptation to different levels of Zn supply and physiological and biochemical mechanisms involved therein (Loneragan et al. 1993; Yang et al. 1999). The genotypes ICPL 87119, BDN 2, JKM 7, Virsa Arhar 1, DT 23 and AAUT 2007–10 had significantly higher grain Zn uptake and harvest index compared to other genotypes under soil application of Zn. These genotypes need to be cultivated with soil Zn application for production of Zn-enriched pigeonpea grain for tackling Zn malnutrition in human. Along with higher gain Zn uptake, these genotypes had significantly higher grain Cu, Fe and Mn uptake. The genotypes ICPL 87119, Hisar Manak, Hisar HO2-60, BDN 2, JKM 7 and Virsa Arhar 1 could be considered for production of higher Zn-enriched grain under soil + foliar application of Zn, as these genotypes had comparatively higher grain Zn uptake. Thus, identified genotypes could be cultivated with soil or soil + foliar application of Zn for production of pigeonpea grain with higher micronutrients (especially Zn) uptake, depending upon the availability of resources with the farmers.

Fig. 8
figure 8

Mean values of grain Zn, Cu, Fe and Mn uptake (g ha−1) by pigeonpea genotypes under different Zn treatments. Error bars indicate standard error of mean. Two means having different letters differ significantly for each parameter

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Conclusions

Both soil and soil + foliar application of Zn enhanced grain yield and grain Zn uptake of pigeonpea genotypes, grown on Zn-deficient soil, which could be helpful in improving human nutrition. Soil Zn application alone may bring sufficient benefits to the pigeonpea farmers due to increased grain yield. However, further benefits could be obtained in terms of additional increase in grain yield and grain micronutrient uptake, by adopting soil + foliar application of Zn, which obviously requires higher workload (for foliar application), time and cost involvement and interference with the calendar of other crops. The genotypes responded differently to soil and soil + foliar application of Zn. The pigeonpea farmers need to grow the identified genotypes with appropriate method of Zn application depending on the availability of resources for obtaining higher grain yield as well as micronutrients enriched grain to tackle human malnutrition. The plant breeders could utilise the genotypes for identification and use of Zn response traits for plant breeding programme. Future research needs to be conducted for different soil-crop situations for standardising Zn application methods and identification of better-performing crop genotypes for obtaining higher crop yield as well as Zn-enriched produce.