Discussion

The concentrations of four microelements (zinc, copper, iron and manganese) and pod yield of 47 Dolichos bean genotypes varied considerably in the present study. Among micronutrients, Cu, Fe, Mn and Zn contents ranged from 10.10–19.95, 77.13–331.93, 22.78–46.40 and 42.03–102.85 mg kg⁻¹ in pole type, while 8.2–18.5, 50.8–99.3, 25.65–53.25 and 37.15–63.25 mg kg⁻¹ in bush type Dolichos bean, respectively (online Supplementary Table S2). In general, pole type genotypes revealed higher average contents for all the micronutrients except manganese. In their study with determinate and indeterminate soybeans, Araújo et al. (Reference Araujo, Santana, Mauad and da Silva2020) observed that the indeterminate varieties had a greater transfer of nutrients than the determinate cultivars. Alam et al. (Reference Alam, Ivy, Islam, Saha and Das2021) reported the Zn and Fe contents as high as 61.86 and 122.22 mg kg⁻¹, respectively. However, in our study, we report the highest Zn and Fe contents of 102.85 and 331.93 mg kg⁻¹, respectively in genotype VRSEM 1000 (online Supplementary Table S2). The magnitude of micronutrients in the present study was substantially higher than those reported by Kumari et al. (Reference Kumari, Naresh, Acharya, Laxminarayana, Singh, Raghu and Aghora2022); thereby, indicating the availability of superior germplasm in our genepool. Differences in micronutrient contents in pods are a function of a gamut of codependent metabolic processes, comprising nutrient uptake dynamics and nutrient partioning (Maganti et al., Reference Maganti, Swaminathan and Parida2020). Pole and bush type genotypes showed normal frequency distributions for most of the parameters studied except for Fe and yield, which did not distribute normally in pole type genotypes (online Supplementary Fig. S1). Some of the pole genotypes like VRSEM-1000, VRDB-01 and VRSEM 109 and bush type genotypes like VRBSEM-3, VRBSEM-35 and VRBSEM- 200 are good source of microelements as well as yield (online Supplementary Table S2 & S3). Such gene sources with enhanced nutrients may be used directly as cultivated forms or indirectly as input material for breeding new, more nutritionally rich varieties of Dolichos bean.

Outcomes of the micronutrients composition and yield in 47 Dolichos bean genotypes were estimated using PCA. In PC-1, Zn (0.545), Fe (0.494) and yield (0.418) contributed the maximum variation (Table 1). Association of some of the pole and bush genotypes in contiguity to each other can be understood by the fact that bush and pole types are cross compatible with each other and involvement of these genotypes in the ongoing breeding programs (online Supplementary Fig. S2). Findings from the loading plot diagram reveals the closer association i.e., smaller angle between the vectors of pod pigmentation, Fe and Zn (online Supplementary Fig. S3). This was in accordance with correlation analysis results that showed significant positive associations between Fe, Cu, Zn, pod colour and yield (Table 2). In our study, Fe and Zn showed strong positive correlation with pod pigmentation. A strong positive correlation between Cu, Fe and Zn concentrations of Dolichos beans in our experiment was concurrent with the findings of the studies by Karakoy et al. (Reference Karakoy, Erdem, Baloch, Toklu, Eker, Kilian and Ozkan2012) and Delfini et al. (Reference Delfini, Moda-Cirino, Dos Santos Neto, Buratto, Ruas and Azeredo Gonçalves2020) in lentil and common bean, respectively. Positive correlations were observed among most micronutrients. This correlation could be advantageous for breeding of nutritionally-superior genotypes as selection for enhancement in one nutrient may be encouraging for other nutrients simultaneously. Pods of Dolichos bean varieties with pigmentation or colouration were found to have higher Fe and Zn contents as evident by the data presented in Table 2. Earlier, Faiz et al. (Reference Faiz, Hanafi, Hakim, Rafii and Akmar Abdullah2015) reported that the pigmented rice genotypes have higher micronutrient contents as compared to white rice genotypes. Evaluation of Dolichos bean genotypes using multivariate analysis based on nutrient content grouped the genotypes into three clusters (online Supplementary Fig. S4). This analysis segregated a maximum of 42 genotypes in Cluster I followed by class II with three genotypes and class III with two genotypes each. Cluster III (VRDB-01 and VRSEM 1000) represents the genotypes, which have higher magnitude of micronutrients (Cu, Fe, Mn and Zn). Selection of parents from Cluster III followed by Cluster II can be utilized in further breeding programme for production of biofortified Dolichos beans. Practically, this means better parental heterosis (superior performance of hybrids over parents), where higher values for traits like micronutrients concentration indicate superior parents. Selection of identified genotypes, from different clusters based on desirable traits, as parent in future breeding programme may generate new recombinants with improved heterosis (Kumari et al., Reference Kumari, Naresh, Acharya, Laxminarayana, Singh, Raghu and Aghora2022).

Table 1. Eigenvalues and proportion of total variability and eigenvectors of three principal components (PCs) for studied Dolichos bean genotypes

Table 2. Coefficients of correlation (r) between concentrations of micronutrients and yield attributes recorded on 19 pole and 28 bush types Dolichos bean genotypes

^a Correlation is significant at the 0.01 level (2-tailed).

^b Correlation is significant at the 0.05 level (2-tailed).