Micronutrient diversity in pole and bush type Dolichos bean germplasm: identification of gene sources from biofortification perspectives

Abstract

Exploring the nutritional potential of underutilized legumes such as Dolichos bean (Lablab purpureus L.) is of great significance, particularly, in view of accomplishing the United Nation's Sustainable Development Goal number two, which emphasizes on improving food and nutrition security by 2030. A thorough understanding of genetic variability is crucial for developing biofortified cultivars of Dolichos bean. In this study, the Dolichos bean genotypes represented by pole and bush types (28 bush and 19 pole types) were assessed for genetic variability for Cu, Mn, Fe and Zn contents. Pole type genotypes had higher average contents for all micronutrients except manganese. Among micronutrients, Cu, Fe, Mn and Zn, content 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, and 8.2–18.5, 50.8–99.3, 25.65–53.25 and 37.15–63.25 mg kg⁻¹ in bush type beans, respectively. Strong positive correlations between Cu, Fe and Zn concentrations occurred, which indicates the possibility of simultaneous improvement of these nutrients. Pod pigmentation was positively correlated with contents of Fe and Zn. There was an association of micronutrients with yield. The pole genotypes VRSEM-1000, VRDB-01 and VRSEM 109 and bush type genotypes VRBSEM-3, VRBSEM-35 and VRBSEM- 200 are good source of microelements and high yielders. Gene sources with enhanced nutrients may be used as cultivated forms or as input material for breeding nutritionally rich biofortified varieties of bean.

Introduction

Micronutrients are vital to healthy life style and wellbeing. Micronutrient deficiencies, responsible for 7% of the global disease burden (Muthayya et al., 2013), affect over 2 billion people globally. Almost half of the micronutrient-deficient population resides in India (Yadav and Maurya, 2022). Fe and Zn deficiencies are widespread, affecting crucial metabolic functions like cell growth, DNA replication and protein synthesis. Copper (Cu) and manganese (Mn) act as enzyme cofactors, supporting growth, cardiovascular health, neuroendocrine function. About 60% of world population is Iron (Fe) -deficient and 33% is zinc (Zn) -deficient (Wessells and Brown, 2012). Furthermore, Indians diets are also limiting in both iron and zinc (Anand et al., 2014).

Legume crops are an important component of human diet. Therefore, it is imperative to explore potentials of underutilized climate-resilient legumes such as Dolichos bean, also known as Indian bean or Lablab bean, in order to avert micronutrient deficiencies among the under-resourced communities. It is seen as one of the most resilient legumes against both biotic and abiotic factors (Naeem et al., 2020); however, it is less popular as human food despite being a nutritious legume (Minde et al., 2020). According to growth habits, Dolichos beans are of two types' viz., Pole and Bush or Determinate type (Naeem et al., 2020); however, both types are easy to grow and taste alike. A thorough understanding of nature and extent of genetic variability is crucial for developing biofortified cultivars (Patel et al., 2022) of Dolichos bean through systematic breeding efforts. Therefore, genetic variability has been investigated for Fe, Zn, Cu and Mn contents in Dolichos bean.

Experimental

Dolichos bean genotypes (28 bush and 19 pole types) were grown under field condition employing definitive cultural practices recommended by the Institute. These genotypes were obtained from various sources (online Supplementary Table S1). The experiment was conducted in randomized complete block design with three replications during 2019–20 and 2020–21. Ten plants were randomly chosen for data recording. The pods were harvested at edible maturity for recording yield per plant (kg). Visual observations recorded growth habits and pod anthocyanin pigmentation. For estimation of micronutrients, the procedures described by Paul and Srivastava (2007) were followed. Contents were expressed in mg/g DW. The results are expressed as means with standard error (S.E.). The significance difference (at P < 0.05) between genotypes was determined by Duncan's multiple range tests at a significance level of 0.05 for all the parameters. Pearson correlation and cluster analysis were done by using SPSS. 16.0 (SPSS Inc., Chicago, IL, USA). Furthermore, Minitab 14 software was used to conduct principal component analysis (PCA) and normal frequency distribution.

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. (2020) observed that the indeterminate varieties had a greater transfer of nutrients than the determinate cultivars. Alam et al. (2021) 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. (2022); 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., 2020). 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. (2012) and Delfini et al. (2020) 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. (2015) 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., 2022).

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).