Influence of mineral zinc application on plant zinc biofortification, protein content, yield and nodule activity of bush bean genotypes.
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Legumes contribute considerably to human dietary mineral requirements and proteins in Sub-Saharan Africa (SSA). Bush bean (Phaseolus vulgaris L.) in particular, is a key staple crop in the region. The crop is recognised as a major avenue for accessing vital micronutrients such as Zinc (Zn) through consumption of tender leaves and pods, and grain. Bean genotypes with unique capacity to biofortify (accumulate) Zn in consumable plant parts exist in different germplasm in a few institutions in east Africa. However, the effect of applied Zn on the nutritional components such as protein and tissue Zn, remain to be investigated. Additionally, the performance of beans in terms nodule activity as a result of Zn accumulation needs to be evaluated. In light of the above scenario, a greenhouse study was conducted at the National Crops Resources Research Institute (NaCRRI), Namulonge in Uganda, to investigate the influence of applied zinc on the content of Zn and protein in leaves and the grain in Zn biofortifier bean genotypes, also known as Zn dense beans. Treatments included Zn at rates of 0, 5, 7.5 and 12.5 mg pot-1 containing 5 kg of soil; and four bush bean Zn biofortifier genotypes, namely; KaboF6-2.8-27, NUA69, NUA99 and RWR2154(37). The study, was laid out in a completely randomised design with three replications. The experiment was repeated three times. Results of this study showed that, here was a significant interaction (p<0.05) among Zn rates and genotypes in terms of content of tissue Zn and protein, nodule activity and grain yield. Zinc application elevated leaf Zn more especially in KaboF2.8-27 bean genotype. Tissue Zn content responded considerably and positively to Zn application, with KaboF2.8-27 superseding the other genotypes with the highest of 43 mg kg-1 of grain and 44.3 mg kg-1 in leaf, in the Zn rate of 7.5 mg pot -1. There was also a slight but significant (P≤0.05) increase in leaf protein, resulting from application of Zn; with the highest level occurring in KaboF2.8-27 genotype at the Zn rate of 7.5 mg pot-1. Seed protein was slightly, but positively influenced (P≤0.05) by Zn application; though KaboF2.8-27 again responded most especially at the Zn rate of 7.5 mg pot-1. Nodule activity was strongly enhanced (P≤0.05) by Zn application, reaching the climax at the Zn rate of 7.5 mg pot-1 for all genotypes. In fact, there was also a positive correlation (r=0.92) between leaf Zn and nodule activity for the study genotypes. The relationship between leaf Zn and leaf protein was also strong and positive (r =0.76). Similarly, Seed Zn and seed protein had a strong and positive correlation (r=0.79). Leaves at vegetative stage accumulated more Zn than the seeds at maturity. Overall, genotype Kabo6F2.8-27 emerged superior to the other genotypes in terms of tissue Zn accumulation and protein yield. Consumption of Zn biofortified beans will considerably contribute towards overcoming Zn nutritional problems among the populations of Africa especially Uganda.