Application of TXRF in the Assessment of Essential Micronutrient Levels in Common Beans (Phaseolus vulgaris)

In this study, essential micronutrient concentrations in common beans (Phaseolus vulgaris) from Muguga, Kiambu County and Kyevaluki, Machakos County were analysed using Total reflection X-ray Fluorescence (TXRF) and Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES). Samples of both bean leaves and dry grains were collected. The results were then assessed for nutritional quality in comparison with sufficiency ranges for high quality yielding bean crops, that is, the comparisons were made from a plant nutrition perspective. A comparison was also made between TXRF and ICP-OES as interchangeable methods of analysis.

The bean leaves from Muguga analysed by TXRF gave mean concentration values of 214 ± 52, 758 ± 219, 2 ± 0.77, 9 ± 1.8 and 65 ± 8.9 mg kg-1 for Mn, Fe, Ni, Cu and Zn respectively.  The Mn, Ni and Cu concentrations were within the sufficiency ranges while 6.2% of the samples had Zn concentrations higher than the sufficiency range. Fe concentrations were consistently high with 97% of the samples having concentrations that were higher than the sufficiency range. 50% of these samples gave Fe concentrations that were higher than the FAO recommended toxic levels of 800 mg kg-1. To compare TXRF and ICP-OES methods of analysis, the samples from Muguga analysed by TXRF were further analysed by ICP-OES. To compare the results obtained by both methods, student t distribution and Analysis of Variance (ANOVA) were used. It was observed that both methods produced results that were similar. This meant that the two methods can be used interchangeably for the analysis of trace elements in plants and especially beans.

The bean leaves from Kyevaluki gave mean concentrations of 76 ± 16, 218 ± 65, 1.5 ± 0.2, 8 ± 0.8 and 27 ± 4 mg kg-1 for Mn, Fe, Ni, Cu and Zn respectively and all these samples were found to have concentrations that were within sufficiency ranges.  In Kenya, the major form in which beans are consumed is the dry grains with less people using bean leaves as vegetables. From this study however, it was found that the analysed bean leaves had high levels of essential micronutrients and thus can be recommended to be widely used as vegetables.

Samples of dry bean grains were also analysed in which seven different local species were analysed. In the analysis of these dry bean grains, which are main bean parts consumed in Kenya, deficiencies were observed in which four bean types, that is Kyevaluki nyayo, Muguga red haricot, Muguga rosecoco and Muguga pinto bean were found to have Mn concentrations lower than the sufficiency range for high quality beans. All the analysed bean species were found to be deficient in Zn. Fe concentrations were above the sufficiency ranges for all the beans except Muguga nyayo but not to toxic levels. 29% of the samples had Cu concentrations that were within the sufficiency range. Fe concentrations were highest in all the samples followed by Mn and then Zn and Cu while the Ni concentrations were the lowest. Therefore, all the samples followed the expected range of Fe>Mn>Zn>Cu>Ni.

Since dry bean grains are consumed in many households in Kenya and deficiencies were observed, interventions need to be made on how to improve their micronutrient levels. This study was done from a crop nutrition perspective and thus further studies can be carried out on the bioavailable micronutrient levels in beans. Further study can also be carried out on associated soils since plant nutrient uptake also depends on soil factors like organic matter and pH.  

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