Agronomic biofortification of selected underutilised solanaceae vegetables for improved dietary intake of potassium (K) in Ghana

Abstract

Agronomic biofortification is the deliberate use of mineral fertilizers to increase the concentration of a target mineral in edible portions of crops to increase dietary intake of the target mineral. Globally, increased dietary intake of potassium (K) is becoming a part of the strategy to address hidden hunger and related non-communicable diseases such as hypertension and cardiac disorders. This study aimed at demonstrating the efficacy of increasing the concentration of K in the edible portions of three commonly consumed but underutilized solanacea vegetables (Solanum aethiopicum, S. macrocarpon and S. torvum) in Ghana. The effects of different types and rates of K fertilizer application on the leaf- and fruit-K contents of the vegetables, as well as the K loss between the raw and cooked fruits were investigated. Five levels of each of three types of K fertilizer (liquid drench of potassium chloride, granular Muriate of potash and Sulphate of potash) were applied to each of the three field-grown vegetables. Yield data were collected and the fruits and leaves were analysed for the content of K, N, P, Ca, Fe, Zn and Cu. The results showed the rate of fertilizer application had significant effect on the yields of S. aethiopicum and macrocarpon but the yield of S. torvum was significantly affected by type, rate and interactive effect of type and rate of fertilizer application. Fruit K concentrations were greatest for S. aethiopicum (2130 mg K kg^-1 DW) and S. torvum (1883 mg K kg^-1 DW) with liquid KCl but with Sulphate of Potash for S. macrocarpon (1801 mg K kg^-1 DW). There were higher K concentrations in leaves than in fruits of all the vegetables. Household cooking of the fruits resulted in the retention of over 70% of the K content in the raw fruits. Potassium fertilization increased the Ca, Fe, and Zn contents of S. aethiopicum and S. torvum. It is concluded that agronomic biofortification may be a useful strategy to increase K intakes and other important elements (e.g. Fe and Zn) in the vegetables studied.

Keywords: Agriculture.

Fig. 1

Effect of type and rate of application of K fertilizer on fruit yield of (A) Solanum aethiopicum; (B) S. macrocarpon; (C) S. torvum. The error bars are standard errors. Differences in fruit fresh weight between fertilizers and rates of application were established using ANOVA and are shown by l.s.d. (P < 0.05), with ‘Fert’ representing type of K fertilizer, ‘Rate’ representing rate of fertilizer application, and ‘Fert × Rate’ representing the interaction of type of K fertilizer and rate of fertilizer application. (D) Relationship between rate of fertilizer application and overall fruit fresh weight for all the three crops, with trend models fitted. Control (no K fertiliser); K100/5 (100 kg granular K²O ha⁻¹ or 100 granular kg KCl ha⁻¹ or 5 mM KCl); K200/10 (200 kg granular K²O ha⁻¹ or 200 granular kg KCl ha⁻¹ or 10 mM KCl); K300/15 (300 kg granular K²O ha⁻¹ or 300 granular kg KCl ha⁻¹ or 15 mM KCl); K450/20 (450 kg granular K²O ha⁻¹ or 450 granular kg KCl ha⁻¹ or 20 mM KCl).

Fig. 2

Concentration of K in leaves (A, C) and fruits (B, D), depending on type of solanaceae vegetable (A, B) and type of K fertilizer applied (C, D) (n = 4). (E) Influence of fertilization with K on leaf and fruit K concentration in solanaceae vegetables grown in a field under rain-fed conditions (±s.e.m).

Fig. 3

Concentration of K in leaves (A, C, E) and fruits (B, D, F), depending on (A, B) the interaction of the crop and the type of K fertilizer applied; (C, D) the interaction of crop and rate of K fertiliser applied; (E, F) the interaction of the type of K fertilizer applied and rate of application. Aeth.: S. aethiopicum; Marc.: S. macrocarpon; Torv.: S. torvum. Data are the mean of 4 plants, with error bars representing the s.e.m. Differences between crop, K fertilizer type and rate of application were established using ANOVA and are shown by l.s.d. (P < 0.05), with ‘Crop’ representing type of solanaceae vegetable, ‘Fert’ representing type of K fertilizer, ‘Rate’ representing rate of K fertilizer application, and their respective interactions indicated by ‘x’.

Fig. 4

Concentration of K in leaves (A) and fruits (B), depending on the interaction of the crop, the type of K fertilizer applied and rate of K fertiliser application. Data are the mean of 4 plants, with error bars representing the s.e.m. Differences between crop, K fertilizer type and rate of application were established using ANOVA and are shown by l.s.d. (P < 0.05), with ‘Crop’ representing type of solanaceae vegetable, ‘Fert’ representing type of K fertilizer, ‘Rate’ representing rate of K fertilizer application, and their respective interactions indicated by ‘x’.

Fig. 5

The K concentration in cooked fruits as affected by (A) type of solanaceae vegetable, (B) the K soil or foliar fertilizer treatments; (C) K concentration of cooked and raw fruits of three solanaceae vegetables averaged across all treatments.

Fig. 6

Effect of crop on fruit and leaf (A) N, (B), P, (C), Ca, (D) Fe, (E) Cu, and (F) Zn concentration of field-grown vegetables. Differences between means were compared by Duncan's Multiple Range Test. Bars with the same letter do not differ significantly (n = 4; +SEM). (G) Effect of K fertilizer rate on fruit N concentration of 3 field-grown vegetables; (H) Relationship between leaf K and N concentration of 3 field-grown vegetables.