Journal of Applied Botany and Food Quality 87, 220 - 226 (2014), DOI:10.5073/JABFQ.2014.087.031 MPED Research Center, Department of Botany, University of Fort Hare, Alice, South Africa Mineral uptake in Solanum nigrum L. cultivated on fertiliser amended soils of the Eastern Cape, South Africa Callistus Bvenura, Anthony J. Afolayan* (Received April 23, 2014) * Corresponding author Summary A considerable interest has been manifested in the cultivation of wild vegetables to combat the ever increasing hunger and micro- nutrient deficiencies especially in children from the developing world. Solanum nigrum, one of the popular wild vegetables con- sumed in the Eastern Cape was cultivated on sandy loam soils to determine the effect of fertilisers on Cu, Fe, Mn and Zn uptake in relation to its growth stages. Five treatments (control; 100 kg N/ha; 8.13 t manure/ha; 100 kg N/ha + 8.13 t manure/ha and 50 kg N/ha + 4.07 t manure/ha) were arranged in plots in a Randomised Complete Block Design with five replicates. Cu (mg/kg) remained variably high throughout the trial, ranging between 7.20-23.50 on the field and 5.30-21.40 in the glasshouse. Fe (mg/kg) also remained variably high, ranging between 213-766 on the field and 178-523 in the glass- house. Mn (mg/kg) increased with increasing plant maturity and ranged between 85-222 on the field and 64-215 in the glasshouse, but Zn (mg/kg) decreased with plant maturity and ranged between 33-78 on the field and 16-74 in the glasshouse. The results indicate that Solanum nigrum has the potential to supply the recommended daily micronutrient intake values throughout all its growth stages. Introduction There is no doubt that one of the causative factors to low nutritional status of food plants is poor soil fertility. Plants obtain their nutrients from the soil on which they are cultivated. The replenishment of nu- trients to low yielding soils through the addition of fertilisers often leads to better performance of food plants. Nutrient uptake and composition of food plants has for a long time been a subject of interest among researchers. Food plants play a critical role in human life by supplying the much needed nutritional elements as well as boosting the human immune system. However, most food plants are currently based on a limited number of crops and these comprise about 103 plant species which contribute 90 % of national per capita supplies of food plants (PRESCOTT-ALLEN and PRESCOTT-ALLEN, 1990). Although wild vegetable plants were an important part of traditional agricultural systems, their consumption has over the years declined in South Africa due to their association with poverty and poverty foods, degree of urbanisation, modernisa- tion of agriculture, distance to fresh produce markets and season of the year, among other factors (FLYMAN and AFOLAYAN, 2007; JANSEN van RENSBERG et al., 2007). According to MODI et al., (2006), the poor utilisation of wild vegetables may be associated with a lack of knowledge about how to access quantities that can satisfy daily human food requirements. Nevertheless, in many parts of the world, including South Africa, the use of wild vegetables is not negligible (MISRA et al., 2008). One of the major health problems affecting children in South Africa is micronutrient deficiency and these have been well docu- mented in South Africa particularly for Fe, Zn and Cu (FABER and WENHOLD, 2007; VOSTER, 2010). The rich nutritional value of wild vegetables has also been documented by many authors (EDMONDS and CHWEYA, 1997; FLYMAN and AFOLAYAN, 2007; ODHAV et al., 2007; LEWU and MAVENGAHAMA, 2010). FABER and WENHOLD (2007) suggested the cultivation of wild vegetables as one of the strategies that can be adopted to address micronutrient malnutrition. Wild vegetables usually grow as volunteer plants alongside con- ventional crops in agricultural fields during the planting season and are mostly viewed as weeds that must be removed usually by me- chanical or chemical means. Although they are viewed as weeds, the cultivation of wild vegetables has been documented in South Africa for some species, for example, Amaranthus and Brassica (JANSEN van RENSBERG et al., 2007; HUSSELMAN and SIZANE, 2006). However, agronomic data required to cultivate wild vegetable plants is scanty. A preliminary survey of wild vegetables consumed in the Eastern Cape Province of South Africa, where the current study was conducted indicated that S. nigrum is one of the popular wild vegeta- bles gathered from the wild for food and consumed by the province’s rural populace (BVENURA and AFOLAYAN, 2014). This wild vegetable was therefore cultivated in the glasshouse and on the field under dif- ferent concentrations of organic and/or inorganic fertilisers with a view to determine the wild vegetable’s response to these fertilisers and also determine the best fertiliser option for its cultivation as well as the best time to harvest the leaves for extraction of the micronutri- ents; Zn, Fe, Cu and Mn. Materials and methods The Experimental site The experiment was conducted in the glasshouse and on the field at the University of Fort Hare, Alice campus, South Africa between September and December 2012. The study area falls under 32o 47’ S and 26o 50’ E and 535 m a.s.l. and is within a semi arid ecologi- cal zone with an average annual rainfall of approximately 575 mm in summer; mean daily temperatures of 22.5 oC during the day and 18.8 oC at night while during the winter the temperature is about 13.6 oC during the day and less than 10.3 oC at night (MARAIS and BRUTSCH, 1994). According to the South African system of soil classification, the soils are deep alluvial; of the Oakleaf form (Oa) and belong to the Jozini series and are texturally sandy loam (SOIL CLASSIFICATION WORKING GROUP, 1991). According to the soil map of the world, the soils are Eutric fluvisols (Fle) (FAO-UNESCO- ISRIC, 1988). The properties of the soil, used for this experiment are shown in Tab. 1. Agronomic practices Ripe and mature S. nigrum berries were harvested between the 3rd and 26th of April 2012 from the wild in Alice. The seeds were se- parated from the pulp, washed in distilled water and dried at room temperature on the laboratory bench for 2 h and kept in sealed bottles until further use. The extracted seeds were planted in cavity trays in the glasshouse and later transplanted to the field when they were Mineral uptake in Solanum nigrum L. 221 about 6 weeks old. For the glasshouse trial, the seedlings were trans- planted into prepared polythene bags containing 5 kg of soil. The soil used in the glasshouse was obtained from the field where the field trial was conducted to ensure consistency in soil properties from the two trials. The organic fertiliser (goat manure) used in this experiment was obtained from the University of Fort Hare animal farm while the inorganic fertilisers (NPK [2:3:4] and Limestone Ammonium Nitrate [LAN]) were purchased from a local fertiliser dealer. The properties of the organic fertiliser used for the experi- ment are shown in Tab. 1. ing the samples in a dust free, forced-draft oven at 40 oC to a con- stant weight. The samples were then ground to a powder using a mortar and pestle and passed through a 2 mm sieve. The samples were kept in plastic val containers and stored in a refrigerator at 4 oC till when needed. The first data were collected on the day of transplanting followed by 3 weeks after transplanting after which data were collected on a weekly basis. The experiment was termi- nated in the 9th week for the glasshouse experiment and 12th week for the field experiment when all the berries on the plant were mature and ripe. Mineral analysis The AGRILASA (2008) method of determining mineral elements in plant samples was followed. About 0.5 g of finely ground veg- etable samples was placed in dry, clean digestion tubes and 5 ml of the digestion mixture comprising 1 part HCIO4 + 2 parts HNO3 added. This mixture was digested at 230 oC on a digestion block for 70 min, allowed to cool down and made up to 100 ml volume with distilled water. The concentration of Cu, Fe, Mn and Zn was then de- termined using the Inductively Coupled Plasma - Optical Emission Spectrometer (ICP OES). Statistical analysis Data of the nutrient concentrations of various treatments were sub- jected to statistical analysis using MNITAB Release 12. A one way analysis of variance was used to compare the means of various nutri- ent concentrations among the treatments and a two way analysis of variance used to determine the interaction between plant age (weeks after transplanting) and treatment on nutrient accumulation in the plant. Means were segregated using Duncan’s multiple range test. The means were treated as significantly different at p < 0.05. Results and discussion Copper (Cu) Cu exponentially increased from the time of transplanting to the 3rd week on the field, after which it varied, but was highest in the 11th week in T3 (Tab. 2a). Treatment means significantly differed (p < 0.05) and ranged between 7.20 and 21.60 mg/kg in the week of transplanting and the 4th week respectively. The means for the dura- tion of the trial were highest in T1 (16.2 mg/kg) and lowest in T4 (15.5 %). Statistical analysis showed an interaction between plant age and the fertiliser treatment on Cu uptake. Regression analysis with Cu as the dependable variable and time (plant age) as the re- gressor showed a coefficient of determination (R2) of 25.7 % indicat- ing that plant age had a minimum effect on Cu. In the glasshouse, Cu exponentially increased from the time of transplanting to the 3rd week after which it decreased (Tab. 2b). Treatment means signifi- cantly differed (p < 0.05) and ranged between 5.30 and 21.40 mg/kg in the 9th and 4th week respectively. The means for the trial period were highest in T3 (13.6 mg/kg) and least in T1 (9.50 %). Statistical analysis showed an interaction between plant age and the fertiliser treatment on Cu. Regression analysis with Cu as the dependable variable and time (plant age) as the regressor showed a coefficient of determination (R2) of 12.7 % indicating that plant age had a mini- mum effect on Cu uptake. The field experiment showed higher concentrations of Cu than the glasshouse experiment. According to SHORROCKS and ALLOWAY (1988), the availability of Cu for uptake by plants is determined by soil pH and organic matter content among other factors. Low pH enhances the absorption of Cu from the soil. The higher concentration of Cu on the field may be due to high organic matter concentrations Tab. 1: The chemical properties of the experimental soil (Upper 0-30 cm depth) and organic fertiliser Soil Organic fertiliser pH(KCI) 6.54 7.17 Bulk density (g cm-3) 1.20 - EC (µS/cm) 162.05 10.75 CECsum (meq/ 100g) 12.10 - Available P (mg kg-1) 71 8 500 Exchangeable K (mg kg-1) 406 26 000 Exchangeable Ca (mg kg-1) 1653 29 700 Exchangeable Mg (mg kg-1) 335 9 900 Exchangeable acidity (cmol/L) 0.06 - Total cations (cmol/L) 12.10 - Saturated acid (%) 0 - Zn (mg kg-1) 10.2 172 Mn (mg kg-1) 17 582 Cu (mg kg-1) 5.7 54 Organic C (mg kg-1) 10000 - N (mg kg-1) 1400 24 800 Clay (%) 17 - Na (mg kg-1) - 1 564 Fe (mg kg-1) - 12 439 Al (mg kg-1) - 5 335 Experimental design The experiments were laid out in a Randomised Complete Block Design (RCBD) with five treatments and five replicates. The treat- ments were: Control (T1); 100 kg N/ha (T2); 8.13 t manure/ha (T3); 100 kg N/ha + 8.13 t manure/ ha (T4) and 50 kg N/ha + 4.07 t manure/ ha (T5). Nitrogen was supplied in the form of NPK and LAN ferti- lisers. The organic fertiliser (goat manure) and NPK were applied at transplanting and LAN fertiliser applied 4 weeks after transplant- ing. These fertilisers were applied in the top 5-7 cm of soil depth by mixing with a spade in plots measuring 3 m × 2 m on the field. In the glasshouse, the experiment was laid out as described in the field except that each treatment had 5 replicates but each replicate had 10 experimental units to ensure a sufficient number of plant samples for the duration of the trial. Each replicate consisted of one S. nigrum plant in a polythene bag containing 5 kg of soil. Data collection The third youngest fully expanded leaves (JONES et al., 1971) were collected from the shoots by uprooting the whole plant; washed in distilled water to remove sediments and other impurities before dry- 222 C. Bvenura, A.J. Afolayan Tab. 2a: Effect of organic and inorganic fertilisers on Cu (mg/kg) of Solanum nigrum L. cultivated in the field Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 10 11 12 T1 7.20±0.18 17.20±0.18a 15.70±0.18a 18.20±0.18a 16.90±0.36 12.40±0.18a 16.80±0.36a 19.80±0.18a 17.40±0.18a 18.70±0.18a 18.00±0.18a T2 7.20±0.18 16.60±0.18b 16.10±0.09b 17.80±0.18b 14.50±0.18a 14.30±0.27b 12.70±0.18b 17.40±0.36b 19.50±0.18b 16.40±0.18b 21.50±0.18b T3 7.20±0.18 16.70±0.09b 21.60±0.27c 15.60±0.27c 16.60±.027 16.60±0.27c 12.70±0.18b 15.60±0.27c 20.60±0.27c 23.50±0.18c 8.10±0.18c T4 7.20±0.18 18.80±0.36c 17.90±0.09d 17.10±0.09d 16.90±0.09 15.40±0.18d 14.40±0.18c 19.00±0.09d 17.70±0.18a 15.00±0.09d 10.93±0.23d T5 7.20±0.18 17.00±0.18ab 17.60±0.27d 16.10±0.09e 16.80±0.18 15.00±0.09e 16.50±0.45a 17.70±0.27e 19.50±0.09b 15.60±0.27e 14.10±0.09e 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. Tab. 2b: Effect of organic and inorganic fertilisers on Cu (mg/kg) of Solanum nigrum L. cultivated in the glasshouse Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 T1 7.20±0.18 17.60±0.27a 13.70±0.18a 12.80±0.18a 6.90±0.09a 5.80±0.18a 6.10±0.18a 5.60±0.27a T2 7.20±0.18 16.20±0.18b 17.90±0.18b 12.50±0.18a 9.00±0.18b 5.50±0.18a 6.20±0.18a 5.50±0.18a T3 7.20±0.18 20.40±0.36c 15.10±0.09c 15.50±0.18b 15.40±0.18c 12.70±0.18b 10.90±0.09b 11.50±0.27b T4 7.20±0.18 18.20±0.18d 14.00±0.18a 11.50±0.45c 7.60±0.18a 6.20±0.18a 7.00±0.18c 5.30±0.27a T5 7.20±0.18 19.70±0.18e 21.40±0.18d 17.00±0.27d 13.10±0.09d 11.30±0.27c 7.30±0.18c 9.20±0.18c 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. and therefore slightly lower soil pH as compared to the glasshouse. In another species, ATTA et al. (2010) observed that Cu was highest during the first stages or the final stages of H. sabdariffa growth depending on the ecotype. However, the concentration of Cu in their study was at least 600 % higher than in the current study. On the other hand, MORILLO et al. (1997) reported lower concentrations of Cu as compared to the current study but in A. gayanus. These results indicate that S. nigrum has the potential to provide more Cu than the needed daily recommended values according to New Zealand and Australian standards (NHMRC, 2005) at any growth phase of the plant if children and adults respectively consume about 150 g and 300 g of the vegetable. Under field conditions, S. nigrum may best be harvested in the final stages of growth while under glasshouse conditions, the early growth stages would be ideal as the micronutrient is at its peak during these stages of growth. Iron (Fe) Fe uptake differed significantly (p < 0.05) and ranged between 213 and 766 mg/kg at the time of transplanting and the 5th week respectively (Tab. 3a). The means for the trial period were highest in T3 (528 mg/kg) and lowest in T5 (440 mg/kg). Statistical analysis showed an interaction between plant age and the fertiliser treatment on Fe. Regression analysis with Fe as the dependable variable and time (plant age) as the regressor showed a coefficient of determina- tion (R2) of 37.7 % indicating that plant age had a minimum effect on Fe uptake. Results of the glasshouse experiment showed lower means which ranged between 178 and 523 mg/kg in the 4th and 5th week respectively except in T1 (Tab. 3b). The means for the duration of the trial were highest in T1 (526 mg/kg) and lowest in T5 (239 mg/kg). Statistical analysis showed an interaction between plant age and the fertiliser treatment on Fe. Regression analysis with Fe as the dependable variable and time (plant age) as the regressor showed a coefficient of determination (R2) of 0.9 % indicating that plant age had a very minimum effect on Fe uptake. Fe concentration was observed to be constant on the field but slightly decreased as the plant aged in the glasshouse experiment. The observation here is different from the report of MORILLO et al. (1997) in A. gayanus. In the same way, FLYMAN and AFOLAYAN (2008) observed variations in Fe concentration on M. balsamina and V. unguiculata. According to GRAHAM and STANGOULIS (2003), solubility of Fe is very low particularly in the presence of moderate oxygen and acidic conditions. The high organic matter content on the field and the continuos release of minerals to the soil may be attri- buted to the differences reported in comparison with the glasshouse in the current study. The variations in concentration between the field and the glasshouse concentration of Fe in S. nigrum remained more than sufficient to supply the required human daily average intake across all age groups and gender according to the New Zealand and Australian standards (NHMRC, 2005). However, harvesting the plant for maximum Fe content would be best during the middle stages of the plant’s growth cycle when Fe will be at its peak. Furthermore, the concentration range reported in the current study is higher than what ODHAV et al. (2007) reported in wild uncultivated S. nigrum leaves. A portion of about 150 g and 300 g of cultivated and cooked S. nigrum therefore has the potential to supplement a starch rich diet of poor rural communities at all stages of growth in children and adults respectively in view of the fact that Fe is one of the most prevalent forms of micronutrient malnutrition in the world (FAO, 2004). Mineral uptake in Solanum nigrum L. 223 Manganese (Mn) Mn concentration increased as the plant matured and ranged between 86 and 159 mg/kg in the 4th and 10th week respectively (Tab. 4a). The means for the trial period were highest in T2 (123 mg/kg) and lowest in T1 (104 mg/kg). Statistical analysis showed an interaction between plant age and the fertiliser treatment on Mn. Regression analysis with Mn as the dependable variable and time (plant age) as the regressor showed a coefficient of determination (R2) of 71.1 % indicating that plant age had a significant effect on Mn. Similarly, the glasshouse means also differed significantly (p < 0.05) and ranged between 64 and 215 mg/kg in the 6th and 9th weeks of respectively (Tab. 4b). The means for the trial were highest in T5 (144 mg/kg) and lowest in T1 (83 mg/kg) and this was lower than what was reported on the field except for T5 which was higher in the glasshouse. Statistical analysis showed an interaction between plant age and the fertiliser treatment on Mn. Regression analysis with Mn as the dependable variable and time (plant age) as the regressor showed a coefficient of determination (R2) of 65.8 % indicating that plant age had a significant effect on Mn. Statistical analysis shows that there is a relationship between the age of the plant and Mn concentration. As the plant matured in age, the concentration of Mn also increased. This observation is in line with the report of FLYMAN and AFOLAYAN (2008) and ATTA et al. (2010) but at variance with that of MORILLO et al. (1997). The continuous increase of Mn in S. nigrum leaves in the present study is an indi- cation of the continuous release of the mineral from the soil for up- take by the plant. According to MCGRATH et al. (1994), during the summer season, the relatively high decomposition rate of organic matter releases Mn in the soil solution for possible uptake by plants and this is a possible reason why concentrations were higher on the field than in the glasshouse where organic matter content is expectedly low. Once taken up into plant tissues, Mn becomes immobile and this is possibly why the mineral continued to increase in the leaves (MCGRATH et al., 1994). Instead of being reassigned to the reproductive parts of the plant, the element remained immobile in the leaves of S. nigrum. The results of this work further indicate the ability of S. nigrum to provide Mn needed to supply the required human daily intake according to New Zealand and Australian standards (NHMRC, 2005) at all stages of the plant’s growth if children and adults respectively consume 150 and 300 g of the cooked leaves. In addition, results of this study indicate that in order to harness the maximum amount of the mineral, leaves of mature plants should be harvested. Zinc (Zn) Zn exponentially increased on the field between the time of trans- planting and the 3rd week and decreased until week 12 (Tab. 5a). The treatment means significantly differed (p < 0.05) and ranged between 33 and 78 mg/kg in the 12th and 3rd week respectively. The means for the duration of the trial were highest in T5 (62 mg/kg) and least in T1 (59 mg/kg). Statistical analysis showed an interaction between plant age and the fertiliser treatment on Zn. Regression analysis with Zn as the dependable variable and time (plant age) as the regressor showed a coefficient of determination (R2) of 59.4 % indicating that plant age had a fairly significant effect on Zn. In the glasshouse, Zn also increased between the time of transplanting and the 4th week after which it decreased (Tab. 5b). The treatment means significantly differed and ranged between 19 and 74 mg/kg in the 9th and 4th week respectively. The means for the trial were highest Tab. 3a: Effect of organic and inorganic fertilisers on Fe (mg/kg) of Solanum nigrum L. cultivated in the field Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 10 11 12 T1 213±2.68 426±2.68a 348±3.58a 591±0.89a 562±1.79a 475±4.47a 439±2.25a 629±1.79a 408±3.58a 570±1.79a 435±4.47a T2 213±2.68 624±3.58b 246±1.79b 766±2.68b 403±2.68b 480±1.79a 374±1.79b 521±0.89b 483±2.68b 540±1.79b 750±1.79b T3 213±2.68 514±1.79c 300±1.79c 587±1.79a 678±1.79c 669±3.58b 548±1.79c 556±2.68c 667±1.79c 677±1.79c 403±2.68a T4 213±2.68 371±0.89d 406±2.68d 617±1.79c 395±4.47b 575±4.47c 671±0.89d 396±2.68d 567±1.79d 578±1.79a 375±4.47c T5 213±2.68 305±4.47e 499±1.37e 655±4.47d 366±2.68d 499±0.89d 414±1.79e 501±0.89b 466±2.68b 567±1.37a 356±3.58c 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. Tab. 3b: Effect of organic and inorganic fertilisers on Fe (mg/kg) of Solanum nigrum L. cultivated in the glasshouse Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 T1 213±2.68 430±1.79a 299±4.03a 337±6.26a 246±1.79a 325±4.47a 327±1.79a 433±2.68a T2 213±2.68 320±1.79b 323±2.68b 212±5.37b 255±4.47b 254±1.79b 302±1.79b 256±2.68b T3 213±2.68 447±1.79a 340±1.79b 265±4.47c 296±2.68c 363±2.68c 269±4.03c 304±3.58c T4 213±2.68 260±1.79c 523±2.68c 178±3.58d 195±4.47d 239±1.79d 224±1.79d 258±7.16b T5 213±2.68 217±1.79d 432±1.79d 191±0.89d 179±3.14e 230±3.58d 209±8.05e 239±0.89b 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. 224 C. Bvenura, A.J. Afolayan Tab. 4a: Effect of organic and inorganic fertilisers on Mn (mg/kg) of Solanum nigrum L. cultivated in the field Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 10 11 12 T1 88.67±3.14 101±0.89a 89±3.14a 90±3.58a 95±2.68a 108±7.16a 102±1.79a 85±4.47a 110±4.47a 137±2.68a 143±2.68a T2 88.67±3.14 116±4.47b 87±3.58a 121±2.68b 110±2.68b 139±4.03b 153±2.68b 114±6.26b 156±2.68b 129±4.03b 134±1.79b T3 88.67±3.14 109±4.93c 86±2.68a 90±4.47a 95±1.79a 124±1.79c 89±4.03c 86±2.68a 114±6.26a 132±1.79b 222±0.89c T4 88.67±3.14 103±1.37d 104±3.58b 104±0.52c 101±0.89c 126±2.68c 132±1.79d 127±6.26c 159±4.03b 121±0.89c 154±3.58d T5 88.67±3.14 106±1.79cd 112±5.37c 108±2.68c 92±1.79a 131±0.89d 136±5.37d 111±1.79b 127±1.79c 149±4.03d 154±2.68d 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. Tab. 4b: Effect of organic and inorganic fertilisers on Mn (mg/kg) of Solanum nigrum L. cultivated in the glasshouse Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 T1 89±3.14 88±3.58bc 78±3.58a 73±0.89a 64±1.79a 79±4.03a 79±4.47a 114±6.26a T2 89±3.14 83±2.68c 108±3.58b 88±1.79b 103±2.25b 101±0.89b 128±3.58bc 148±1.79b T3 89±3.14 94±1.79b 79±4.03a 90±4.47b 118±2.68c 125±1.79c 125±4.47c 167±6.26c T4 89±3.14 86±2.68bc 98±3.58c 78±3.58a 90±4.47d 102±1.79b 138±5.37b 147±1.79b T5 89±3.14 117±13.42a 153±2.68d 119±4.47c 160±4.47e 164±3.58d 134±3.58b 215±1.79d 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. in T2 (49 mg/kg) and least in T5 (37 mg/kg) and these values were lower than those reported on the field. Statistical analysis showed an interaction between plant age and the fertiliser treatment on Zn. Regression analysis with Zn as the dependable variable and time (plant age) as the regressor showed a coefficient of determination (R2) of 81.7 % indicating that plant age had a very significant effect on Zn. Zn was found to be gradually decreasing as the plant matured. This observation is in line with the report by AFOLAYAN and FLYMAN (2008). However, ATTA et al. (2010) reported variations in Zn con- centration in H. Sabdariffa as it grew older while MORILLO et al. (1997) observed no change in Zn concentration in A. gayanus. According to KABATA-PENDIAS (2001), about 75 % of total Zn taken up by plants is stored in the shoots of young plants whereas about 20-30 % occurs in the shoots of old plants. This phenomenon is a possible explanation to the decline in Zn concentration with advancing plant maturity in the present study. Furthermore, KABATA- PENDIAS (2001) proposed that the roots often contain more Zn than the shoots. The results of the current study indicate that the minimum value of Zn reported in the present study (19 mg/kg in the glasshouse) can potentially supply only 51.8 % of the daily recommended human intake values of the mineral in women according to the New Zealand and Australian standards (NHMRC, 2005), however, the maximum value reported (78 mg/kg in the field) can potentially supply about 213 % of the recommended daily human intake in women. Although the values reported in this trial declined with increasing plant maturity, the concentrations have the potential to supply the daily recommended human intake of the mineral across all age groups and gender up to the 12th week on the field while in the glasshouse, the supply is adequate across all age groups and gender up to the 5th week and begins to vary. The optimum time for harvesting S. nigrum leaves for Zn would therefore be during early stages of its growth. Conclusion This study revealed that Solanum nigrum is a high micro mineral yielding wild vegetable that can be cultivated and incorporated into the diets of marginalised poor rural communities whose diets are mainly starch based. The best fertiliser to apply and best time to harvest the plant leaves for food may be a challenge as different minerals respond differently to different fertiliser options, therefore it is critical to recommend the best fertiliser and time of harvesting based on specific micronutrient requirements. However, 50 Kg N/ ha + 4.07 t manure/ha increased the concentration of Zn on the field and Mn in the glasshouse while 8.13 t/ha goat manure increased the uptake of Mn as well as Fe on the field and Cu in the glasshouse. In addition, 100 Kg N/ha increased the concentration of Ca and Zn in the glasshouse while the field and glasshouse controls increased the concentration of Cu and Fe respectively. Cu and Fe from both the field and glasshouse varied up and down throughout the trail while Mn concentration increased steadily but Zn decreased. In general, the nutrient values recorded indicate the ability of the wild vegetable to supply the molarity of recommended daily mineral intakes of micronutrients at all stages of the plant’s growth if children consume about 150 g of the cooked vegetable and adults consume 300 g. However, in order to exploit the maximum potential amounts of the minerals, the results indicate that for Cu and Zn, the leaves are best harvested during the early stages of growth and the middle stages for Fe while the final stages of the plant’s life cycle would be ideal for Mn. Mineral uptake in Solanum nigrum L. 225 Tab. 5a: Effect of organic and inorganic fertilisers on Zn (mg/kg) of Solanum nigrum L. cultivated in the field Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 10 11 12 T1 60±3.00 78±1.67 68±8.37 66±5.86 62±2.51a 67±3.35 60±4.67ab 56±3.45 43±3.35a 39±4.18ab 42±6.69 T2 60±3.00 76±1.53 72±4.00 66±2.00 66±3.00ab 69±10.00 59±4.00ab 53±5.00 59±4.00b 37±1.00a 43±3.00 T3 60±3.00 75±5.00 74±2.00 64±4.00 71±1.53b 59±4.00 56±3.00ab 56±2.00 47±2.00a 47±2.00b 33±3.00 T4 60±3.00 78±4.00 71±2.52 65±5.00 70±5.00b 70±6.00 52±4.00b 53±3.00 49±9.00ab 45±6.00ab 37±2.00 T5 60±3.00 77±2.00 73±3.00 67±6.51 73±3.00b 67±5.00 63±3.00a 61±2.00 57±4.00b 41±3.00ab 39±3.51 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. Tab. 5b: Effect of organic and inorganic fertilisers on Zn (mg/kg) of Solanum nigrum L. cultivated in the glasshouse Sodium Plant age (Weeks after transplanting) 0 3 4 5 6 7 8 9 T1 60±2.68 59±7.15a 48±3.58a 46±5.37 26±4.93a 16±5.37a 23±4.00a 19±2.68a T2 60±2.68 72±1.78b 74±3.58b 46±2.68 46±2.68c 32±3.58a 32±4.00ab 33±2.68b T3 60±2.68 59±8.05a 38±3.58c 37±3.58a 34±3.58b 21±1.79b 23±3.00a 24±3.58c T4 60±2.68 68±0.52b 53±2.68a 50±4.47 41±3.14bc 33±2.68b 34±4.00b 34±3.58b T5 60±2.68 63±2.68ab 60±4.47d 46±2.25 39±4.03b 29±3.57b 32±3.51b 31±1.79b 0 indicates readings taken at the time of transplanting Values shown are mean ± S.D. Means with different letters down the same column represent significant differences at p < 0.05. Acknowledgements We thank the Govan Mbeki Research and Development Center of the University of Fort Hare for funding this project. 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