Effect of different plant substrates and growth hormones on the development characteristics and nutrient content of Pleurotus ostreatus in semi-arid conditions

Abstract

With the global and local need to identify low-cost sustainable sources of protein and essential minerals for human nutrition the study was undertaken to investigate effect of different plant growth substrates and hormones on the yield and nutrient content of Pleurotus ostreatus. The other objectives were to analyse the transfer efficiency of macro- and micro-nutrients by the respective substrates and plant growth hormones and also investigate possible post-harvest mushroom processing technologies. The experiment was laid out in a 3 X 4 factorial treatment combinations. The experimental factors were as follows: three (3) substrates - Urochloa panicoides, Zea mays and Datura stramonium and four (4) hormones - cytokinins; auxins; gibberellins and control (no hormone). The treatment combinations were replicated three times making a total of thirty six (36) experimental units. Treatments were arranged in a Randomized Complete Block Design (RCBD). The amount of substrate used on each mushroom dome was determined by the volume of the mushroom dome structure and their texture i.e. Urochloa panicoides (1kg), Zea mays (1.23kg) and Datura stramonium (1.1kg). Dosage of hormonal treatment used to treat the respective substrates were as follows: auxins (PoMaxa): 31.6%, cytokinins (MaxCel): 31.6% and gibberellins: 11.1% and for the control - no hormones. The pH levels on respective substrates were analysed using 1:25 probe and meter procedure with water. Macro- and micro-nutrient content on various substrates were analyzed using the dry-ashing macro- and micro-nutrient procedure provided by AgriLASA. Pleurotus ostreatus production process on various substrates with different hormonal treatment was evaluated by assessing mycelial development, fruiting body initiation and total biomass. Fruit cap diameter and style length of mushrooms were measured using a tape measure at each harvest and conversion efficiency ratio from different treatment combinations into total biomass was determined by using the following formula: Mushroom Biomass / Substrate biomass X 100 = Convention Ratio (%). Analysis of variance indicated that there was a significant difference on effect of substrates and hormones on total biomass however, there was no significant difference from the ‘substrates X hormones’ interaction (P≤0.05). Hormones also had significant effect on fruit cap size and style length (P≤0.05). Transfer efficiency of macro- and micro-nutrient, protein content and heavy metals by the respective substrates and plant growth hormones were analyzed using the dry-ashing macro- and micro-nutrient procedure provided by AgriLASA. Polyphenols content of mushrooms fruiting bodies were analyzed using the Folin-Ciocalteu colorimetric method and flavonoidsand determined by a colorimetric method. The results on transfer efficiency of macro- and micro-nutrient content of Pleurotus ostreatus fruiting bodies indicated that pH, substrate and phytochemicals have a significant effect on nutrients transfer efficiency of Pleurotus ostreatus. Mushrooms harvested from Urochloa panicoides had high levels of micro-nutrient composition on Fe and Zn compared to Zea mays and Datura stramonium. All the hormones had no significant influence on transfer efficiency ratio of Pleurotus ostreatus. Protein content percentage (per 100g) was relatively high on treatment combination S1H3 (Urochloa panicoides + Gibberellins) and S2H2 (Zea mays + Auxins). The levels of total polyphenols and flavonoids on mushrooms harvested from treatment combination S3H2 (Datura stramonium + Gibberellins) were found to be significantly higher than treatment combination S2H1 (Zea mays + Auxins) and all other treatment combinations. Accumulation coefficient of macro- and micro-nutrient content and heavy metals in Pleurotus ostreatus fruiting bodies had high levels of Fe, Zn and Mn on mushrooms harvested from substrate Urochloa panicoides. The nutrient content of the processed solar-dried Pleurotus ostreatus was analyzed using the dry-ashing macro- and micro-nutrient procedure provided by AgriLASA. A participatory assessment technique was used to determine the palatability and acceptability of the three different mushroom biscuits. The biscuits were made from the following ingredients: one egg, 250g of wheat-flour, 2.5g salt, and 200ml of water with three different ratios of solar-dried oyster mushrooms i.e. 50g, 100g and 150g. A random selection of 33% of students was carried out in a residence of a total one hundred and fifty. The selected students were used for the palatability and acceptability survey. The results on the macro- and micro-nutrient content of the different biscuits grades, i.e., ‘Mushroom-Wheat Flour’ mixing ratios, indicated an increase in levels of micro-nutrient Mn in all biscuits compared to the solar-dried mushrooms. The palatability survey on taste and acceptability of the biscuits indicated respondents preferred biscuits A which had the lowest mushroom nutrient content compared to biscuit B and C. In conclusion, the general finding of this study was that there is potential to improve oyster mushroom yield by identifying and use of appropriate substrates. It also showed that there is potential to improve nutritive value of mushrooms by amending the respective substrates by use of plant growth hormones. Furthermore, the study showed that oyster mushrooms had relatively low levels of heavy metals, i.e., Cr, Cd and Pd. These heavy metal levels were noted to be of acceptable safe standard for human consumption. This was noted for both the solar-dried mushrooms and the mushroom biscuits.