Abstract
Secondary metabolites are bioactive compounds which are synthesized naturally in all plant parts. The quality and quantity of secondary metabolites produced by plants differ depending on the plant and environmental conditions under which they are produced. The purpose of the study was to investigate the effects of nitrogen, phosphorous and potassium (NPK) fertilizer (17:17:17) rates (0, 100, 200, 300 and 400 kg ha-1) on the production of secondary metabolites in field and greenhouse grown pepino melons (Solanum muricatum Aiton). The experimental design was randomized complete block design with five NPK fertilizer treatments replicated three times. Results indicated that an increase in NPK fertilizer rate led to an increase of carotenoids (lutein, lycopene and β-carotene) up to a maximum at 200 kg NPK ha-1 after which the contents decreased in both growing environments and trials. The control (no fertilizer application) favored the accumulation of total phenolic content (TPC) in both growing environments and trials. Greenhouse grown pepino melon fruits which were not supplied with fertilizer (control) had a TPC content of 174.3 and 145.5 mg GAE 100g-1 fresh weight (FW) in trial one and two, respectively. Fertilizers could not enhance production of TPC in pepino melon fruits and application of 200 kg NPK ha-1 is recommended for maximum accumulation of carotenoids (lycopene, lutein and β-carotene). Key words: Secondary metabolites, NPK fertilizer, greenhouse, field, pepino.
Highlights
Plants produce a wide variety of organic compounds which can be grouped as primary and secondary metabolites
The current study revealed that an increase in NPK fertilizer led to an increase in lutein content of pepino melon fruits
Plants growing in areas with intermediate resources including fertilizers will have the highest allocation of secondary metabolites and this could explain why lutein content was high and reached its peak in fruits which were supplied with 200 kg NPK ha-1 compared to plants supplied with higher fertilizer rates
Summary
Plants produce a wide variety of organic compounds which can be grouped as primary and secondary metabolites. Primary metabolites include organic acids, amino acids and phytosterols and they play vital roles in respiration, photosynthesis, growth and development in plants. Secondary metabolites empower plants to adjust to biotic and abiotic stresses and as a method of correspondence with symbiotic microorganisms as well as to attract pollinators and seed dispersal agents (Wink, 2003). Plant secondary metabolites are classified based on their chemical structure and they include flavonoids, terpenoids, steroids and alkaloids (Yang et al, 2018a). Secondary metabolites have been used as conventional medicine, perfumery and as raw materials for industries (Balandrin et al, 1985)
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