The effects of soil temperature from soil mulching and harvest age on phenol, flavonoid and antioxidant contents of Java tea (Orthosiphon aristatus B.)

Abstract

Background The environmental conditions resulted by the agronomic management practices may govern the secondary metabolite contents of medicinal plants, including Java tea (Orthosiphon aristatus B). Abiotic factors such as temperatures have been known to determine the secondary metabolite contents of Java tea. This study aimed at evaluating the effects of soil temperature resulting from soil mulching and harvest age on total phenol, flavonoid and antioxidant contents of Java tea. Methods The research was arranged using nested (hierarchy design) with completely randomized design under a screen house at Karanganyar, Indonesia, from July to December 2019. The main factor was soil mulching (control; black plastic mulch, transparent plastic mulch, biodegradable mulch and rice straw mulch) with three replicates. The main factor was nested in the temporal hierarchy factor, namely harvest age which consisted of two levels, i.e., 80 and 100 days. The observation parameters were soil temperature of 10 min (maximum; mean; minimum and soil temperature-based Growing Degree Days, GDD) with sensors and logger; plant growth (plant height, number of leaves, fresh and dry weight); and secondary metabolites including phenol, flavonoids and antioxidant. Results The results confirmed the order of the highest to the lowest mean soil temperature was resulted under the transparent plastic mulch > straw > black plastic mulch > control > biodegradable plastic mulch (26.69 > 26.29 > 26.10 > 26.07 > 25.68 °C, respectively). Overall, the harvest age 100 days resulted in higher plant growth, indicated by the higher fresh and dry weight of biomass, higher phenol and antioxidant contents than 80 days. Soil mulching, especially with plastic and biodegradable plastic mulches with long harvest age (100 days) effected into lower fresh and dry weight of plants. On the other hand, soil mulching indirectly resulted in lower phenol but higher flavonoid contents through higher soil temperature, while antioxidant contents were higher under the big soil temperature-based Growing Degree Day (GDD). The total phenol, flavonoids and antioxidant produced ranging from 193.75 to 412.50 mg GAE/ 100 g DW; 81.13 to 141.47 mg QE/ 100 g DW; and 1875.5–2144.4 µmol TE/g DW. Conclusion Higher maximum soil temperature resulted in lower phenol content, while higher minimum soil temperature and shorter harvest age increased total flavonoid. Longer harvest age produced more total phenol and antioxidant due to bigger soil temperature-based Growing Degree Day (GDD). Graphical Abstract

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