The phenological growth stages, heat unit requirement and anti‐tyrosinase activity of Greyia radlkoferi Szyszyl

Greyia radlkoferi Szyszyl (Melianthaceae) is an indigenous southern African medicinal tree whose ethanolic leaf extracts have been shown to have anti‐tyrosinase activity that can treat skin hyperpigmentation in humans. However, there are differences in activity levels among the populations of plants, with some trees having higher anti‐tyrosinase activity than others. It is critical to generate high‐quality materials with consistent anti‐tyrosinase activity for long‐term production of cosmeceutical products. To overcome the variation observed, determining the appropriate phenological growth stages to harvest quality leaf materials for better activity could be a way to address the observed variation. Detailed phenological growth stages of G. radlkoferi were described according to the BBCH scale to identify the phenological developmental stages of the plant, such as bud development, leaf development, inflorescence emergence, heading, flowering, development of fruit, ripening or maturity of fruit and seed, and senescence. Temperature and precipitation affect the phenological growth stages of G. radlkoferi. Lower temperatures and precipitation in 2018 resulted in lower development of leaf and flower buds, whereas higher temperatures in 2019 and 2020 led to increased leaf and flower buds. It was observed that more accumulated heat units were needed for leaf development, flowering, and fruiting than for bud development, fruit ripening, and senescence. The highest anti‐tyrosinase activity was achieved in leaves harvested during the flowering growth stage (half maximal concentration) (IC 50 19.50 ± 2.710 μg/mL) compared to budding (IC 50 23.91 ± 4.017 μg/mL) and vegetative growth stage (IC 50 27.88 ± 3.100 μg/mL); lower IC 50 value indicates higher anti‐tyrosinase activity.