Ultraviolet Radiation and Selenium Nutrition on Yield, Biochemical Composition and Sensory Quality of Selected Tea Varieties in Kericho County, Kenya
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Ultraviolet (UVB) radiation (280–315 nm) and interaction with Selenium (Se) have potential promoting plant tolerance to UVB stress in plants and modify metabolites that influence taste and functional properties of tea. The objectives of this study were to assess sensitivity of two tea varieties to ambient solar UVB radiation and Selenium nutrition on 1) yield components,2) secondary metabolites, antioxidant activity, theanine and mineral composition of tea and 3) non-aerated (green) tea quality. The field experiment was undertaken in Chepgoiben, Kericho County 0’, 24’16” S and 2160 masl. The experiment was conducted in RCBD laid out in split-split plot with three replications and 3 factors under evaluation (Variety as main plot, Selenium in subplots and UVB radiation as experimental unit plots). Three levels of UVB radiation; UV ambient (+UVB amb), UVB blocked (-UVB) and +UVB transparent filter control (+UVB-FC), without (−Se) or with (+Se) foliar Se application (10 g L−1 chelated Se) were tested for two seasons S1 (Jan-Mar) and S2 (April to July) 2019.Upon harvest of shoots ad processing, tea brew was prepared and evaluated based on color, flavour, bitterness and astringency with reference to commercial standards of Japanese green tea and Kenyan non-aerated purple tea. Results showed that -UVB caused average accumulation of ∼11%, ~15% and 28% more fresh leaf weight, dry weight and shoot density respectively compared to UVB-FC plants irrespective of Selenium nutrition in both varieties. In SMK 30/52, highest levels of chlorophyll SPAD values were found in –UVB (353) followed by +UVB ambient (341) and lowest in +UVB-FC (293). The results revealed that TRFK 306 has distinct catechin, caffeine and theanine profiles compared to regular green tea while Catechins (EGC, GC, GCg and ECg) were influenced by UVB radiation regardless of Se nutrition.It was observed that TRFK 306 (purple tea) is less sensitive to UVB radiation perharps due presence of additional UV absorbing anthocyanins. However, both varieties had similar range of antioxidant capacity (p<0.05). Selenium nutrition did not significantly change N, P, K, S and Se content in tissues in both varieties. Sensory evaluation based on hedonic scale showed quality differences were affected by varieties and UVB radiation with least astringent and bitter being in the order –UVB (5.8) > +UVB FC (5.5) > +UVB ambient (5.3). Tasters preferred flavour in -UVB blocked (5.9) and UVB-FC (5.9) teas, the latter having unique bakey and fruity note in SMK 30/52. Teas from +UVB and UVB-FC treatments had appealing colour (6.3) and least in UVB ambient (5.8) tea samples. Correlation analyses showed that astringency, bitterness and flavour were most influenced by GC, EGC, EGCg, EC, Cg and Caffeine. It was determined that bitterness and astringency of teas is a central attribute in determining taster’s perception of sensory quality. The overall metabolic fluxes of some key secondary metabolites by solar UV exclusion in green tea variety SMK 30/52 demonstrated that ambient UVB exclusion can potentially improve quality and yield of non-aerated tea. UVB exclusion is therefore proposed as an alternative agronomic management method to improve tea yield while minimizing undesirable bitterness and astringency and overall marketing quality characteristics of non-aerated (green) tea.