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The impact of water stress on the biochemical constituents that determine black tea quality was investigated. Phenylalanine ammonia lyase (PAL) activity was highest in the drought tolerant 'Assam' cultivar UPASI-2, followed by UPASI-8 and UPASI-9, under non-stress conditions. Under soil moisture stress a reduction in PAL activity was found in all three clones investigated. A strong positive correlation was observed between an increase in soil moisture deficit and a decrease in PAL activity. Lower PAL activity correlated well with lower synthesis of flavanols such as epigallocatechin gallate (EGCG) and epicatechin gallate (ECG), which are important precursors of theaflavin-3,3 -digallate that determines final tea quality. Altered synthesis of EGCG and ECG could be due to their molecular rearrangement at elevated leaf temperature during drought. Synthesis of quality constituents such as gallic acid and caffeine declined significantly owing to both drought and waterlogging stress. The reduction in gallic acid due to water stress could lead to lower synthesis of theaflavin fractions such as epitheaflavic acid, epitheaflavic acid-3 -gallate and theaflavic acid and, thereby, quality deterioration. Similarly to drought, flooding stress was also found to alter the biochemical constituents necessary for tea quality.

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