The Role of Stress in Tea Growth and Manufacture

by SANWAR M. CHANGOIWALA

Stresses & Their Relation to Flavour

Let us have a look at some of the factors observed by tea planters that give more flavorful tea.

• High-elevation tea bushes give more flavour
• Non-fertilised tea bushes give more flavour
• Tea bushes on rocky soils give more flavour
• A slow-growth period gives more flavour than a free-flush period
• Frost-damaged tea leaves give more flavour. Tea bushes affected by frost injury, have metabolites such as pyruvate, acetaldehyde, and ethanol accumulated in the leaves of tea manufactured from frost injury leaves gives better flavour.
• Insect-damaged flush in general gives more flavour
• Droughty conditions give more flavour

Stress Exerted on the Tea Leaf

My professional training in engineering led me to surmise that the common factor in all the situations listed above is stress. The stress exerted on a tea leaf can be of various types, as follows:

• Low-temperature stress (as found in high-elevation teas)
• Nutrition stress (as in the case of underfertilised teas, or teas grown on bad soils like rocky soils.
• Ultraviolet radiation stress: UV radiation is more concentrated in high elevation and on clear sky days. In India, it is also highest in south-facing sections of Darjeeling, and least in north-facing sections. Due to this factor only, Darjeeling’s sequence of initial or first of First Flushes follows the acronym “NEWS”: i.e. north-facing sections will flush first (since north-facing sections initially receive the least sunlight, and thus also the least UV rays), followed secondly by east-facing sections, followed thirdly by west-facing sections. Fourth and last to flush are the south-facing sections, due to the UV rays of the sun during the sunshine hours.
• Moisture-related stress, as in case of droughty conditions
• Mechanical-damage stress, as in case of frost-damaged leaf or insect-damaged leaf
• (And so on).

But the following fact must be kept in mind: Any time the tea plant faces stresses -- biotic and/or abiotic -- it tries to adjust itself to these: outwardly, by slowing the growth of the tea flush; and inwardly, in the quality and quantity of the biochemicals present in the tea leaf. If the stresses continue, the growing apical bud stops growing and thus becomes what in India is called a Banjhi Bud. (This term is derived from the Hindi word Banjh, which means a woman who can not have children: a non-fertile woman.)

I knew that the stress was the agent that causes the increase in a tea's flavour, but I did not know exactly how this occurred.

Long ago I had discussion on this topic with a Japanese scientist, Professor Kanzo Sakata of Kyoto University. It has now been proven by Professor Sakata and his team that stresses during processing, while the leaf is alive, induce the expression of various new genes in the tea-plant. These induced genes result in the enhancement of the aroma formation of oolong teas. (ref: Proceedings of the 2005 International Symposium on Innovation in Tea Science and Sustainable Development in Tea Industry, pp. 541-545)

Moisture Stress on the Plucked Leaf

When the moisture content in the tea-leaf is reduced to around 70%, the moisture stress developed in the leaf produces reactive species of oxygen such as super-oxides, peroxides, and other free radicals. If these do damage to the cell membranes, the cell structure will begin to deteriorate.

There is a minimum critical moisture level below which the cells of the plucked tea shoot cannot survive. The best telltale sign of the dead tea-leaf cell is that it not able to take atmospheric oxygen for respiration. Very hard withers will lower the moisture-level in tea-leaf cells to the point that some or all of them will die.

[TO BE CONTINUED ...]