Not all tea is created equal — and the reason is older than any human tradition. The species Camellia sinensis comprises two major botanical varieties that represent genuinely different answers to the same evolutionary question: how should a plant invest its limited metabolic resources to survive?

One variety chose umami. The other chose astringency. Understanding this split is essential to understanding why matcha can only be made from one of them.

1. The Two Varieties: CSS and CSA

2. The Umami Shield: L-Theanine as Survival Molecule

CSS varieties evolved in the cool, montane environments of southern China and highland east Asia. These regions subject plants to late spring frosts, cold winters, and the daily temperature swings characteristic of high-altitude valleys — the same conditions found today in Uji, Nishio, and Yame.

L-theanine functions as a cryoprotectant in these conditions. During cold exposure, theanine accumulates in leaf cells and helps stabilize cellular membranes against freeze damage. The mechanism is broadly analogous to how some plants accumulate soluble sugars (like trehalose or raffinose) for cold tolerance, but tea chose an amino acid instead. This is unusual in the plant kingdom and directly tied to the neofunctionalization of CsTSI (see Part 4).

Beyond cold protection, L-theanine serves as a nitrogen storage compound. Tea is an exceptionally heavy feeder on soil nitrogen. Storing nitrogen in the stable form of theanine allows the plant to bank it during autumn and redeploy it rapidly when new shoots emerge in spring. The first flush — the most prized harvest for matcha — benefits directly from this stored nitrogen reserve. Approximately 75% of the nitrogen in spring shoots was actually assimilated the previous autumn (see Part 7).

From the plant's perspective, theanine is a survival molecule. From the consumer's perspective, it is the source of matcha's defining umami — that deep, savory sweetness that separates premium tencha from ordinary green tea.

3. The Astringency Edge: Catechins as UV-B Shield

CSA varieties evolved at lower altitudes in tropical and subtropical environments — the Assam plains of India, lowland Yunnan, and southeast Asia. These environments are characterized by intense solar radiation, high humidity, and diverse herbivore and pathogen pressure.

Their primary metabolic investment is in catechins, particularly epigallocatechin gallate (EGCG). Catechins accumulate in cell vacuoles where they function as UV-B screens — absorbing damaging ultraviolet radiation before it reaches the photosynthetic machinery in chloroplasts. In high-radiation tropical environments, this represents a survival advantage of the first order.

Catechins also serve as broad-spectrum defense compounds:

• Herbivore deterrence: Pronounced astringency and bitterness discourage insect feeding.
• Antimicrobial activity: Catechins disrupt bacterial cell membranes, protecting damaged leaf tissue in warm, humid conditions where fungal and bacterial infection risk is high.
• Oxidative buffering: As antioxidants, catechins scavenge reactive oxygen species generated by intense photosynthesis under high light.

In sensory terms, catechins produce the astringency and bitterness associated with strong black tea and heavily sun-exposed green tea. These are not defects from the plant's perspective — they are the taste of survival chemistry.

4. The CE/TA Ratio: A Quality Metric Written in Molecular Ink

The relative concentration of catechins (CE) to theanine (TA) in a tea leaf is not merely a laboratory measurement. It is the single most informative ratio for predicting the sensory character of the cup.

The CE/TA ratio is influenced by four factors: cultivar genetics (CSS vs. CSA baseline), shading duration, harvest timing, and terroir (soil nitrogen, altitude, microclimate). All four must align to produce the low CE/TA ratio that defines ceremonial matcha.

This ratio is what tea professionals evaluate — consciously or intuitively — when they taste matcha. A low ratio reads as depth and richness. A high ratio reads as sharpness and thinness. The number puts precision behind the palate.

5. Why Matcha Can Only Come from CSS

Given the metabolic strategies outlined above, it becomes clear why matcha production is restricted to CSS varieties:

• Baseline theanine capacity: CSS varieties have higher CsTSI expression and theanine accumulation even before shading. The genetic starting point is different.
• Shading response: CSS varieties respond to shade by further upregulating the theanine pathway (via CsPIF7 activation) and downregulating catechin synthesis (via HY5 degradation). CSA varieties, optimized for high-light tropical conditions, do not exhibit the same magnitude of response.
• Cold tolerance: The cryoprotective function of theanine matters in Japanese tea-growing regions. CSA varieties, bred for tropical conditions, suffer in the cold winters that are standard in Uji, Nishio, and Yame.
• Leaf architecture: CSS leaves are smaller, thicker, and more suited to the stone-milling process. CSA leaves are larger and thinner — physically less suited to tencha production.

Attempting to make matcha from a CSA variety is not merely a quality compromise. It is a category error — asking a plant that evolved to produce astringency to produce umami. The genome is different. The metabolic strategy is different. The output is different.

Summary

• CSS (var. sinensis) and CSA (var. assamica) represent two divergent metabolic strategies within the same species: theanine accumulation for cold tolerance vs. catechin accumulation for UV-B defense.
• L-theanine functions as a cryoprotectant and nitrogen storage compound in CSS varieties — the biological basis of matcha's umami.
• Catechins function as UV-B screens, antimicrobial agents, and herbivore deterrents in CSA varieties — the biological basis of black tea's astringency.
• The CE/TA (catechin-to-theanine) ratio is the most informative single metric for predicting matcha quality: lower ratios correspond to richer umami character.
• Matcha can only be produced from CSS varieties because the genetic, metabolic, and morphological characteristics required for premium tencha are specific to the CSS lineage.

Frequently Asked Questions

Can CSA varieties be used for matcha production?

Technically, any tea leaf can be ground into powder. But the resulting product from a CSA variety would not exhibit the umami depth, vivid green color, or smooth mouthfeel that defines matcha. CSA varieties evolved to prioritize catechin production over theanine, and their response to shading does not shift the CE/TA ratio as dramatically as CSS varieties. The powder would be more bitter, more astringent, and lack the amino acid density that ceremonial matcha requires. Labeling such a product as matcha would be misleading.

What is cryoprotection and why does it matter for tea?

Cryoprotection refers to any biochemical mechanism that protects cells from freeze damage. In CSS tea varieties, L-theanine accumulates in leaf cells during cold exposure and helps stabilize cellular membranes — preventing the ice crystal formation and membrane rupture that kills cells during frost. This adaptation allowed CSS varieties to thrive in the cool montane environments of Japan and highland China, while CSA varieties remained confined to warmer tropical lowlands. The same theanine that protects the plant in winter produces the umami that defines premium matcha in spring.

Is a lower CE/TA ratio always better?

For matcha, generally yes — a lower ratio indicates more umami relative to astringency, which is the sensory target for ceremonial-grade matcha. However, some astringency provides structural complexity to the cup. A ratio of zero (pure theanine, no catechins) would taste flat and one-dimensional. The best matcha achieves a low ratio where umami is clearly dominant but catechin astringency is present as a subtle structural element — depth without bitterness. The optimal ratio also depends on intended use: culinary matcha may tolerate higher ratios than ceremonial grade.