How Aspirin Affects Plant Growth: Exploring Stomatal Density in Watercress

Aspirin—known chemically as acetylsalicylic acid (ASA)—is usually associated with pain relief in humans, but it also plays surprising roles in plant biology. Once dissolved in water, ASA converts into salicylic acid (SA), a plant hormone involved in growth, stress response, and immune signaling. SA influences leaf formation, photosynthesis, and cellular activity, meaning even small changes in its concentration could reshape how a plant grows.

To explore these effects, this study focused on Nasturtium officinale, commonly known as watercress, a semi-aquatic vegetable valued for its exceptionally high nutrient density. Understanding how ASA influences this crop isn’t just academically interesting—it also has practical agricultural relevance, particularly in improving plant growth and nutrient output.

Why Study Aspirin in Plants?

The Research Question

How do increasing concentrations of ASA (0 to 0.001 mol/L) affect stomatal density in watercress leaves after 21 days of growth?

Stomata are tiny pores on leaves that regulate CO₂ uptake and water loss. Their density influences how efficiently a plant can photosynthesize and adapt to stress. If ASA affects photosynthesis or growth, stomatal density may be one of the first indicators to change.

The Experiment Design

Watercress seeds were grown for 21 days across five ASA concentration levels: 0 mol/L (control), 0.00005 mol/L, 0.0001 mol/L, 0.0005 mol/L, 0.001 mol/L. Each condition received the same amount of water, light, and environmental exposure. After 21 days, leaf impressions were taken using clear nail polish, and stomatal density was measured under a microscope at 400x magnification.

What We Saw as the Plants Grew

As the watercress sprouts pushed through the cotton, the effects of aspirin were visible long before any microscope work. The plants given just a tiny bit of ASA grew taller and looked healthier, almost as if the low dose gave them a gentle boost. But when the aspirin concentration increased, the plants reacted very differently—shoots stayed short, leaves began to yellow, and a few even developed purple-tinged stems, which is usually a sign of stress in plants. Even germination slowed down. These early clues suggested that plants don’t mind a small nudge from aspirin, but too much quickly becomes overwhelming.

Low Doses Lead to Increased Stomata

When it came time to examine the leaves, the pattern became even clearer. The plants that received the tiniest dose of ASA had the most stomata—tiny pores that help plants breathe and take in carbon dioxide. But as the aspirin levels increased, the number of stomata dropped. The higher the dose, the fewer pores the plants developed. The trend was unmistakable: low aspirin seemed helpful, while high aspirin held the plants back. Even without knowing exact numbers, you can easily see the story—plants thrive with gentleness, not intensity.

Why Aspirin Changes How Plants Grow

This whole reaction comes down to how plants respond to salicylic acid, which is what aspirin turns into once it dissolves in water. In small amounts, this compound helps plants grow and stay healthy. But in large amounts, the plant interprets it as a danger signal. When that happens, it shifts into protection mode, slowing growth, tightening its pores, and reducing new cell formation. It’s like the plant saying, “Something feels wrong—better conserve energy and defend myself.” So instead of growing lush leaves with lots of stomata, the plant becomes cautious and conservative.