Stressed Plants, Stronger Medicine

A plant cannot run from drought, hide from frost or swat away a hungry caterpillar. Its only defence is chemistry. When life gets hard, a plant responds by making more of the compounds that protect it: natural sunscreens, antioxidants, antimicrobials and bitter deterrents. Many of these are the very compounds that make foods and herbs so valuable to our health. It turns out that a plant's struggle may be our gain.

How plants defend themselves

Plants make two kinds of compounds. Primary metabolites, such as sugars, proteins and fats, keep them alive and growing. Secondary metabolites, the polyphenols, flavonoids, terpenes, alkaloids and glucosinolates, are made mainly for defence and communication. (See The Inner Life of Plants.)

Making these defensive compounds costs a plant energy, so it makes more of them when it needs them most:

• Strong sunlight and UV trigger flavonoids and anthocyanins, the deep red, purple and blue pigments that act as a natural sunscreen.
• Drought concentrates flavour, colour and polyphenols. In grapes, water shortage switches on the genes that make anthocyanins and other flavonoids in the skins.¹ (See Not Written in Stone.)
• Cold prompts many plants to build up antioxidants and natural sugars, which is why parsnips and kale taste sweeter after a frost.
• Insects, fungi and damage set off chemical alarms. Grapevines, for example, make resveratrol when they are attacked by fungi or injured.²
• Poor soils slow a plant's growth, so it puts more of its energy into defence than into size.

Xenohormesis: borrowing a plant's resilience

Hormesis is the idea that a small, manageable dose of stress makes a living thing stronger.³ We see it in our own bodies: exercise, a cold swim, a short fast and even a hot sauna each place the body under mild stress, and it responds by switching on repair and protection, becoming more resilient in the process.

Xenohormesis ("xeno" means "foreign") takes this one step further. It suggests that when we eat plants that have been through stress, their defensive compounds act as signals inside us too, switching on our own protective pathways.⁴ Many plant compounds, including sulforaphane from broccoli, curcumin from turmeric and resveratrol from grapes, are gentle stressors that activate a master switch in our cells called Nrf2, which turns on hundreds of genes involved in antioxidant defence, detoxification and repair. It may be one reason why the colourful, bitter and pungent compounds in plants are so good for us, even though they are not "nutrients" in the classic sense.

A plant's struggle may be our gain.

What this means on your plate

Organic food

Conventionally grown crops are protected from pests by pesticides and fed generously with fast-acting fertilisers, so they have less need to defend themselves. Organic crops, by contrast, must fend off pests and work harder for their nutrients. A major review of 343 studies found that organic crops contained substantially higher levels of antioxidants and polyphenols, between 18 and 69% higher for several important groups, as well as less cadmium and fewer pesticide residues.⁵ In a ten-year study comparing tomatoes grown side by side, organic tomatoes contained 79% more quercetin and 97% more kaempferol, two protective flavonoids, than conventional ones.⁶ (See Why Eat Organic?)

Wild and foraged plants

Wild plants live with no one to water, feed or protect them, and many are far richer in protective compounds than their cultivated cousins. Wild rocket is more pungent than salad rocket, wild garlic more potent than many cultivated varieties, and dandelion and nettle leaves are packed with minerals and polyphenols. (See Closing the Divide for foraging safely, and Bitter Is Better.)

Sprouts and seedlings

Young plants are at their most vulnerable, so they are packed with defences. Three-day-old broccoli sprouts contain 20 to 50 times more glucoraphanin, the compound that becomes sulforaphane, than mature broccoli heads.⁷

Colour, bitterness and bite

A plant's defences are often things we can see, taste and smell: deep colours, bitter flavours, pungent aromas and a peppery bite. Darker, stronger-tasting varieties are usually richer in protective compounds than pale, mild ones.

Herbs grown the hard way

Herbalists have long known that where and how a herb is grown matters. Mediterranean herbs, such as rosemary, thyme, sage and oregano, evolved on dry, rocky, sun-baked hillsides. Grown in rich, well-watered soil, they become lush and leafy but often weaker in scent. Grown "lean", in poor, free-draining soil and full sun, they produce more of the aromatic oils that give them their flavour and medicinal power.

The same is true of many adaptogens, the herbs that help us cope with stress. Rhodiola grows on cold, windswept Arctic and mountain slopes, including Scotland's sea cliffs, where it's known as roseroot. Maral root grows in the high meadows of the Siberian mountains, and ashwagandha thrives in hot, dry soils. Shilajit, prized in Ayurveda, isn't a plant at all but a mineral-rich resin, formed over centuries from plants pressed deep in Himalayan rock, which seeps from the cliffs in the summer heat. Many traditional healers believed that the toughest plants made the strongest medicine, and modern science is beginning to agree. (See The Inner Life of Plants.)

This does raise a question of sustainability. Demand for some wild herbs, including rhodiola, has led to over-harvesting in parts of their native range. Choose herbs from suppliers who grow them responsibly or harvest them sustainably, and look for certified organic sources where possible.

Applying it in daily life

• Choose organic where you can, especially for the fruit and vegetables you eat most often.
• Eat seasonally and locally. Plants grown in their natural season, and picked ripe, have had time to build their full range of compounds.
• Pick heritage and deeply coloured varieties, such as purple carrots, red onions, black rice, dark leafy greens and red-leaved lettuces.
• Embrace bitter leaves and pungent flavours: rocket, watercress, radicchio, garlic, onions, ginger and mustard.
• Eat the skins and outer leaves. They face the sun and the pests, so they are often richest in protective compounds. Scrub rather than peel organic potatoes, carrots and apples.
• Grow your own sprouts on the windowsill: broccoli, alfalfa and radish are easy and cost very little.
• Forage a little, safely and responsibly: nettles, dandelion leaves, wild garlic, blackberries and rosehips.
• Grow herbs with "tough love". Plant rosemary, thyme, sage and oregano in sunny spots with gritty, poor soil, and water them sparingly.
• Handle with care. Chop broccoli, garlic and onions and wait a few minutes before cooking to let their defensive compounds form.

At Rootwise, we care about where our foods and herbs come from, not just what they are. A plant that has weathered its environment brings that resilience to the table, and helps us build our own.