AAAS: Stress Can Make Plants More Nutritious

Most people know the benefits of diets rich in fruits and veggies. They’ve been linked to lower rates of heart disease, diabetes and stroke, for instance. Yet Americans typically ignore those data, preferentially chowing down instead on meats, other foods rich in fats, and starches. Realizing this, researchers at the University of California—Davis decided to see if they could identify types of produce that generate a relative abundance per serving of flavonoids, vitamins and other beneficial plant micronutrients.

And they did. They’re in many old-style cultivated varieties of crops — ones that commercial growers have abandoned because these plants don’t hold as well to the vicissitudes of climate, transport to market and pests.

One cultivar of white-fleshed nectarines, for instance, can produce six times the quantity of total phenolic antioxidants as another under the same field conditions, Alyson Mitchell of UC-Davis reported this morning. Such field data show, she says, that cultivar selection is an important but “oft overlooked” factor not only in crop selection but also in nutrition. Mitchell reviewed her team’s findings this morning at an opening session of the American Association for the Advancement of Science’s annual meeting, in Chicago

Some of the bonus antioxidants her group has focused on have been linked to potent health-promoting properties. The problem, she argues, is that the marketplace today rewards farmers for yield and disease resistance in their crops, not for how much of these beneficial micronutrients a crop may contain.

Mitchell’s team has also been investigating whether organically grown fruits and veggies differ from those produced by conventional farming.

Her team compared identical cultivars grown on certified organic plots versus those where standard fertilizers and pesticides were being applied. And as a rule, organics far surpassed their conventionally grown kin for vitamins and beneficial micronutrients, such as the antioxidant flavonoids quercetin and kaempferol, Mitchell reported

She also thinks she knows why that is. Plant nutrients tend to fall into two broadly defined categories: primary and secondary plant metabolites. We know the first category better. It includes fats (or oils), carbohydrates, amino acids and simple sugars. The second group includes the phenolic acids, flavonoids, alkaloids and terpenoids.

Conventional farming has optimized its practices and crop amendments to maximize a plant’s production of the primary metabolites. These are the ones listed on food labels. However, plants normally have a fairly balanced ratio of both primary and secondary metabolites: the primary ones don’t dominate.

And that makes sense, Mitchell points out, since many of the secondary metabolites are defense compounds — essentially a plant’s natural pesticides or sun screens, for instance.

When plants aren’t stressed, they produce fewer of these compounds. But the relative paucity of plant-protective agents available to organic farmers means that crops on their farms tend to suffer more damage from pests and the weather. And they respond by revving up production of defensive secondary metabolites.

The extra stress that organically grown plants typically experience may lead to less attractive veggies — like spinach greens with a hole in each leaf. But the resulting nutritional value of each gram of spinach from moth-eaten plants can be superior.

It’s time, Mitchell argued, that we consumers — and the marketplace generally — find ways to reward farmers for the nutritional quality of their crops.

And one potential bonus: Better taste. Some of the secondary plant metabolites break down into flavor compounds, Mitchell says.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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