Growing our own fruits and veggies gives us access to the freshest crops possible, which is one way to ensure we get the maximum boost of both flavor and beneficial phytochemicals.
Many of our indoor gardening favorites such as tomatoes, capsicums, strawberries, arugula and watercress are natural sources of important bioactive compounds that can be increased within the plant under hydroponic growing conditions.
Using hydroponics, growers are able to fully manipulate their growing environments and nutrient regimens to boost certain plant bioactives well above those found in mass-produced commercial produce.
So, what are bioactive compounds? Bioactive compounds are extra nutritional compounds that occur in small quantities in food. They are present in addition to the fiber, carbohydrates, vitamins and minerals we associate with a healthy diet, and many bioactives are still undergoing intense investigation.
Types of Bioactive Compounds
There are several types of bioactive compounds found in a wide variety of plant foods, which can be broken down into categories such as flavonoids, carotenoids, plant sterols and glucosinolates.
Flavonoids are a large group of phenolic bioactive compounds involved in a diverse range of plant processes, including UV protection, coloration and disease resistance. Flavonols are the most widespread flavonoids and are found in onions, kale, broccoli, apples, cherries, berries, tea and red wine.
Flavones are similar to flavonols but are less common. They are found in parsley, thyme and celery. Capsaicin from chili peppers is a phenolic compound as are tannins found in teas and many other types of plant material. Phenolic compounds include anthocyanins, which give berries and flowers their characteristic red, blue and purple colors.
Carotenoids are well-known bioactive compounds found in commonly grown plants such as carrots, tomatoes, peas, spinach and peppers. They give fruits and vegetables their distinctive yellow, orange and red pigments. One of the most well-known carotenoids is lycopene, found in tomato skin. Others include lutein in yellow peppers and carotene in carrots.
Plant sterols help reduce cholesterol levels when taken in sufficient amounts. They are largely obtained from nuts, seeds, grains and avocadoes, and nowadays are incorporated into a range of plant sterol-enriched margarines and spreads.
Plants containing glucosinolates are easy to grow and take up little space. Vegetables containing high levels of glucosinolates include broccoli, cabbage, kale, watercress, wasabi, Brussels sprouts, arugula, salad cress, mustard, horseradish and other cruciferous crops.
These compounds, which are converted into isothiocyanates during food preparation, are continually being studied for their cancer-fighting and other medicinal properties.
Allium species such as onions, shallots, garlic and chives contain sulfur compounds that are reportedly anti-inflammatory and powerful antioxidants good for lowering cholesterol, protecting against cancers and boosting the immune system. Shallots have the most antioxidant activity of all the alliums.
Top Crops for Bioactive Compounds
Many superfoods contain more than one category of bioactive compound and deserve more attention when it comes to hydroponic production. Apart from the brassicas, such as broccoli and watercress, and alliums, such as onions and garlic, the main hydroponic crops of interest are berry fruits, capsicums and tomatoes.
Berries contain phenolic compounds, including anthocyanins, phenolic acids, tannins, carotenoids and vitamin C, which represent a wide range of antioxidants. Berries are also said to have anti-carcinogenic and anti-inflammatory properties. They can assist with maintaining good eyesight.
Tomatoes are a perennial favorite amongst hydroponic growers. Not only do they taste great and contain a good range of vitamins and minerals, they also contain important bioactive compounds.
Of these, lycopene, the bright red pigment in tomatoes, has a number of positive properties, like protecting human skin from sun damage. It is also said to be a powerful antioxidant and an anti-carcinogen. Lycopene is fat-soluble, so to extract and absorb as much lycopene as possible, eating tomatoes with some healthy oils is recommended.
Capsicums, bell peppers and hot chilies are well-suited to indoor gardens and have diverse levels of bioactive compounds, depending on their type, cultivar, growing conditions and degree of ripeness at harvest.
Generally, capsicums contain good levels of carotenes, phenols, capsaicinoids, xanthophylls and flavonoids. Hot chilies contain much higher levels of the phytochemical capsaicin—a potent antioxidant—than sweet bell peppers, but bell peppers have good levels of flavonoids and phenolic acids.
For those with limited space, growing plants specifically for their bioactive compounds can still be achieved with microgreens. While there is a diverse range of microgreen species with equally diverse levels of different phytochemicals, those with high levels of bioactivity are red cabbage, red radish, garnet amaranth and cilantro.
Overall, microgreens generally have similar or higher levels of bioactive compounds than their mature versions, while only requiring a fraction of the growing area.
Increasing Bioactive Compounds
Determining the growth factors that will have the greatest effect on increasing the health-giving compounds in hydroponic plants is not as simple as just providing optimal growing conditions.
In fact, many of the beneficial phytochemicals in plants are produced in greater concentrations when the plants are under stress, something we usually try to avoid. For example, lycopene in tomatoes increases when tomatoes are grown hydroponically under osmotic or salt stress created with a high nutrient solution EC in the root zone.
Tomato plants grown with a higher EC commonly see a 30-40% increase in lycopene concentration. If we couple this with the fact that different tomato cultivars have widely varying, naturally occurring levels of lycopene already, it is possible to grow tomato fruit with a superfood status.
Lycopene is generally highest in tomato cultivars with the darkest-colored skin, which includes many of the varieties listed as black types, which have purplish skin. While some of these darker-skinned tomato types are grown commercially, there is a wide range of older, heirloom varieties that are many times higher in lycopene than the standard red, commercial, hybrid tomato fruits.
By combining a tomato variety with a naturally high level of lycopene with increased nutrient EC conditions to supply a slight amount of stress, the bioactive levels in the fruits could be potentially boosted many times higher than the standard red homegrown tomato.
Fortunately, commercial tomato producers have started to take advantage of the increased consumer awareness of lycopene levels and some are growing hybrids specifically bred for higher levels of this compound. These high-lycopene tomatoes are marketed and sold in mainstream grocery stores as a “functional food,” making them a potentially profitable hydroponic niche crop for greenhouse growers.
Stressing plants out to increase bioactive compounds also works for hydroponically grown broccoli. Levels of phytochemicals with biological activity in broccoli are a result of both the genetics of the plant and altered environmental factors.
When hydroponic broccoli plants are grown with some applied stress (increasing the EC of the nutrient with sodium chloride), levels of certain bioactive compounds increase, resulting in a healthier crop.
The organosulfur compounds in onions, garlic and other alliums can also be manipulated with the use of hydroponic techniques. The concentration of organosulfur compounds in alliums is due to the large quantities of sulfur that accumulate as the plants grow. Sulfur concentrations in excess of 1% on a dry-weight basis have been reported in bulbing alliums, and much of this is partitioned into the healthy, organosulfur compounds.
The concentration of these important organosulfur compounds in alliums can be increased by increasing sulfur and nitrogen fertilization. In hydroponic trials, certain organosulfur compounds increased when nitrogen levels were increased, and garlic also responds in a similar way to increasing sulfur and nitrogen in the nutrient solution when grown hydroponically. This not only produces healthier bulbs, but also ones with increased flavor and pungency.
Genetics also plays a major role in the overall composition of fruits, vegetables and herbs, and this includes their levels of bioactive compounds. In the past, commercial varieties have largely been bred for yields, appearance and storage life.
These days, researchers are much more focused on the compositional quality of certain produce, with plant-breeding programs now in effect to enhance the levels of many of the commonly known health-giving compounds such as carotenoids and anthocyanins. These new varieties will be worth trialing as they come onto the seed market.
Growing plants specifically for their bioactive compounds and health benefits may seem like a new approach to hydroponic production, but with soilless growing methods, we have the ability to fully manipulate growing conditions to induce sufficient plant stress to boost their production of these naturally occurring compounds.
To maximize the bioactive compounds in crops, home gardeners and commercial growers need to consider which types of plants to grow, what the right cultivars are and the ideal conditions for increasing bioactive compounds.