Light is needed as a driver of growth, development, physiology, and metabolism. By providing different wavelengths at different stages of plant life, it is possible to get kales with desirable features.
Different Kale Species
Different varieties of Brassica oleracea, give us many of the common vegetables like broccoli cauliflower, kohlrabi, and kale. However, it should be noted that not all kale varieties come from Brassica oleracea; Russian red kale is Brassica napus (1, 2). Russian red kale differs from the green of varieties in its color and also tastes sweeter.
Kale is popular as it has various vitamins- C, A, K, magnesium, and fiber. It contains more antioxidant polyphenols, effective against effects of aging, as well as glucosinolates, ßcarotene, and carotenoids than the other Brassica vegetables (1, 3). In terms of nutrients, both red and green kales have similar properties (4). These beneficial chemicals or nutraceuticals are produced as secondary metabolites in kales (1).
Red, blue, and white light have similar effects on seedling growth, so any one of them can be use in this stage. As with other crops it is not advisable to use far-red, as it can inhibit germination (1).
During the seedling stage, far-red light can inhibit stem growth rate (1). While using Blue light for 21 days or more can decrease leaf length in kale (5).
Colors of Kale
Anthocyanins are the compounds which produce the red color in leaves and fruits and make kale more nutritious. The green is derived from chlorophyll.
Far-red increases anthocyanins contents. The longer kale is exposed to light and absorbs it, the darker it becomes in color. Since anthocyanins affect plant pigmentation using far-red light alone should be enough to get the red colored desired in Russian kale. In case of other kale varieties which are green, therefore far-red is not suitable (1).
Chlorophyll accumulation is good under red light with lower fluence rates in seedlings (1). However, a combination of blue and red lights which increases chlorophyll content by 50% than red light alone would be the ideal combination in case of green kale varieties in the seedling stage. Once the plants are bigger, accumulation of chlorophyll was maximum under red light (3). Far-red alone has a detrimental effect on chlorophyll accumulation (1). Far-red and red in the ratio of 3:1 produces the maximum level of anthocyanins and chlorophylls, making them suitable for green kales (1).
Secondary Metabolite Production in Kale
The different secondary metabolites in kale show a complex pattern of response to different light wavelengths.
Anthocyanins accumulation was similar under white, red, and blue light. The anthocyanins concentrations can be increased further to what is obtained under far-red by adding blue light with red. When blue light was used with far-red, anthocyanin production was improved by 30% than when far-red was used alone (1).
Similarly, far-red light helps in the accumulation of 25% more of antioxidants, than white or blue light. Red light resulted in the lowest concentration of antioxidants. White light alone can also be used to improve antioxidant levels (1).
Far-red light also increased the levels of total GLs by 15-42% over darkness, while red, and blue had no effect at all (1). Mixtures of light are not advisable for GLs, as adding blue light to far-red can decrease GL levels (1).
Lutein is the so-called eye vitamin because it prevents eye diseases associated with age (6). Sinigrin is a natural aliphatic glucosinolate that is anti-cancer, antibacterial, antifungal, antioxidant, and anti-inflammatory (7). Increasing levels of these two chemicals beneficial to people has been attempted with LEDs.
Lutein and sinigrin levels responded to LED grow light treatment, and reached maximum levels under red light, and did moderately well under blue. Lutein production was the least under far-red (3). Other secondary metabolites were not affected by irradiance, especially ß-carotene levels (3). Green light can enhance levels of vitamin C and phenolic content (9).
So if far-red can increase levels of anthocyanins, GLs and antioxidants, they decrease some of the vitamins.
In general, UV radiation can be used to enhance the levels of secondary metabolites which are responsible for increasing resistance to pathogens such as fungi and other external stresses (9).
LED grow lights can be used not only during the cultivation but also in the post-harvest phase to keep the kales fresh, safe and nutritious.
After harvest, it is better to retain the light and dark cycles for kales, as this maintains tissue integrity, green coloration and chlorophyll content, similar to storage in cool refrigerated temperatures. By maintaining chlorophyll content, the appearance of kale is also retained. Moreover, the levels of glucosinolates in kale also remain high when the biological rhythm is maintained, instead of storing in completely dark or light conditions. A 12 hour light and 12 dark periods are recommended (8). Since LEDs produce less heat, it is advisable to use them instead of any other light source to maintain the biological rhythm to enhance preservation of the crops (9).
Shelf-life of kales during storage can be extended by using red light, as it delays senescence in kales (9). Red light also improves water retention in vegetables in general by closing stomata, but blue light will have the opposite effect (9).
Blue light becomes important for protecting the kale from diseases in the post-harvest stage Blue light offers protection to vegetables against fungal infection and bacteria post-harvest (9). So a combination of more red and some blue lights are advisable in storage.
It is interesting to note that it is not just the wavelength or the intensity that is important but also fluency rates, that is the amount of light the plants absorb over a period of time. By changing light regimes, different kinds of plants can be obtained from the same variety of kale, especially Russian red kale as its shows plasticity in its response (1).