One of the problems with growing tomato plants in artificial environments especially under artificial lighting is a physiological disorder that negatively affects their growth and ultimately also the tomato fruit yield. The disorder manifests itself through the appearance of callouses or bumps on the leaves and veins of the plants, and rarely occurs in tomato plants that have been grown in greenhouses. We at LED Grow Lights HQ, have personally experienced this issue when growing cherry tomato plants indoors under the TaoTronics LED grow light - no longer sold - (which emitted light in three light wavelengths, namely, red (630nm), orange (610nm) and blue (460nm)). Fortunately in our case the problem was not too serious, and we were able to get a decent enough yield of tomato fruit. However, the disorder is likely to be an important issue for any growers thinking of using artificial environments to grow tomatoes, and it would be nice to avoid this problem in any of our own future indoor tomato grows.
To this end, a recent study from researchers in Japan (PDF link) have deduced that this ‘bumpy’ physiological disorder is likely due to inappropriate levels of certain environmental variables, namely, humidity, carbon dioxide (CO2) and light quality. In the case of humidity and carbon dioxide levels, too high a humidity level or too high a carbon dioxide level results in the appearance of these bumpy callouses on the tomato plants. The researchers found that reducing humidity levels to 60% - 80% relative humidity (RH) and keeping carbon dioxide levels down to between 400ppm and 500ppm was enough to significantly reduce the appearance of these callouses on the leaves of the plants. In our case, the humidity and carbon dioxide levels in our test grow were close to normal, and may be one of the reasons why the disorder was not too bad in our plants.
The study also identified another causative factor for this physiological disorder in tomato plants as being light quality. The researchers demonstrated that the lack of ultraviolet light specifically is likely to be the reason for the appearance of the bumpy physiological disorder in tomato plants grown under artificial lighting. This is because most artificial lighting lacks the ultraviolet part of the light spectrum that plants are normally exposed to when growing outside under natural sunlight. The researchers in this study found that by applying a small amount of exogenous UV-B light almost completely eradicated any sign of the physiological disorder developing on the leaves of the young plants. Moreover, when the UV-B light was removed, the physiological disorder returned a few days later, reinforcing the evidence that UV-B was playing an important role in keeping the tomato plants healthy. One thing to note however, applying too much UV-B tended to retard growth of the plants, so getting the right balance of UV levels is necessary for healthy growing seedlings.
Takeaway for the indoor gardener
So if you're thinking of growing tomatoes indoors especially under LED grow lights, you probably want to first check your humidity and CO2 levels are both within acceptable ranges (60-80% RH, less than 1000ppm CO2). Then and perhaps most importantly, make sure your grow lighting includes a small amount of UV-B light.