A small amount of UV radiation is needed for tomato plant health

Original paper: Kubota, C., T. Eguchi, and M. Kroggel. 2017. UV-B radiation dose requirement for suppressing intumescence injury on tomato plants. Scientia Horticulturae. 226:366-371. https://doi.org/10.1016/j.scienta.2017.09.006

UV-B radiation (300-320 nm) has the shortest wavelength of the sunlight spectrum in the natural environment. Too strong UV radiation can cause issues such as leaf burn, but too little UV radiation also becomes problematic for certain species of plants including tomato. The sensitivity varies among varieties of tomato and a particular rootstock tomato used for grafting, for example, is a very sensitive one that must be grown under light including UV-B.  A typical disorder caused by lack of UV-B radiation are leaf tumors called intumescence (or oedema). In severe cases, plants cannot grow and usually die. This becomes problematic when plants are grown under protected environments (such as tunnels) covered with UV-blocking plastic material or under sole source electric lighting that does not emit UV radiation (such as LEDs). A research group led by Chieri Kubota (currently at the Ohio State University) identified the needed amount of UV radiation to maintain plants without causing such disorder.  This group also worked on other innovative approaches to mitigate intumescence injury, including a discovery of an effective LED lighting protocol to mitigate the intumescence injury.

In their experiment, they grew rootstock tomato ‘Beaufort’ plants as their experimental plant material for its high sensitivity to induce intumescence under UV-B deficient light environment.  The lamps that they tested were red and blue LEDs or cool-white T5 fluorescent lamps. While red and blue LEDs do not emit UV radiation at all, T5 fluorescent lamps do emit small amount of UV-B radiation. When ‘Beaufort’ plants were grown under LEDs, plants developed massive amounts of intumescence, but the injury was less severe under T5 fluorescent lamps due to the UV radiation.  UV-B supplementation was examined for the plants under LEDs at varied hours of UV-B exposure (1.5, 3.0 and 6.0 hours) at a very low intensity of UV-B of 0.12 W m^-2 (or 0.31 μmol m^-2 s^-1). The UV-B exposure was conducted during the night time (every night), rather than day time. Kubota’s group showed that an increasing amount (dose) of UV-B exposure drastically improved ‘Beaufort’ tomato plant health.  The highest daily dose of UV-B they examined was 2.6 kJ m^-2 (or 6.7 mmol m^-2). This UV-level is less than one-tenth of what you can expect outdoors during the summer.  Under this dose of 2.6 kJ m^-2, plants still exhibited a minor injury of intumescence. So the group used a mathematical approach (linear regression) to estimate the dose of UV-B that would have eliminated the intumescence injury, which was 5 kJ m^-2 (or 14 mmol m^-2), about twice of the highest examined dose in this study, but still far below the natural UV level of sunlight.

The impact of this study includes possible contributions to the design of plant growing facilities that use LED lighting.  The linear dose response they described in this study is particularly useful as one can choose a combination of UV-B intensity and exposure time suitable for their production setting.  A lower intensity for long exposure and a higher intensity for shorter exposure could yield the same effective dose.  Kubota group also showed that night-time application of UV-B is effective and this is helpful for growers who do not want to expose workers to UV-B light while they are in the facility.  The known/Knowing the dose response also makes the application methods more flexible, including using a movable light source over the plant canopy. The finding will be helpful for Indoor farming as well as greenhouse crop production.