Like lots of millennials, I like to spend some time on a smartphone or tablet before going to sleep. It has been a habit deeply ingrained thanks to incredible advances in mobile technology. However, it brings about one negative side effect: the light emitted from those electronic devices interferes with humans’ circadian rhythms and causes difficulty to fall asleep. It turns out the culprit is blue light, the electromagnetic radiation roughly between 400 and 500 nm. Perceived by the photoreceptors like rods and cones in our eyes, blue light can inhibit the concentration of melatonin, which is a pigment essential to signal the onset of darkness in our circadian rhythms (Kathleen et al., 2011). As a result, blue light promotes alertness and makes it challenging to fall asleep. Apple is clearly aware of this. A new feature called Night Shift came with the latest operating systems since iOS 9.3. You can set a time in the evening to switch on Night Shift, which blocks blue light and thus creates a warm color on the screen. The goal of this feature is to ameliorate the user’s sleep quality if the phone is a sleep companion.
Without exposure to regular daily patterns of light and darkness, circadian rhythms can become irregular and cause sleep disorders in humans. Unsurprisingly, plants have circadian rhythms, too. For them, is it really a bad thing to have some light at night? Aaron Rose, a film director and writer, once said, “In the right light, at the right time, everything is extraordinary.” The right light at the right time that disrupts plants’ sleep can indeed create benefits in greenhouse production of ornamental crops from bedding plants like petunia to potted plants like poinsettia.
Plants are classified into three categories: long-day, short-day and day-neutral, depending on their responses to the duration of light and dark. Long-day plants flower most rapidly when the daylength is sufficiently long, and short-day plants flower most rapidly when the daylength is sufficiently short. When natural days are short, we can use electric lighting at night to promote flowering of long-day plants to shorten production time or delay flowering of short-day plants to obtain sufficient vegetative growth. This strategy is called photoperiodic lighting, which occurs at the beginning of the night (day extension, DE) or in the middle of the night (night interruption, NI) (Fig. 1).
Fig. 1. Lighting at night during short days can promote flowering of long-day plants and inhibit flowering of short-day plants.
Plants’ flowering responses to daylength are attributed to two main factors, circadian rhythm and genes that mediate flowering. For long-day plants like Arabidopsis, CONSTANS (CO) is a transcription factor that ultimately triggers flowering. The expression of CO oscillates in a circadian rhythm; the level of CO protein is fairly low during the day but high in the evening. When days are short in winter and early spring, the accumulation of CO protein is so minimal that flowering is barely induced. However, if light is provided in the evening or at night, CO is expressed, leading to CO protein accumulation. CO protein then activates a flowering hormone (florigen) like Flowering Locus T (FT), which induces flowering. Therefore, circadian rhythm and daylength must coincide to induce flowering. The coincidence model is illustrated in Fig. 2 (Taiz and Zeiger, 2009).
Fig. 2. The coincidence model for photoperiodic control of flowering of Arabidopsis.
The genes for regulation of flowering are different in short-day plants, but the principle is similar. A variety of plant photoreceptors are responsible for perception and transduction of light in regulate of flowering. Phytochrome and cryptochrome are two examples. Phytochromes primarily absorb red (600 to 700 nm) and far-red (700 to 800 nm) light, whereas cryptochromes blue light. Multiple members exist in each of the photoreceptor families, including phytochrome A, B, C, D and E as well as cryptochrome 1 and 2.
As mentioned previously, blue light is the waveband that disrupts humans’ sleep. However, it is mainly red and far-red light that disrupt plants’ sleep. Stay tuned with LightHort as we’ll soon talk more about how different colors of light at night influence flowering.