It has been known for many years that some plants respond to day length to control certain activities such as flowering, dormancy, and general growth, etc. The ability is called photoperiodism, and it is not limited to plants. Many organisms take cues from the number of hours of daylight (or number of hours of darkness). In more recent years a number of proteins have been identified called photoreceptors; these molecules respond to different wavelengths of light and have been shown to effect some of the plant responses to light (e.g., shade avoidance and choloroplast movement within cells). One of the more obvious photoperiodic responses of plants is flowering. How do plants know when to flower? Many plants can grow in cool temperatures and even flower, but it would be folly to flower too early in the season—a late frost could destroy all of the flowers, which tend to be tender. Thus, many plants have evolved a mechanism to delay flowering till days are longer, hence the season is more advanced and chances of a frost are reduced. Of course, there are many other reasons plants bloom at a certain time, one major factor is the presence of a pollinator. There are many plants that bloom only at the precise time their pollinating insect can be expected to be present. In fact some insect-plant pollinator relationships are very fine tuned, and the flowers and insects are only active for a few weeks each year. If they miss each other it might spell doom for both species; no seeds set for the plant and no pollen or nectar for the insects. Anyway, a molecular mechanism that controls flowering in Arabidopsis has been worked out in the past few years. The mechanism turns out to be fairly simple. Photoreceptors are activated by light, and when they are active they can prevent the degradation of a protein called CONSTANS. In turn, CONSTANS can activate certain genes that promote flowering. As with any fine tuned system there are opposing controls. Other photoreceptors cause the degradation of CONSTANS. So there is a balance between the gain of CONSTANS and the loss of CONSTANS. Finally, it turns out that the two opposing mechanisms occur late in the day (stabilizing CONSTANS) or early in the day (degrading CONSTANS). The effect is that levels of CONSTANS rise and fall each day. However, the stabilizing effect is enhanced as days become longer and more CONSTANS is able to survive. Finally, when the hours of light reach a critical value, enough CONSTANS accumulates to activate flowering genes. Plants are clever indeed. From what we know about them it seems likely that CONSTANS is only one way plants regulate flowering by day length. Probably we’ ll know more in the next few years. LINK 1: Photoreceptor Regulation of CONSTANS (Abstract) http://ncbi.nlm.nih.gov/entrez/query.fcgi/… list_uids=14963328&dopt=Abstract LINK 2: Science Update Article http://www.nature.com/nsu/020916/020916-12.html LINK 3: Photoreceptors in Arabidopsis http://www.pnas.org/cgi/reprint/100/4/2140.pdf Cordially, Joe , zone 9, Snowdrops emerged this week (Galanthus hybrid)