The length of the day relative to night, or photoperiod, is a strong determining factor for the induction of flowering in many plant species. Short day (SD) plants require a short day length (or more precisely, a long night) in order to flower. These are plants that flower as the days grow shorter, such as in the fall in temperate regions. Long day (LD) plants will flower when nights are short (and days are long), and typically flower in late spring or early summer. SD crops include rice and maize, and LD crops include wheat, barley, oats and peas. Day-neutral plants will flower under either long or short days. In addition to its fundamental importance in basic plant biology, understanding and manipulating the photoperiodic control of flowering time is an important objective in crop breeding and development programs, because it can aid in optimizing crop yields and other traits for local environmental conditions.
Experimental evidence indicates that a flowering-inducing substance, known as florigen, is produced in the leaves of a plant under inducing conditions, and then is transported through the phloem to the floral meristems, where it acts together with other factors to induce flowering. The precise nature of the florigenic signal has eluded plant biologists for over 70 years, owing to the difficulties attendant with the accurate detection and measurement of compounds in phloem sap, and designing experiments to allow accurate monitoring of long-distance transport of potential signalling compounds in plants. Some of the principal factors associated with the photoperiodic induction of flowering are proteins known as CONSTANS (CO) and FLOWERING LOCUS T (FT). CO does not appear to travel long distances through the phloem, but rather, it strongly influences the production of another substance (i.e. florigen) that does. Recent work in a number of laboratories has focused on a role for FT protein and/or FT mRNA as the florigenic signal, and evidence is buil
Contact: William J. Lucas
American Society of Plant Biologists