Floral Evocation Internal And External Cues

A plant may flower within a few weeks after germinating, as in annual plants such as groundsel (Senecio vulgaris). Alternatively, some perennial plants, such as many forest trees, may grow for 20 or more years before they begin to produce flowers. Different species flower at widely different ages, indicating that the age, or perhaps the size, of the plant is an internal factor controlling the switch to reproductive development. The case in which flowering occurs strictly in response to internal developmental factors and does not depend on any particular environmental conditions is referred to as autonomous regulation.

In contrast to plants that flower entirely through an autonomous pathway, some plants exhibit an absolute requirement for the proper environmental cues in order to flower. This condition is termed an obligate or qualitative response to an environmental cue. In other plant species, flowering is promoted by certain environmental cues but will eventually occur in the absence of such cues. This is called a facultative or quantitative response to an environmental cue. The flowering of this latter group of plants, which includes Arabidopsis, thus relies on both environmental and autonomous flowering systems.

Photoperiodism and vernalization are two of the most important mechanisms underlying seasonal responses. Photoperiodism is a response to the length of day; vernaliza-

tion is the promotion of flowering—at subsequent higher temperatures—brought about by exposure to cold. Other signals, such as total light radiation and water availability, can also be important external cues.

The evolution of both internal (autonomous) and external (environment-sensing) control systems enables plants to carefully regulate flowering at the optimal time for reproductive success. For example, in many populations of a particular species, flowering is synchronized. This synchrony favors crossbreeding and allows seeds to be produced in favorable environments, particularly with respect to water and temperature.

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