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09/01/1995 07:00:00

Caltech Science Question of the Month: When Do Plants Flower?

Question: How does a plant "know" when it is time to bloom?

Answer: Many factors work together to "tell" a plant when it is time to flower, including the amount of food and water available, light, temperature, and a plant's age. But just how a plant senses many of these things remains a puzzle.

To begin with, food and water regulate flowering time in many plants. For example, starved plants often flower earlier than well-fed ones. Somehow the lack of nutrients forces early blooming, as the plant tries to reproduce before it dies an early death.

Daylight affects all plants. Most people know that green chlorophyll absorbs light to make energy for the plant, but the light sensors that influence flowering are independent of photosynthesis. In fact, it is these other sensors that tell a seedling that it is above soil after it has sprouted, so that it can turn green and begin to photosynthesize.

There are several classes of plant photoreceptors—proteins that sense light. Proteins called phytochromes sense red and far-red light, and are the major sensor for greening, avoiding shade, measuring day length, sensing the proximity of other plants, and knowing when to flower. There are several related phytochromes, and scientists are just now figuring out the roles they play in these different processes.

Other proteins sense blue light and ultraviolet light, but these don't play such a key role in blooming. A protein called cryptochrome, whose structure was just figured out in the last year, responds to blue light, but the structure of the ultraviolet light-sensing protein remains unknown.

Plants also sense temperature, and we know from experience that they won't bloom until the air and soil are warm enough, but almost nothing is known about how they do this. Some scientists study how plants tolerate cold, but how the plants sense the cold and respond to it is unknown. Similarly, measurements of cotton leaves on hot days have shown that they maintain a constant temperature by regulating the rate at which water evaporates through stomata—the tiny pores in the leaf—but again, no one knows how they do this.

Temperature is also sometimes important to blooming in a way that is not as obvious. Some plants have to go through a cold spell, called vernalization, before they will flower, no matter what other conditions are like. For example, my plum tree only produces plums during the summer after a cold winter here in Pasadena. There is quite a bit of research in this field right now, but so far no one understands the molecular mechanism behind it. The plant's age also plays a key role in some species. It is a simple matter of maturity, though how plants measure time is not known. Botanists use the term juvenility to describe youth in plants, and it goes away gradually. In some plants the leaves gradually change shape, in others the leaf texture changes, until the plant is fully mature and able to flower.

For most common garden plants the major factors are food and water, light, temperature, and age. But for some plants there are additional critical factors that are a complete mystery. Some species of bamboo, for example, flower only at long intervals, with all plants flowering at the same time. The signal in this case in unknown.

This is a monthly feature produced by the Media Relations Office at the California Institute of Technology, in collaboration with Caltech's faculty, to answer commonly asked or particularly intriguing questions about science and the natural world. The Media Relations Office will accept any questions you may have.