Too Much

The following are selected quotes indicating what effects color of light (spectrum) and quantity of light have on plant growth.

Spectrum and plant growth

Terrestrial plants are extremely sensitive to the red/far-red ratio (called the zeta ratio). Changes in the zeta ratio can completely alter the structure and growth of plants. Aquatic plants, however, are likely far less sensitive to the zeta ratio because of the rapid and variable attenuation of light in fresh waters.

 

Darkness period and plant flowering

The distinction between describing a plant as a short day plant or a long night plant is not important as long as the plant is on a 24 hour cycle. If it gets short days, it will automatically get long nights. The distinction was made because it was found that plants measure the night length, not the day length. There is a pigment in plants called phytochrome that exists in two forms, phytochrome red (P660) and phytochorme far red (P700). Plants begin their nights with most of the pigment in the P700 form, which slowly converts to P660 during the night. The amount converted is the measure of the night length.

P660 absorbs red light, with a peak absorbance at a wavelength of 660 micrometers. When P660 absorbs red light, it converts to P700. P700 absorbs far red light, with a peak absorbance at 700 micrometers. When P700 absorbs far red light, it converts back to P660. Daylight has a lot more red light than far red light, and that is why the plant starts off its night with mostly P700, the form that slowly reverts to P660. A short day (long night) plant needs a long night to accumulate enough P660 to trigger the hormonal sequence that leads to blooming. If the night is too short, P660 does not build up to high enough levels to trigger blooming. The two phytochromes are quite sensitive to light, and even room lighting has enough red light to keep the "clock" from running, i.e., keep any P660 from building up. Even the relatively dim light from street lights has enough red light to slow down the clock and give plants the "misinformation" that the night is a lot shorter than it really is. Every November I see weeds growing near street lights that delayed blooming and got killed by the frosts while still in the vegetative state. Further away from the lights, the weeds have gone to seed in plenty of time.

Steve Pushiak mentioned on Jan. 22 that his Ocelot sword bloomed after he had been on vacation for a week. While the house was unoccupied, there were no room lights on after dark to prevent the clock from running, and the plant got the long nights required for blooming. Room light strong enough to keep the clock from running is not strong enough for any meaningful photosynthesis.

"Normal" light that plants are likely to encounter has much more red than far red light, and so the effect is always to reset the clock to the point where nearly all the phytochrome is in the P700 form. With just a brief flash of red light in the middle of a long night, the clock will be reset, and the plant starts counting from the beginning. With a special filter that only allows far red light through, it is possible, with a flash of far red light, to run the clock to the end and create the effect of a long night.

Offered by Mike.