Download Plants Seeing Red

Connection to Biology
Plants Seeing Red
V
egetable growers have long used plastic sheeting (mulch) in the rows between plants to retain moisture, retard weeds, and provide warmth
for the roots of young plants. It is now becoming
apparent that the color of the plastic used has a significant effect on the plants. How can this be? Why
does the color of their mulch matter to plants?
Michael J. Kasperbauer of the Department of
Agriculture’s Coastal Plains Soil, Water, and Plant
Research Laboratory in Florence, South Carolina,
has spent most of his 40-year career studying the
responses of plants to various colors of light. Plants
use proteins called phytochromes to sense light in
the red (640–670 nm) and the far red (700–
750 nm) ranges. Although far red light is not photosynthetically active—it does not provide energy
for plant growth—this light gives plants important
information about their environment. For example,
green leaves reflect a lot of light in the far red
region. Therefore, when a given plant’s phytochromes sense a high ratio of far red to red light
wavelengths, the plant knows that it has many
neighbors—many other plants around it are reflecting red light. Because these neighbors are competitors for the lifegiving light from the sun—their
leaves will shade neighboring plants—a plant sensing this situation tends to direct its growth above
ground, producing a taller, thinner structure that
can compete more successfully for sunlight. In fact,
Kasperbauer and his colleagues have found that by
using a red plastic mulch they can fool tomatoes
into “thinking” they are crowded, leading to faster
seedling growth and eventually producing earlier,
larger fruit.
Since the opposite effect should benefit root
crops, the Department of Agriculture scientists have
Red plastic mulch being used in an experimental plot.
grown turnips in soil covered by an orange mulch.
These turnips proved much bigger than those
mulched with black or red plastic, presumably because of increased reflection of red light by the orange plastic. The increased red light signaled no
significant competition for light from other plants,
encouraging growth of roots rather than aboveground foliage.
Besides affecting the plant’s structure, the type
of reflected light influences the nature of the waxy
coating on the leaves and the taste of the plant
product. Surprisingly, the color of the reflected light
also seems to affect the plant’s response to insect
damage.
This research shows that plants are very sensitive
to the type of red light that bathes them. Our plants
may benefit from rose-colored glasses as much as
we do.