Plants and Light
In the darkness of December, light is important in fact and symbolism.
Plants have three basic responses or reactions to light. They are
photosynthesis, phototropism and photoperiodism.
Photosynthesis is, of course, the process on which all life on
earth depends. Radiant energy from the sun is converted into chemical
energy. The energy is stored in chemical bonds in sugars like glucose
Phototropism is the plant's movement in response to light. All
of us have seen the houseplant that leans toward the window. That
is phototropism. Growth hormones are produced which cause the stem
cells on the side away from the light to multiply causing the stem
to tilt. The leaves are then closer to the light source and aligned
to intercept the most light.
The most interesting response is photoperiodism. This is the plant's
reaction to dark and is controlled by the phytochrome pigment in
the leaves. The pigment shifts between two forms based on whether
it receives more red or far red light. The reaction controls several
different plant reactions including seed germination, stem elongation,
dormancy, and blooming in day length sensitive plants.
Some of our common ornamental plants are day length sensitive plants
requiring specific light conditions to initiate bloom. Poinsettia
and chrysanthemums are short-day plants. To set bloom, the night
(dark period) must be longer than 12 consecutive hours. Some plants
are so sensitive that if the dark is interrupted by even a blink
of light the plants will not bloom. Conversely, long day plants
require over 12 hours of light to bloom. A common long-day plant
is the Easter lily. Many plants do not have this requirement and
are called day-neutral. Asters have a combination requirement of
long-day followed by short-day.
Some seeds are also light sensitive. Germination is controlled
by the reaction in the phytochrome pigment. Many lettuce varieties
must have light to germinate. Lettuce is packaged and distributed
in foil packets to prevent sprouting before planting. Most weed
seeds are in this category. Have you noticed how every time you
till the soil more weeds shoot up? Weed seeds lie dormant in the
soil for years waiting for you to stir up the soil so they get enough
light to germinate.
Phytochrome also controls lengthening or elongation of stems. Leggy
plants in low light are one example. The light reaction in phytochrome
also guides the germinating seedling stem through the soil toward
The last photoperiod response is stimulation of dormancy. Several
things trigger dormancy, but a major one is the shortening day length.
This is critical when we move plants out of the area where they
evolved. For example, a sugar maple grown in the north but from
southern seed will not become dormant early enough to escape winter
cold injury. Therefore it is important to buy perennial plants from
seed sources at similar latitudes to our own.
The ways plants respond to light is a fascinating topic. On these
dark days remember your plants are reacting to the light or dark
in multiple ways.
December 2000 - January 2001: Winter
Gardening Tips | Plants and Light | Botrytis
(Gray Mold): A Disease for Many Plants | Choosng
a Christmas Tree Variety | Key Questions
for Garden Catalogs