[Pollinator] SF Chronicle: Plants send and receive signals in living color

[Jennifer Tsang]

http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/11/27/HOA2TI0H4.DTL

 


Plants send and receive signals in living color


Ron Sullivan,Joe Eaton

Tuesday, November 27, 2007

Enjoy that sunset or that Rothko painting? Thank a plant. 

Speaking metaphorically, mammals opted out of color vision when we were
scurrying around in the dark, dodging dinosaurs; your dog doesn't see
rainbows as well as you do because our common ancestor didn't. But our
closer primate ancestors had an advantage if they could recognize ripe fruit
in a rain forest, and plants advertise their fruits' mature sweetness and
their seeds' readiness for distribution by suffusing them with color. Color
vision had evolved previously in invertebrates and vertebrates both, and
some mammal lineages tapped back into that latent capacity. 

In fact, as we animals co-evolved with plants, well-advertised fruits gained
better seed distribution via mammals and birds, and poster-bright blooms
further strengthened the partnership by helping pollen get distributed to
conceive those seeds in the first place. 

Flowers are a plant's pollination advertising campaign, and every color
sends a message to a particular kind of pollinator. Biologists recognize
different "pollination syndromes," combinations of flower color, shape,
scent and blooming time, fine-tuned to the senses of bees, butterflies,
birds or bats.

Birds have excellent color vision. Bird-pollinated flowers (California
fuchsias, for example) are often red, not so much because birds prefer red
as because bees don't. Bees aren't red-blind, but they have more trouble
picking out red flowers from green foliage than they do blue ones, so it's
more efficient for them to specialize in blue flowers.

Bees can also see into the ultraviolet. Some flowers that look yellow or
white to us appear purple or blue-green to them. Bee flowers often have
nectar guides - petal marks pointing to anthers and pistils, like arrows on
a runway - that are invisible to humans, but clear as day to a bee. Although
hummingbirds and some flower-visiting bats have UV vision, "their" flowers
lack such signals. Bird flowers are also less likely to be scented, since
most birds lack a well-developed sense of smell.

Butterflies vary; some prefer yellow and blue, others blue to purple. Bats
and hawk moths go for white flowers. Red-flowering scarlet gilias produce
paler flowers when migrant hummingbirds depart and moths replace them as
pollinators. North American columbines, sorting themselves into new species,
have evolved new colors to attract new kinds of pollination partners.

Color signals can be switched off as well as on. In plants as diverse as
trillium, lantana and Texas bluebonnet, flowers change color once they've
been pollinated, advising prospective customers that the nectar bar is
closed.

Sometimes plants use specialized leaves to attract pollinators. The showy
"flowers" of poinsettia, heliconia and bougainvillea are not flowers at all
but pseudanthia made of bracts - modified leaves.

Leaf colors have other functions. The default is green, of course, from the
chlorophyll that manufactures a plant's food. Some variations on green are
not a good thing: a chlorotic (yellow) leaf usually signals a deficiency of
nitrogen, magnesium or iron. But healthy plants can have vividly
multicolored leaves: Consider the coleus and the croton.

Those colors derive from pigments stored in microscopic vacuoles within the
cells of the leaf. Most common are anthocyanins, which range from pink
through red and purple to blue and occur in flowers and fruits as well as
leaves. The pigment overlies a leaf's chlorophyll and doesn't interfere with
photosynthesis. Anthocyanins are believed to screen out harmful levels of UV
radiation.

In a minority of plants, the active ingredients are betalains, used by
botanists as markers of relationships among diverse-looking forms.
Nepenthes, those strange carnivorous hanging-pitcher plants, got moved all
the way to a different order because their red mottlings are betalains.
These pigments, usually purple, red or orange, are also what beets
(including Swiss chard), ice plants, purple prickly pear cacti and
bougainvillea have in common. Their function is obscure, but they might be
antifungals. 

Some leaves are two-toned - green above, purple below. This may allow
forest-floor plants to make the best of low-light conditions, bouncing light
waves back into the leaf's photosynthetic cells.

Don't forget the structural colors. Silvery leaves get their color from
hairs called trichomes, which reflect infrared radiation that might
otherwise cook the leaf. They're often associated with hot or arid
locations, as with the Hawaiian silverswords. Some bluish leaves have a waxy
coating that may waterproof the leaf, filter UV radiation or reflect
infrared radiation.

A new exhibition at the San Francisco Conservatory of Flowers, "Color: A
Winter Carnivale," showcases the colors of flowers, fruit and foliage. It
features ambient calliope music, carousel animals (from local carver Oscar
Pivaral) and a Wheel of Botanical Fortune. 

Two years in the making, "Color" represents a collaboration between Curator
of Education Lisa Van Cleef and designer Anjel Van Slyke. "I ran around to
all the Bay Area nurseries asking, 'What's in bloom?'" says Van Cleef. "It's
not just about the flowers - the foliage contains so much color."

Van Cleef says the exhibition is designed to appeal to kids as well as
adults: "How do we get kids as excited about plants as they are about
animals? We need to show them how to look at the ordinary. When they get out
of here, they'll see plants differently."

Joe Eaton and Ron Sullivan are freelance nature and garden writers in
Berkeley. E-mail them at home at sfchronicle.com. 

http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/11/27/HOA2TI0H4.DTL

This article appeared on page G - 8 of the San Francisco Chronicle

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