Many
of us omnivores like to eat our steak or chicken drumsticks, but get rather
squeamish when it comes getting it from the field to our plate. Prepackaged from
the grocery store seems a much nicer option than chasing a chicken round the
yard even if it means that we have less exercise that we will eventually have
to make up on the treadmill in the gym.
Plants
on the other hand induce no sign of squeamishness at all. We happily pick
leaves off “pick and come again” lettuce plants. Many different species of
herbivores will also munch on our plants, but we should remember that the
plants are going to do their best to combat this.
The
problem of how a plant recognizes that it is wounded has been studied recently
by Heil et al of CINEVESTAV in Mexico
and published in the Public Library of Science (1). Their study was mainly on
lima beans, but addressed a variety of other plants too. Plants had their
leaves wounded mechanically with a wire brush and by a lighter flame.
Wounding
was found to cause the release of the plant hormone Jasmonic acid. The
application of sucrose or adenosine triphosphate (cellular metabolites) were
found to trigger the release of Jasmonic acid as well. Jasmonic acid is the
frontline defense for the plants.
As
Jasmonic acid is released, the extrafloral nectaries of the plant start
secreting nectar. The strategy here is to attract insects such as ants and
recruit them into a plant defense force.
This
is an excellent strategy against small herbivores such as caterpillars, but it
won’t work quite as well with you or I damaging the leaves as we start to
harvest the beans. Although, perhaps if there are a lot of plants and we are
being slow, we may get put off if a large number of angry ants start patrolling
the leaves.
The
paper makes the interesting comparison that we also recognize the presence adenosine
triphosphate outside our skin cells as a signal of wounded skin. So it seems
that we have more in common with that tomato plant than we might have thought,
so talk to them nicely as you tend them in your greenhouses before stealing
their fruit. Pinching out their growing tips is likely to upset them.
