This post was contributed by QBIC Sophomore, Renata Gallegos.
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Jones IM and Koptur S (2015). Dynamic extrafloral nectar production: The timing of leaf damage affects the defensive response in Senna mexicana var. chapmanii (Fabaceae). American Journal of Botany 101(1): 58-66.
Plants are sessile organisms incapable of
movement and unable to escape their predators, of which they have many. If left
unprotected, particular plants can lose up to 40% of their biomass to a single
species of insect alone. To compensate for their immobility, plants have
developed a variety of indirect defenses, one such example being extrafloral nectar
(EFNs). An EFN is a solution of sugars secreted by glands found outside of the
flower. While a large variety of arthropods (animals such as insects) are attracted
to this sweet solution, its potential is truly exploited only when discovered
by ants. EFN is produced for the ants in exchange for their services in fending
off herbivores, a widespread mutualism. Understanding the factors that affect
this phenomenon is essential to understanding how plants manage their defenses,
and that is exactly what scientists in Florida International University are
doing.
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| Picture of S. chapmanii, an herbaceous legume native to the pine rocklands. Photo by Ian Jones. |
Compared to direct defenses, such as the spines
found on a rose, the production of extrafloral nectar is less metabolically inexpensive,
but costly nevertheless. In response to this expense, plants adjust their
production of extrafloral nectar over time and in response to the herbivory level
they experience. In general, EFN production increases as herbivory increases.
There are differences in peak extrafloral nectar production, with different
species experiencing different peak times. These differences are most likely a
result of different selection pressures on these plants, as well as the
differences in suitable partner availability. An example of this is seen in the
common vetch, a plant studied by one of the authors, Dr. Koptur. When found in
California, vetches with ants present were healthier and fitter -more likely to
have offspring- than vetches with ants absent. In England, the common vetch's
native home, ants protected the herbivores found on the plant, rather than
protecting the plant (Koptur 1979). Data collected from experiments with the
common vetch reveals that EFN production should be studied in the context of
community.
Variations can also occur over plant
development. There are two prominent, and sometimes contradicting, hypotheses
that predict this variation. The first one is optimal defense theory (ODT),
which predicts that tissues that are young and developing, and therefore the
most vulnerable to herbivory, are the ones that should have stronger defenses.
The second prediction is given by the growth differentiation balance hypothesis
(GDBH), which states that tissues that are already grown will experience the
necessary allocation of resources for the development of indirect defenses.
A factor affecting ontogenetic (life) variation
in EFN production is the type of mutualism in which the plant acts as mediator.
A plant that provides food and shelter for its ant partners usually begins to
produce EFN once it reaches a mature stage, whereas plants that provide only
EFN do not follow this trend, but instead have varying stages at which they
produce EFN. While a great deal of literature on EFN has appeared over the last
couple decades, little attention has been focused on temporal variation, a
factor which the authors of the article claim is pertinent for understanding
the extent to which EFN-producing plants manipulate their ant partners.
Senna mexicana var. chapmanii
The plant of choice for the
study was Senna mexicana var. chapmanii (Jacq.), an herbaceous legume
native to south Florida and the Caribbean, possessing globular extrafloral
nectaries, which are frequently patrolled by ants. It is a threatened plant
residing in the pine rocklands habitat, an endangered habitat in the regions of
South Florida and the Florida Keys. Experiments were carried out in the
greenhouse at Florida International University in order to answer two
questions: (1) Does the time of day during which a plant is damaged affect the
amount of extrafloral nectar produced? (2) Are these responses the same for old
and young leaves? A field study was carried out in order to
determine a relationship that has infrequently been studied: does leaf damage
and the resulting rise in extrafloral nectar production in existing nectaries affect
ant attendance on S. chapmanii?
The first experiment, to
determine differences in temporal variation, was conducted using old and young
seedlings. Seedlings were injured mechanically by using scissors to cut 50% of
each of five leaves chosen from each plant. It was found that old seedlings
produced more sugar than did young seedlings. Both young and old seedlings
produced more sugar during the night than during the day. These results held
true for both damaged and undamaged seedlings. Although 60 hours post treatment
there were no longer differences in response between any of the seedlings. These
results on seedling damage agree with growth differentiation balance
hypothesis, with the older seedling producing the most sugars, as they were the
ones that could allocate an excess of resources into indirect defenses.
The second experiment was carried out to determine if the location of
damage on leaves affected response. Younger leaves with 50% damage (incurred by
cutting leaves with scissors) produced more sugar than did older leaves with
50% damage, as well as more than the control (plants without leaf damage).
These results support the assumption that extrafloral nectar production
increases in response to an increase in damage. They also agree with optimal
defense theory, where younger leaves produce the most sugars in response to
damage.The third experiment was carried out in the pine rocklands
section of the nature preserve, to determine if plants with more damage and,
therefore, more sugar production attracted more ants. A higher number of ants
were found on plants with induced damage than in plants without damage.
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| Senna chapmanii's extrafloral nectaries are located on the pedicels, and on the rachis between the first pair of leaflets. This picture depicts a nectary with a nectar droplet. Photo provided by Ian Jones. |
The third experiment was carried out in the pine rocklands
section of the nature preserve, to determine if plants with more damage and,
therefore, more sugar production attracted more ants. A higher number of ants
were found on plants with induced damage than in plants without damage.
Variations in EFN production
occur as a response to damage and due to differences in ontogenetic stages. Studying
these factors can increase understanding of how plants manipulate their ant
partners and in optimizing EFN production in agricultural settings. Rather than
potentially harming crops and other organisms (such as the humans that eat
them) with pesticides, agricultural scientists have found it beneficial to grow
EFN producing plants next to crops. The production of EFN next to valuable food
sources attracts ant partners, which eat the harmful pests preying upon the
potential harvest.
Sources:
I'd like to graciously thank the author, Ian Jones, for the two pictures he provided.
Koptur S (1979). Facultative
mutualism between weedy vetches bearing extrafloral nectaries and weedy ants in
California. American Journal of Botany 66:1016-1020
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