Grasses That Feign Infestation

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Given the option, most of us would rather avoid a salad riddled with insects or an apple chock full of worms. Much as we prefer to avoid insect-infested fruits and vegetables, so too do many herbivores. Some plants seem to be taking advantage of this. In response to strong herbivore pressure, some plant species have evolved insect mimicry. One such case involves grass and aphids. 

Paspalum paspaloides can be found growing in tropical regions around the globe. In many ways they are similar to other C4 grasses. When they flower, however, one may notice something interesting. All of the flowers appear to be covered in aphids. Close inspection would reveal that this is not the case. Those clusters of dark specks swaying the breeze are simply the numerous dark anthers of the inflorescence. This has led some to hypothesize that these plants may be mimicking an aphid infestation.

This observation begs the question: "what benefit is there in mimicking aphids?" There are two major hypotheses that have been proposed in order to explain this phenomenon. The first is defense against herbivory. As stated above, herbivores often avoid plant material that has been infested with insects. Aside from any potential palatability issues, large populations of insect pests can signal a decrease in the nutritional value of a potential food source. Why waste time eating something that is already being eaten? Evidence in support of this hypothesis has come from other systems. A wide array of herbivores, both mammalian and insect, have been shown to avoid aphid-infested plant material.

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The second hypothesis is one of avoiding future infestations. Aphids are clonal organisms with a short generation time. It does not take long for a few aphids to become many, and many to become an infestation. As such, aphids looking for a new plant to colonize habitually avoid plants that already have aphids on them. It could very well be that such aphid mimicry is a means by which the grass keeps actual aphids at bay.

If this is a form of true mimicry then the question is not a matter of which hypothesis but the relative influence of each. It seems that it very well could be driven by a mixture of both strategies. Still, all of this is speculative until actual experiments are carried out. Those who originally put forth these ideas have identified similar potential mimicry systems in other plants as well. The idea is ripe for the testing!

Photo Credits: [1]

Further Reading: [1] [2] [3]

 

Floral Mucilage

Spend enough time around various Bromeliads and you will undoubtedly notice that some species have a rather gooey inflorescence. Indeed, floral mucilage is a well documented phenomenon within this family, with something like 30 species known to exhibit this trait. It is an odd thing to experience to say the least.

The goo takes on an interesting consistency. It reminds me a bit of finding frog spawn as a kid. Their brightly colored flowers erupt from this gooey coating upon maturity and the seeds of some species actually develop within the slimy coating. Needless to say, the presence of mucilage in these genera has generated some attention. Why do these plants do this?

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Some have suggested that it is a type of reward for visiting pollinators. Analysis of the goo revealed that it is 99% water and 1% carbohydrate matrix with no detectable sugars or any other biologically useful compounds. As such, it probably doesn't do much in the way of attracting or rewarding flower visitors. Another hypothesis is that it could offer antimicrobial properties. Bromeliads are most often found in warm, humid climates where fungi and bacteria can really do a number. Again, no antimicrobial compounds were discovered nor did the mucilage show any sort of growth inhibition when placed in bacterial cultures.

It is far more likely that the mucilage offers protection from hungry herbivores. Flowers are everything to a flowering plant. They are, after all, the sexual organs. They take a lot of energy to produce and are often brightly colored, making them prime targets for a meal. Anything that protects the flowers during development would be a boon for any species. Indeed, it appears that the mucilage acts as a physical barrier, protecting the developing flowers and seeds. One study found that flowers protected by mucilage received significantly less damage from weevils than those without mucilage.

The mucilage could also provide another benefit to Bromeliads. Because these plants rely on water stored in the middle of their rosette (the tank, as it is sometimes called), some species may also gain a nutritional benefit as well. Bromeliad flowers emerge from this central tank so anything that gets stuck in the mucilage may eventually end up decomposing in the water. Since nutrients are absorbed along with the water, this could be an added meal for the plant. To date, this has not been confirmed. More work is needed before we can say for sure.

Photo Credit: [1] [2]

Further Reading: [1] [2] [3] [4]