Should Plants Protect Their Flowers?
Plants only have so many nutrients in the soil.
Photosynthesis only generates as many sugars as the enzymes and sunlight allow.
Plants have these two inputs of raw material (nutrients from the soil and sugars from photosynthesis) to do all of their plant things: grow, reproduce, and protect themselves from being devoured.
Understanding how plants prioritize all of these tasks is an important challenge in biology. Many plant scientists seek to understand the patterns of plant defense allocation by asking: how, when, and where should plants protect themselves?
Not all plant parts are equal. Damage to leaves versus a flower or a seed pod likely affects plants differently, so which organs are most important to protect?
One hypothesis, the Optimal Defense Hypothesis (ODH), predicts that the organs with the most defense within a plant depend on:
Value of the organ.
The risk of it being attacked.
The benefit of defending it.
So we tested all three: we measured levels of toxic cyanide in different plant organs, allowed herbivores to choose among these organs, and we removed a portion of certain plant organs to measure the negative impact losing some flowers or young leaves could have on plant fitness. Fitness in biology means contributing genetic material to the next generation, aka reproduction.
When we designed this experiment, we had assumed flowers—plant sex organs—would be the most valuable to the plant for their role in reproduction, and would be highly armed with cyanide.
But instead, young leaves produced the most cyanide, and my research team and I were surprised to find low levels of toxic cyanide in flowers, especially after bioassays showed us that herbivores ate more flowers than any other plant part. At this point we were prepared to write a paper showing how the ODH does not hold up in this case.
But before we were satisfied with this conclusion, we decided to actually test whether seed production is affected by flower herbivory, which, by the way, has the greatest name: florivory.
After simulating florivory (removing 0-75% of all flowers), we counted seeds. We found that our plants could produce similar numbers of seeds even after we removed most of their flowers. Without any benefit from defending flowers, the cyanide and bioassay data support the ODH. Why should plants pack delicate petals full of cyanide-releasing precursors if doing so does not help the plant’s genetic material make it to the next generation?
Cyanogenesis is an expensive way for plants to protect their tissues, and the rationale behind the ODH is to only use resources for defense where it is worth it.
Because young leaves are well-protected, we then assumed that these organs representing the future site of photosynthesis must be important for plants to make more seeds. When we first submitted this paper, we suggested that removing young leaf tissue would probably reduce seed production.
The peer reviewers agreed, but suggested that we actually do the simulated folivory (leaf herbivory) experiments.
So we did, and plants did produce fewer and fewer seeds as we removed more and more young leaf tissue. This revealed that young leaves are valuable.
And with the addition of those data, all was well in the optimal defense hypothesis once again, with plants protecting the organs most valuable for fitness (leaves)—even though the most important organs in this case are not the ones we initially assumed to be important (flowers).
Now we have a map of cyanide precursors in lima bean plants. We also know that this within-plant pattern supports the ODH, but only because we tested this from multiple angles.
What I find striking about this paper is the way our notion of supporting a particular hypothesis was drastically different depending on which pieces of information we collected.
If we want to understand the plant defense theory: how, when and where plants use their resources for protection, it is important to be sure our data really show us whatever it is the hypothesis aims to test.
-Adrienne L. Godschalx