OTS leftovers: Does leaf pH influence herbivory?

My research focuses on how species have adapted to environmental variation and how these adaptations influence species’ niche breadths and geographic distributions. Although IUntitled focus on tropical plants, and insect herbivores are a substantial biotic selection pressure, herbivory is not a topic I actively pursued until I participated in an OTS course last summer. On my OTS course I was able to investigate an array of topics from plant defense to plant-mediated tri-trophic interactions, all of which are documented in our 2015 OTS coursebookOf all the projects I participated in, the last project of the course remains my favorite because it has provoked new questions, some of which have informed my thesis research.

In this last project, my classmate and I were following-up a previous study in which we investigated the ability of leaf pH to predict herbivory in plants. Literature suggested that low pH values deter herbivores in the sub-arctic where ungulates, not insects, are the dominant herbivores. Consequently, the influence of leaf pH on herbivory in tropical to sub-tropical regions where insects are the dominant herbivores is unclear. In our first project, my classmate and I found no effect of leaf pH on standing herbivore damage in the 5 Piper species we measured. For our follow-up project we were interested in determining the sources of variation that may have influenced leaf pH, and thus, the results of our previous study. Ultimately, we decided to investigate diurnal and ontogenic changes in leaf pH for two Piper species. 

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Figure 1: Ontogenic changes in leaf pH of P. multiplinervum & P. hispidum

Interestingly, our immature leaves exhibited the lowest pH values (Fig 1), and immature leaves have previously been shown to be the most palatable to insect herbivores. Compared to mature leaves, immature leaves tend to receive more insect herbivory because they are softer, are higher in nutrients, but also produce more chemical defense compounds. Therefore, leaf acidity may be correlated with leaf age and linked to chemical defenses that deter herbivores. If so, leaf pH could be an easily-measured trait used to understand the susceptibility of plants to insect herbivory.

In addition to ontogeny, leaves also exhibited diurnal changes in their chemistry (Fig 2). We found that both immature and mature leaves that were measured in the morning (~9am) had lower pHs than leaves measured in the the afternoon (~4pm). These diurnal fluctuations may have resulted from changes in carbonic acid and cytoplasmic proton gradients throughout the day. However, given the potential relationship of pH to chemical defenses, it is easy to speculate that some chemical defenses could fluctuate throughout the day in addition to ontogenic phase. 

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Figure 2: Diurnal changes in leaf pH of P. multiplinervum & P. hispidum

Since our first study did not properly measure herbivore damage, the effect of pH on insect herbivory remains an unanswered question. The observations from our follow-up study indicate pH is probably correlated with ontogeny, which has been shown to influence the production of chemical defenses that deter herbivory. pH was also observed to fluctuate diurnally, and may indicate diurnal changes in herbivore deterrence.

What I think is exciting about these results is that if pH does influence herbivory, insects may feed upon leaves according to temporal differences in leaf chemistry. In turn, these short-term temporal feeding preferences could promote co-existence of insect species, which greatly outnumber plant species, and are likely to overlap in diet-breadth. There are many unanswered questions that this project has induced that warrant further investigation before any concrete conclusions can be made about the influence of pH on insect herbivory. 

My uncharacteristic foray into plant-insect interactions has led me to a new understanding of niche-breadth and coexistence theories through concepts such as the storage effect that incorporate temporal niche partitioning – a topic of great relevance to my thesis. Moreover, my newfound interest in herbivory highlights the benefit of stepping outside the territory of your own research in order to gain a better understanding of it. 

Now that I am in Florida, where Pipers are introduced and rare,  I’ve been
gathering information on the native species Psychotria nervosa to address some of the questions I developed while on my OTS course. I’m currently collecting and identifying insects that feed on P. nervosa, and have already quantified ontogenic changes in leaf nutrients. As my course-load dwindles and the resources at the International Center for Tropical Botany 
become available, I hope to dedicate more time to understanding how leaf chemistry influences insect herbivory in the sub-tropics as a side-project…so stay tuned.

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Me (Tim Perez) happily collecting Piper cenocladum at La Selva Biological Station, Costa Rica.

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