Friday, April 8, 2011

Get in Shape


Lately I've been reading papers on leaf morphology, and in the grand art of being lazy I decided to post about a paper I've already read rather than reading a whole new one.

I think that reading a paper about leaf morphology is kinda difficult if you don't know the basic parts of a leaf, and although this paper doesn't get into the itty-bitty differences in leaf structure I'm still gonna go through some of the basics. This is a leaf:

It consists of a (usually) flat photosynthetic portion called the blade where the very tip is called the apex, the edges the margins, and includes both a midvein and lateral or net veins. The margins of some leaves are serrated or have "teeth" with the low, in-between areas called the sinuses. At the base of the leaf the blade is attached to a supportive stalk called the petiole. Where the petiole and the stem meet is the stipule.

Morphology refers to the study of the forms of things and the relationships between their structures. In botany, leaf morphology is simply the characterization of leaf shape. The traits that you see on plants are determined by a combination of genetic heritage (genotype) and the capacity for a single genotype to respond to environmental variation (phenotypic plasticity). There has been quite a lot of research into the roles of genotype and plasticity in order to figure out how an organism responds to change. Studies such as these typically grow plants in a common garden experiment where all of the individuals are grown under exactly the same conditions so as to see what traits are different. These types of experiments show that in most species both plastic and genetic factors are important for a number of plant functions such as stomatal distributions, photosynthetic efficiency, leaf area, water availability etc. However, little is known about the plastic response of some leaf traits to temperature.Why does that matter? In terms of global climate change it is important to know both how plants responded to temperature in the past, using fossilized plants, and how they are responding to changing temperature now. And because many of these traits are plastic they can tell us a lot about rapid climate change. For example, it is known that in colder climates plants have more highly dissected leaves, meaning they have a low shape factor and a high compactness and perimeter ratio, and that many species show a temperature effect on leaf shape.

A paper from 2009, published in PLoS ONE, takes a look at leaf size and shape in Red Maple (Acer rubrum) growing in contrasting climates. They started by collecting seeds across a broad temperature gradient, across the eastern U.S. and Canada. Then two common garden experiments were set up in Rhode Island and Florida. These were common garden experiments in that they had the same set up in terms of plot layout, plant spacing, etc. but as they are in different locations there are differences in soil composition, precipitation, etc. Once the plants had grown then two leaves per plant were collected and the petioles removed, then they were dried, pressed, and photographed against a black background. The researchers used Photoshop to analyze their leaf images. They measured/counted the number of teeth, leaf area with teeth, and leaf area without teeth. They also measured a range of leaf size and shape variables using a program called ImageJ (very useful freeware from NIH that I use all the time). There were three categories of variables:

1. Leaf Dissection - Shape Factor: This was calculated as 4pi times the leaf area divided by the perimeter squared.
2. Compactness: This was calculated as the perimeter squared divided by the area.
3. Perimeter Ratio: This was calculated as the perimeter divided by the internal perimeter. The internal perimeter being without the teeth.

In this study, phenotypic plasticity was used analogously with growth site, and they found that growth site explained 5 to 19% of the variance they observed for traits related to the number of teeth on a leaf and the leaf dissection. They also found that seed source accounted for 69 to 87% of the variance. The plants from cold climates had more teeth, but smaller teeth, and were more highly dissected. The researchers concluded that while the size of the teeth is probably due to genetics, leaf dissection and tooth number likely respond plastically to their environment. These results are consistent with other studies that found a functional link between leaf teeth and climate. As the trees studied here were only grown for two years before sampling, this study shows that plants can respond quickly to environmental change. That is good news for paleobotanists who study leaf morphology in fossilized plants, it can give them an idea of the environmental conditions with greater resolution. However, it is important to note that plasticity isn't the only process that determines the distribution of leaf traits. Other process operate on slower timescales and include evolutionary changes within a population and species. Also, this study does not look at environmental factors such as the concentration of atmospheric carbon dioxide and the UV-impact plants receive.

If you would like to read this paper you can find it for free through PLoS ONE:

Royer, Dana L. Laura A. Meyerson, Kevin M. Robertson, and Jonathan M. Adams. (2009) Phenotypic plasticity of leaf shape along a temperature gradient in Acer rubrum. PLoS ONE: 4(10), e7653. (DOI: 10.1371/journal.pone.0007653)

(images from pendernursery.com and teacherbridge.org, respectively)
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