Leaf Area : Perimeter Ratio

Shade Tolerance and

Photosynthesis

Brigette Jones and

Dunya Onen

Physiological Ecology

Spring 2007

Background• One of the simplest questions to ask

in plant physiological ecology is “Why are the leaves shaped like that?” – Leaf morphology is highly variable, and

leaf functionality is directly related to plant fitness• Strong selective pressure• Additionally many studies have shown that

leaf morphology, rather than biochemistry often governs photosynthesis acclimation

–Many factors potentially influence shape, and many measures of shape• Incident radiation determining Ps

Sassafras

• In light-limiting environments-– Potentially advantageous to extend the

leaf in many directions (increase lobbing??, decrease area: edge)• Maximize angle and place variety for light

interception

–Niklas 1988 showed that Sassafras leaves were more strongly, and more frequently lobed on the interior parts of the branches, where light was intermittent and limiting

• Question is mostly unanswered-studies tend to look at other morphological parameters, or don’t connect perimeter to physiological responses, such as photosynthesis– Or are limited to one species, or don’t

cover a range of leaf shapes• There must be a function for leaf

shape, or such variance would be unlikely-an ideal would have evolved

Hypothesis• Photosynthesis will decrease with

increasing Area : Perimeter ratio independent of tree species

Predictions• Tree species with greater shade

tolerance will have lower A:P ratios

• Within species with plastic leaf morphology, A:P will be lower for shade leaves than sun leaves

• Study species were chosen from the avaliable local pool to represent a range of shade tolerance and leaf shape

Tree Species

• Paper Birch (Betula papyrifera)- low shade tolerance, teardrop shaped leaf

• American Elm (Ulmus americana)- moderate shade tolerance, egg-shaped leaf

• Northern Red Oak (Quercus rubra)- high shade tolerance, moderately pinnately lobed leaf

• Sugar Maple (Acer saccharum)- very high shade tolerance, moderately palmately lobed leaf