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Apocynum cannabinum: variation in leaf traits

Beth Drewes, Betsy Feighner, and Dana Dudle
Biology Department and Science Research Fellows Program, DePauw University
Summer 2006

Introduction
Apocynum cannabinum grows in a variety of environments in the Quarry Bottom of the DePauw Nature Park.  The plant shows a high degree of phenotypic plasticity, with variation in leaf size, shape, hairiness, and angle to the stem.  The goal of our study was to identify relationships between environmental conditions and leaf traits of A. cannabinum.

Hypothesis
We hypothesized that leaf traits of A. cannabinum would correlate with the environmental conditions where the plants were growing. 

Methods
We measured soil water content and type of material at the base of plants, size of leaves, trichome and stomatal densities of leaves, and angle of leaf inclination.

Results
There was significant heterogeneity in environmental conditions in the Quarry Bottom.  Out of 246 plants, 45 percent were growing in sites covered by rocks greater than 2 cm in diameter. Soil water content was higher at the base of plants growing in the shade early in the growing season.  The angle of leaf inclination varied more in response to light conditions than time of day.  Leaves were angled more steeply in plants growing in the sun.  There was no relationship between environmental conditions, trichome density, stomata density, or leaf thickness.  Plants growing in the shade had significantly longer leaves than plants growing in the sun.  Plants with longer leaves were more likely to flower and produced more fruits than plants with shorter leaves.

Discussion
Many questions remain about the relationship between environmental conditions, leaf traits, and reproductive success of Apocynum cannabinum. Morphological variables did not sufficiently explain reproductive success of the plants. While 50 percent of the plants were flowering, it is unknown which factors contributed to flowering.  Factors that may affect flowering include photosynthetic rate, water-use efficiency, and herbivory. Many traits also may have a strong genetic component that overrules other environmental influences.