General biology article summary

¶ … Leaf Water Repellency as an Adaptation to Tropical Montane Cloud Forest Environments by Curtis D. Holder.

Experimental Hypothesis:

The research into leaf water repellency was designed to investigate the ability of leaves to repel rain water and moisture deposited by atmospherically humid environments. The experimental hypothesis was that leaves that evolved within extremely moist environments naturally selected the ability to minimize water residue on their surfaces as a mechanism of ensuring photosynthetic efficiency. Excess water remaining on the surfaces of leaves is thought to decrease photosynthetic efficiency by reducing the rate of gas exchange. Specifically, carbon dioxide diffuses more slowly in water than in air, suggesting that the greater the surface area of a leaf covered by water, the more photosynthesis efficiency will be reduced by inhibited carbon dioxide absorption. The precise mechanism of leaf repellency relates less to the ability to shed water droplets than to the relative ability of leaves to promote the beading of individual droplets on its surface. Generally, the ability of leaves to accomplish this is determined by both the inclination contact angle of the leaf with respect to the horizontal and by the microstructure of the leaf's surface and it secretions that promote the beading of water remaining on its surfaces. Higher angles between the exterior dimensions of the water droplet and the surface of the leaf correspond to less surface area of water coverage per constant volume of water.

The Experimental Design:

To test the experimental hypothesis that leaves found in extremely humid climates have evolved hyper-repellency to compensate for their increased exposure to water droplets as a means of ensuring efficient photosynthesis, the researchers collected leaves from two environments with different seasonal moisture and precipitation rates in Guatemala and one in Colorado. The researchers collected samples of different species of leaves and measured their repellency by comparing the contact angles of water droplets of a uniform size deposited on their surfaces by the use of pipettes under laboratory conditions.

Experimental Results:

Contrary to the experimental expectations, the results of the experiment did not support the initial hypothesis: some of the leaves collected from the drier environments exhibited greater water repellency than some of the leaves collected from the wettest environments. After analyzing the data, the researchers proposed an alternate evolutionary purpose for the mechanism behind water repellency of different leaves.