THE EFFECT ON STOMATAL PROXIES AS A FUNCTION OF GEOGRAPHICAL POSITION RELATIVE TO GROWING SEASON SUNLIGHT

Stomatal frequencies of fossil-plant species are used to estimate past pCO₂ levels based on the physiological functions of living taxa. Numerous studies have shown that there is an inverse relationship between pCO₂ and stomatal frequency paramters. As levels of pCO₂ increase, the Stomatal Density (SD) and Stomatal Index (SI) decrease. However, pCO₂ is not the only factor affecting Stomatal Density and Stomatal Index, which are a product of leaf growth and expansion. Stomatal characteristics differ between genera, and studies also have shown that SD and light intensity have a positive correlation. This study hypothesizes that SD and SI are not influenced by a leaf's physical orientation relative to the sun during the growing season.

Leaves of Northern Red Oak, Quercus rubra, were collected from trees on lake margins around six lakes in the Belgrade Lakes Region in Central Maine. Lakes oriented in NE/SW, NW/SE, and E/W directions allowed for sampling of trees exposed to varying light intensities throughout the day. Sun leaves were cleared in chromic acid, mounted on slides, where after stomata and epidermal cells were counted using a Zeiss Axioskop and AxioVision software.

The SD and SI of each tree were calculated and comparisons made between populations exposed to morning, mid-day, or afternoon light intensities. Data from Salmon Pond, oriented in a NW/SE direction, show a statistically significant difference between the SD of trees on either side of the lake. However, the SI shows no statistically significant difference between trees located on either shore and exposed to different light regimes throughout the day. These preliminary data suggest that exposure to various sunlight regimes on opposite sides of lakes may not play a role in the stomatal response as reflected in SI of plants during a growth season.