Abstract:

In petroleum-producing territories of West Siberia (Russia), oilwell gas flares have a thermal effect on nearby plant communities. Such communities can be used as models for studying plant acclimation to global warming. In the present study on the effect of the hydrothermal regime at the flare sites on mesophyll and stomatal functional traits of Betula pubescens, leaves were collected from trees at 250 m (control site [CS]), 200, 150 and 100 m (maximum impact site [MIS]) from a flare. From the CS to MIS site, the average annual air temperature increased by 0.5 °C and bog water level decreased by 17 cm. On plants from the MIS, stomata were 16% smaller and density was 20% lower compared to those at the CS, resulting in lower maximum stomatal conductance in plants from the MIS (mean ± SE: MIS 0.84 ± 0.05 mol·m⁻² s⁻¹, CS 1.24 ± 0.06 mol·m⁻² s⁻¹; F = 12.6, P < 0.01). Mesophyll cell volume was 1.9 times lower at MIS than at CS. Chloroplast numbers per cell also declined with distance from the flares, from 21 (MIS) to 18 (CS; F = 15.6, P < 0.001), and chloroplast volume was 24% higher at the CS, whereas the number of mesophyll cells and chloroplasts numbers per unit leaf area were 1.9 and 1.8 times higher at the MIS than at the CS, respectively. As a result, leaves from the MIS had a large total mesophyll cell (A _mes/A) and chloroplast (A _chl/A) surface area per unit leaf area, resulting in a 46% increase in mesophyll conductance in plants from the MIS. Thus, structural changes in leaf epidermis consisted of a decrease in stomatal size and number, could lower transpiration losses with higher temperatures and less water. To compensate for the reduction in leaf conductance due to a decrease in stomatal conductance under these conditions, an increase in the number of mesophyll cells and chloroplasts per unit area provides a greater gas-exchange area and mesophyll conductance.

Key words: Leaf traits, Mesophyll cell volume, Chloroplast numbers, Mesophyll conductance, Stomatal conductance