Medeiros, Juliana , Becklin, Katie M. , Ward, Joy K. .
Glacial conifers exhibited photosynthetic adaptation to low atmospheric CO2concentration.
When modern plants are exposed to atmospheric CO2 concentrations (ca) ranging from current values to those of the last glacial period they often maintain a constant ratio of leaf intercellular CO2 concentration (ci) relative to ca (ci/ca). This results in low ci at glacial ca, suggesting reduced photosynthesis in glacial compared to modern plants. Studies with modern plants may not always be representative of glacial plants, however, if glacial plants were adapted low ca. Adaptation to low ca could have occurred in a number of ways: first, plants could have maintained higher stomatal conductance (gs), which would elevate ci/ca in glacial compared to modern plants. Second, maximum photosynthetic capacity (Amax) could have been higher in glacial plants. This would reduce ci/ca, but also increase the magnitude of the CO2 concentration gradient (ca - ci) in glacial compared to modern plants. To address these hypotheses we determined temporal changes in leaf delta13C for 8 conifer species native to the Snake Range, Nevada, USA. From d13Cwe calculated ci/ca, and then using ice core records of ca,we calculated ci and ca - ci. We compared sub-fossil leaves from pack rat middens (30 - 4 ky old) with herbarium (76 - 27y old) and modern samples, representing a range of ca from 186 - 392ppm. We found that ci/ca has increased significantly for all 8 species since glacial times, providing no support the hypothesis that gs was higher for Snake Range conifers in the past. Rather, it suggests that gs was lower or the same compared to modern plants from the same location. In addition, we found that ci increased significantly from glacial to modern times, suggesting that low ca constrained photosynthesis in these conifers relative to the present. But, we also saw that ca - ci decreased from glacial to modern times. Combined with data from a companion study demonstrating that leaf nitrogen content was generally higher in glacial conifers, this result suggests that ci was reduced at least in part because Amax was higher during the last glacial period than at present day. Finally, Pinus longaeva,the dominant glacial species, exhibited significantly steeper changes in all three variables compared to the other species, indicating larger changes in gsand Amax since glacial times. From these results we conclude that 1) glacial conifers exhibited photosynthetic adaptation to low ca,and 2) closely related species differ in the degree to which they responded to changing ca.
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1 - University Of Kansas, Ecology And Evolutionary Biology, 1200 Sunnyside Ave, Lawrence, KS, 66045, USA
Presentation Type: Oral Paper:Papers for Topics
Date: Tuesday, July 10th, 2012
Time: 11:00 AM