Open Gas Exchange Research Articles

Inter- and intraspecific variation in the response of photosynthesis to elevated [CO2] in a Florida scrub oak community

Understanding inter- and intraspecific responses to rising [CO2] is critical for assessing possible future shifts in competition and species dominance, and for accurately predicting future carbon exchange in forest ecosystems. Two clonal, co-dominant evergreen oaks within a Florida scrub-oak ecosystem have regrown entirely under elevated [CO2], providing a rare opportunity to assess inter- and intraspecific variation during canopy closure. Photosynthesis was measured with an open-gas exchange system to assess acclimation of photosynthetic capacity in two co-dominant oaks. There was no acclimatory loss of photosynthetic capacity in Quercus myrtifolia; therefore, with decreased photorespiration in elevated [CO2], a 72% stimulation of net photosynthetic rate was observed. Quercus geminata showed a large acclimatory loss of photosynthetic capacity sufficient to offset any stimulation of light-saturated net photosynthesis due to decreased photorespiration. The photosynthetic rate of Q. geminata leaves grown and measured at elevated [CO2] was no higher than leaves grown at ambient [CO2]. Q. myrtifolia also exhibited an increase in photosynthetic nitrogen use efficiency, which was still present at canopy closure. Intraspecific variation in the response of photosynthesis to elevated [CO2] exists in both species. The role that individual genotypes play in that variation is currently unknown. DNA microsatellite analysis of Q. myrtifolia and Q. geminata is being used to estimate genetic distances between individuals. The results show that at canopy closure in a woody community, elevation of [CO2] causes a substantial difference in the capacity of co-dominants to acquire carbon and energy, with important implications for fitness and future composition of this community.

A Highly Sensitive, Flow Through H2 Gas Analyzer for Use in Nitrogen Fixation Studies

Studies of H(2) evolution by N(2) fixing systems are frequently limited by an inability to accurately measure H(2) gas concentrations of less than about 10 microliters per liter. In this study, a H(2) gas analyzer is described which is able to accurately and reproducibly detect up to 100 times lower H(2) concentrations than most thermal conductivity gas chromatographs or other conventional instruments used for the measurement of H(2) gas. This high level of sensitivity (maximum of about 0.02 microliter per liter H(2) per millivolt output) and the ability to continuously monitor H(2) concentration directly in a flowing gas stream, makes this instrument well suited for use in an open gas exchange system.Since the sensor used in the instrument was also sensitive to other combustible gases, it was necessary to demonstrate that H(2) was the only combustible gas produced by the N(2) fixing system being studied. When an air stream was passed through a pot containing nodulated soybean (Glycine max L.) roots, gas chromatographic analysis of the effluent gas stream revealed that H(2) was the only combustible gas present. These results were supported by other studies in which no combustible gases were detected in the effluent gas stream from soybean roots nodulated with USDA 110, a Rhizobium strain which displays active uptake hydrogenase activity.