Short term carbon isotopic fractionation in plants

A system was developed for measuring carbon isotopic fractionation in plants over a time interval of 1-3 hours, in contrast to leaf combustion studies which give long-term, integrated discrimination measurements. The system was used to study environmental effects on soybean (Glycine max) and corn (Zea mays) discrimination. Changes in leaf temperature, photon flux density (PFD), O{dollar}sb2{dollar} concentration, and CO{dollar}sb2{dollar} concentration produced little or no change in measured discrimination ({dollar}Delta{dollar}). For soybean, {dollar}Delta{dollar} increased with decreasing PFD. For corn, {dollar}Delta{dollar} decreased with decreasing O{dollar}sb2{dollar} concentration. For both soybean and corn, {dollar}Delta{dollar} increased with increasing CO{dollar}sb2{dollar} concentration. These changes in {dollar}Delta{dollar} were interpreted as environmental effects on stomatal conductance and photosynthetic capacity, which indirectly affect {dollar}Delta{dollar} by altering C{dollar}sb{lcub}rm i{rcub}{dollar}/C{dollar}sb{lcub}rm a{rcub}{dollar}.; Respiratory discrimination in the dark and light was also investigated. Respired CO{dollar}sb2{dollar} was 5{dollar}perthous{dollar} and 0-1{dollar}perthous{dollar} more positive than leaf carbon for soybean and corn, respectively. Photorespiratory discrimination was 6-7{dollar}perthous{dollar} for soybean, supporting the contention that glycine decarboxylase may be the source of discrimination in the photorespiratory pathway.; No significant increase in {dollar}Delta{dollar} during corn leaf development was found using 6-13 day old corn seedlings, suggesting any direct C{dollar}sb3{dollar} fixation occurring in young corn leaves must be {dollar}<{dollar}10% of total carbon fixation. Immature leaves at higher rank were {dollar}sim{dollar}1{dollar}perthous{dollar} more positive isotopically than mature leaves of lower rank, as measured by leaf combustion analysis. The difference in the {dollar}deltasp{lcub}13{rcub}{dollar}C values of leaf carbon without a corresponding difference in measured discrimination implies processes such as respiration or metabolite transport lead to further fractionation.; Discrimination during the four CAM phases for Kalanchoe daigremontiana was similar to expected values for the pathways operative during each phase. The CO{dollar}sb2{dollar} leaked during the deacidification phase was positive (+7 to +11{dollar}perthous{dollar}) relative to leaf carbon ({dollar}-{dollar}17{dollar}perthous{dollar}); this loss of heavy CO{dollar}sb2{dollar} from the leaf may account for the discrepancy between leaf combustion measurements and the observed nighttime discrimination.