Functional Group Difference And Environmental Regulation On The Nocturnal Variation In Carbon Isotope Composition Of Leaf Respired CO₂

In recent years, stable carbon isotope technique has already become an important approach in ecological research. Stable carbon isotope (δ13C) in plant tissue and respiratory released CO2can be used to reveal effects of variation in external conditions on plant metabolic activities, as well as on the partitioning of carbon exchange between ecosystem and the atmosphere into its components. Differences in plant photosynthetic pathway, variation in environmental conditions may alter the carbon isotope composition of leaf dark-respired CO2and the magnitude of nocturnal variation. To date, several species of C3species have been studied on their dark respired CO2isotopic composition and the magnitude of diurnal variation; however information on C4plants are scarce. In this study, we conducted2experiments.In the first experiment, we measured photosynthetic features and carbon isotope of leaf dark respired CO2and respiratory substrates in22plant species with either C3or C4photosynthetic pathway in Songnen grassland. In the second experiment, we studied the effects of changes in water condition on the isotopic composition and the magnitude of nocturnal variation in leaf dark-respired CO2in2C4species and a C3grass. The main results are as follows:1. For both C3and C4plants, leaf dark-respired CO2varied apparently with13C enriched in CO2of respiration at8pm and13C become depleted in CO2respired at4am.2. There were no statistical significant difference in the magnitude of nocturnal variation in δ13C of leaf dark-respired CO2.3. The magnitude of nocturnal variation in leaf dark-respired CO2were sensitive to changes in water conditions. With the progress of drought, leaf CO2assimilation decreased gradually, while the carbon isotope composition of photosynthates were enriched in13C.4. The magnitude of nocturnal variation in δ13C of leaf dark-respired CO2was positively correlated with the amount of accumulated photosynthates.5. The carbon isotope of leaf dark-respired CO2was enriched in13C compared to root respired CO2. 6. With the progress of drought,δ1C of leaf dark-respired CO2become enriched in13C in C3plant, while it showed depleted in13C in C4grasses.In summary, there were significant differences in the13C composition of leaf dark-respired CO2between C3and C4plants; however there were no differences between the two photosynthetic pathways in the magnitude of nocturnal shift in δ13C of leaf respired CO2. For both C3and C4plant, value and magnitude of nocturnal shift in leaf respired CO2were sensitive to changes in soil water conditions. The observed positive correlation between the magnitude of nocturnal shift in813C of leaf respired CO2suggested that the allocation of respiratory intermediates are the potential controller of813C of leaf respired CO2. This study provided more information about the δBC of leaf dark-respired CO2in C4plants. The results are useful for the application of stable carbon isotope to the partition of ecosystem carbon flux into its components.