Enriquecimento e alocação de 13C em plantas de eucalipto

Abstract

Bolsistas do CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico. In the last decades the use of stable isotopes has gained importance in several research areas, e.g., for metabolic flux and pathway analysis and studies on effects of biotic and abiotic plant stresses and on soil organic matter (SOM). Studies on the alterations and dynamics of SOM based on the variation in natural 13C abundance require variations in the C isotopic composition, which are not always observed. One alternative is to label the plant material with 13C by 13CO 2 fixation, so that the C isotope ratio is different from that of native SOM. The objective of this study was to investigate the magnitude and homogeneity of 13C labeling in the different plant parts of eucalypts. Three 4-month-old eucalypt plants grown in nutrient solution were labeled, using a pulse technique. Plants were exposed to a 13CO 2 enriched atmosphere in a 448 dm 13 glass chamber, at around 24°C. The CO 2 concentration and the 13C/ 12C ratio were monitored by gas sample analysis in an isotope ratio mass spectrometer (IRMS) during the pulse-labeling period (126 days, three 13CO 12C pulses per week). After the labeling period the plants were separated in leaves (leaf-source and leaf-sink), branches, bark, stem, and roots and analyzed in an IRMS. The results were expressed in parts per one thousand (‰) relative to the Pee-Dee Belemnite standard (δ 13C PDB). The δ 13C PDB were: 828.07 ‰ for young leaves, 645.7 ‰ for mature leaves, 672.5 ‰ for branches, 691.9 ‰ for bark, 632.02 ‰ for stem, and 536.6 ‰ for roots. The 13C allocation and enrichment pattern among plant parts was homogeneous (statistically equal), although variations in δ13CPDB of 291 ‰ between plant parts were observed. The eucalypt plants maintained a high CO 2 absorption, and consequently photosynthetic rates well above the normal atmospheric CO 2 concentration (8.64 mmol L -1 - 380 ppmv), even at high CO 2 concentrations (180.4 mmol L -1 - 7.934 ppmv). The 13 C fixed during the day was proportionally less respired at night in comparison to 12C. The degree of 13C enrichment obtained indicates that the technique allows a sufficient plant enrichment to trace C in studies of litter decomposition and eucalypt litter stabilization in SOM fractions.