The effect of chemical processing on the δC value of plant tissue

The effect of standard processing techniques on the δ¹³C value of plant tissue was tested using species representing the three photosynthetic pathways, including angiosperms and gymnosperms within the C₃ taxonomic division. The species include Cowania mexicana (C₃ angiosperm), Juniperus osteosperma (C₃ gymnosperm), Opuntia spp. (crassulacean acid metabolism CAM] angiosperm), and Atriplex canescens (C₄ angiosperm). Each species is represented by 5 plants collected at two different sites, for a total of 10 samples. The samples were processed to whole plant tissue, holocellulose, α-cellulose, and nitrocellulose. An additional process was added with the discovery of residual Ca-oxalate crystals in holocellulose samples. Both C₃ species show δ¹³C values becoming ¹³C enriched with increased processing. The CAM representative shows the opposite trend, with ¹³C depletion during the progression of treatments. The greatest range of values and most inconsistent trends occur in the C₄ representative. Removal of the Ca-oxalate fraction resulted in different mean weight percentages and δ¹³C values among the species. Calculated δ¹³C values of the Ca-oxalate crystals show depletion from the tissue values in the two C₃ species and enrichment in the C₄ and CAM representatives. The C. mexicana samples show the greatest change between the tissue and Ca-oxalates (7.3‰) but the least mean weight percentage (11%), whereas A. canescens shows the greatest overall change, with a −2.8‰ isotopic shift and over 48% mean weight percentage. Variability within the samples undergoing each treatment remained relatively unchanged even with increased cellulose purity. This paper provides estimates of isotopic offsets necessary to correct from one treatment to another. Significant differences in δ¹³C among different treatments confirm the need to state the tissue fraction analyzed when reporting δ¹³C results.