Variation in bulk tissue, fatty acid and monosaccharide δ13C values between autotrophic and heterotrophic plant organs

Abstract

The flowers of 23 species of grass and herb plants were collected from a mesotrophic grassland to assess natural variability in bulk, monosaccharide and fatty acid δ 13C values from one plant community and were compared with previous analyses of leaves from the same species. The total mean bulk δ 13C value of flower tissues was −28.1‰, and there was no significant difference between the mean δ 13C flower values for grass (−27.8‰) and herb (−28.2‰) species. On average bulk δ 13C flower values were 1.1‰ higher than bulk δ 13C leaf values, however, the δ 13C flower and δ 13C leaf values of grasses did not differ between organs suggesting that carbon isotope discrimination is different in grass and herb species. The abundance of different monosaccharides abundance varied between plant types, i.e. xylose concentrations in the grass flowers were as high as 40%, compared with up to 15% in the herb species, but the general relationship δ 13C arabinose > δ 13C xylose > δ 13C glucose > δ 13C galactose which had been observed in leaves was similar in flowers (total mean δ 13C values = −25.9‰, −27.2‰, −28.8‰ and −28.1‰, respectively). However, the average 5.4‰ depletion in the δ 13C values of the C 16:0, C 18:2 and C 18:3 fatty acids in flowers compared to bulk tissue was significantly greater than observed for leaves. The trend C 16:0 < C 18:2 < C 18:3 previously observed in leaves was also observed in grass flowers ( δ 13C C16:0 = −33.8‰; δ 13C C18:2 = −33.1‰; δ 13C C18:3 = −34.2‰) but not herb flowers ( δ 13C C16:0 = −34.1‰; δ 13C C18:2 = −32.4‰; δ 13C C18:3 = −34.5‰). We conclude: (i) that the biological processes influencing carbon isotope discrimination in grass flowers are different from herbs flowers; and, (ii) that a range of post-photosynthetic fractionation effects caused the observed differences between flower and leaf δ 13C values, especially the significant 13C-depletion in flower fatty acid δ 13C values.