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

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(13)C values from one plant community and were compared with previous analyses of leaves from the same species. The total mean bulk δ(13)C value of flower tissues was -28.1‰, and there was no significant difference between the mean δ(13)C(flower) values for grass (-27.8‰) and herb (-28.2‰) species. On average bulk δ(13)C(flower) values were 1.1‰ higher than bulk δ(13)C(leaf) values, however, the δ(13)C(flower) and δ(13)C(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(13)C(arabinose)>δ(13)C(xylose)>δ(13)C(glucose)>δ(13)C(galactose) which had been observed in leaves was similar in flowers (total mean δ(13)C values=-25.9‰, -27.2‰, -28.8‰ and -28.1‰, respectively). However, the average 5.4‰ depletion in the δ(13)C 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(13)C(C16:0)=-33.8‰; δ(13)C(C18:2)=-33.1‰; δ(13)C(C18:3)=-34.2‰) but not herb flowers (δ(13)C(C16:0)=-34.1‰; δ(13)C(C18:2)=-32.4‰; δ(13)C(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(13)C values, especially the significant (13)C-depletion in flower fatty acid δ(13)C values.