| dc.description.abstract | The effects of elevated atmospheric CO2 concentrations on plant polyphenolic concentration, tannin concentration and chemical composition were investigated in leaves of sub-tropical grass species and fynbos shrubs. The working hypothesis was based on predictions that carbon based secondary compounds (polyphenolics and tannins) would increase when carbon in excess of growth requirements accumulate in plant leaves under nutrient imbalanced conditions. This imbalance would arise due to an increase in atmospheric CO2 level. Furthermore, empirical evidence suggests that nutrient paucity would enhance carbon-based secondary compound production.
This hypothesis was tested in two different systems involving plants with differential photosynthetic mechanisms and growth strategies. Polyphenolics, tannins and chemical composition (N, P, C and TNC) were quantified in grass species from a natural, C4 dominated, sub-tropical grassland in KwaZulu/Natal. Three plots were subjected to different freeair CO2 enrichment treatments, i.e. elevated (550-800 ppm), intermediate no more than 400 ppm) and ambient CO2 (currently at 365 ppm). One of the seven grass species, Alloteropsis semialata, had C3 photosynthetic mechanism. Polyphenolics, tannins and chemical composition (N, P, C and TNC) were quantified in three fynbos species grown in open-top chambers under controlled greenhouse conditions. The plants were grown under ambient (360 ppm) and ambient+ 350 ppm CO2 in typical low nutrient acid sands of the fynbos biome.
This study shows that despite some of the grasses having the capacity to produce tannin-like substances, polyphenolics and tannins do not increase in the grass species studied. Polyphenolic and tannin concentrations were increased only in Leucadendron laureolum amongst the fynbos species. Its sister species Leucadendron xanthoconus did not show any change in phenolic or tannin concentrations.
Chemical composition in grasses were largely unaffected by elevated CO2, however, some species-specific responses were observed. The C3 A. semialata showed a decrease in P concentration and a consequent increase in C:P ratio at elevated CO2. Only L. laureolum showed a response in chemical composition at elevated CO2, whereas its sister species did not show any response except a decrease in N concentration. In conclusion, fast growing grass species invest extra carbon into growth rather than polyphenolics and tannins and show small species-specific chemical changes at elevated atmospheric CO2 concentrations. Increased investment into phenolics and tannins as well as changes in chemical composition in fynbos species were species-specific even within plants from the same genus. Thus generalizations about plant responses to elevated CO2 based on theoretical principles cannot be directly applied. This is especially true in complex natural environments where ecophysiological processes may dictate phytochemical responses. | en_US |
