Stephanie J.E. Wand

Growth responses to elevated CO2 in NADP-ME, NAD-ME and PCK C4 grasses and a C3 grass from South Africa

The potential use of C4 biochemical subtypes [nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME), nicotinamide adenine dinucleotide-malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PCK)] as delimiters of plant functional types (PFTs) with distinct responses to rising atmospheric CO2 concentrations was investigated in South African grass species. Gas exchange and above-ground growth in ambient and elevated CO2 (360 and 660 µmol mol–1 , respectively) were determined in three NADP-ME species, two NAD-ME species, two PCK species and one C3 species, all excavated from the same field site. Plants were grown in open-top chambers in a greenhouse for 178 d. Net CO2 assimilation rates were only significantly increased in one NAD-ME species, but stomatal conductances decreased (in six out of eight species, by a mean of 46%) and instantaneous leaf water-use efficiency increased (in all species, by a mean of 89%) in elevated CO2. These responses did not differ between photosynthetic pathways. Parameters derived from photosynthetic CO2 and light response curves were also not differentially influenced by CO2 treatment between pathways. Gas exchange responses were generally poorly related to CO2 responsiveness. Significant increases in leaf growth and canopy leaf area in elevated CO2 were found in two NADP-ME species, whereas increases in non-leaf above-ground growth were measured in three species representing all three C4 subtypes. Growth responses in elevated CO2 were apparently not simply correlated with biochemical subtype characteristics, although the most significant responses (particularly at the leaf level) were found for the NADP-ME pathway. This result was more likely attributable to the significant positive correlation found between CO2 responsiveness of leaf growth and relative leaf regrowth potential of individual species, the latter being higher in the two responsive NADP-ME species. Therefore, categorisation of PFTs according to relative growth potential may be more appropriate for predictions of CO2 responsiveness in C4 grasses.