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

Abstract

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.