Limitations of net CO2 uptake in plant species of a temperate dry loess grassland

Possible limitations of net CO2 assimilation (PN) in four drought stressed loess grassland species (Festuca rupicola, Salvia nemorosa, Euphorbia pannonica, all three C3 plants, and Bothriochloa ischaemum, a C4 plant) were characterised using data from measurements of CO2 gas exchange (PN, intercellular CO2 concentration Ci and stomatal conductance Gs) and the slow kinetics of chlorophyll fluorescence (variable Chl fluorescence decrease ratio, Rfd). The limitation imposed by Rubisco capacity was estimated from PN/Ci curves. In leaves of the C3 plant F. rupicola, PN was mainly limited by the mesophyll diffusion resistance, most probably due to its sclerophyllous leaf structure. In S. nemorosa (C3) leaves, PN was significantly affected by all investigated factors with well balanced weights. In E. pannonica (C3), the order of limitations was stomatal-mesophyll without a photochemical limitation. In the C4-plant B. ischaemum, the limitation of PN was mesophyll-stomatal including a significant photochemical limitation. The most characteristic difference in the limitation of PN by the factors considered occurred in species in which photochemical reactions (Rfd-values) were not limiting (F. rupicola) or only to a small extent (E. pannonica) and where either mesophyll (F. rupicola) or stomatal (E. pannonica) limitations of PN were decisive. These species had either very low or very high CO2 assimilation rates and are either the maintainer of the original grassland vegetation (F. rupicola) or represent species associated with the degradation of the grassland (E. pannonica, B. ischaemum). Plant species with either a deep root system and succulent leaves (E. pannonica) or with the traits, such as high water use efficiency (WUE) associated with C4 photosynthesis (B. ischaemum), might be successful in an increasingly arid and disturbed environment. Photochemical limitation was significant in the invader B. ischaemum and the characteristic species S. nemorosa. These species exhibited their tolerance through a coordinated stomatal mesophyll and photochemical control.