LIMITATIONS OF NET CO2 UPTAKE IN PLANT-SPECIES OF A TEMPERATE DRY LOESS GRASSLAND

Possible limitations of net CO2 assimilation (PN) in four drought stre ssed loess grassland species (Festuca rupicola, Salvia nemorosa, Eupho rbia pannonica, all three C-3 plants, and Bothriochloa ischaemum, a C- 4 plant) were characterised using data from measurements of CO2 gas ex change (P-N, intercellular CO2 concentration C-i and stomatal conducta nce G(5)) and the slow kinetics of chlorophyll fluorescence (variable Chi fluorescence decrease ratio, Rfd). The limitation imposed by Rubis co capacity was estimated from P-N/C-i curves. In leaves of the C-3 pl ant F. rupicola, P-N was mainly limited by the mesophyll diffusion res istance, most probably due to its sclerophyllous leaf structure. In S. nemorosa (C3) leaves, P-N was significantly affected by all investiga ted factors with well balanced weights. In E. pannonica (C-3), the ord er of limitations was stomatal-mesophyll without a photochemical limit ation. In the C-4-plant B. ischaemum, the limitation of P-N was mesoph yll-stomatal including a significant photochemical limitation. The mos t characteristic difference in the limitation of P-N by the factors co nsidered occurred in species in which photochemical reactions (Rfd-val ues) were not limiting (F. rupicola) or only to a small extent (E. pan nonica) and where either mesophyll (F. rupicola) or stomatal (E. panno nica) limitations of PN Were decisive. These species had either very l ow or very high CO2 assimilation rates and are either the maintainer o f the original grassland vegetation (E. rupicola) or represent species associated with the degradation of the grassland (E. pannonica, B. is chaemum). Plant species with either a deep root system and succulent l eaves (E. pannonica) or with the traits, such as high water use effici ency (WUE) associated with C-4 photosynthesis (B. ischaemum), might be successful in an increasingly arid and disturbed environment. Photoch emical limitation was significant in the invader B. ischaemum and the characteristic species S. nemorosa. These species exhibited their tole rance through a coordinated stomatal mesophyll and photochemical contr ol. (C) Elsevier, Paris.