Elevated CO₂ , water relations and biophysics of leaf extension in four chalk grassland herbs

Diurnal measurements of leaf or leaflet, extension, water relations and cell wall extensibility (ø) were made on young growing leaves of four chalk downland herbs (Sanguisorba minor Scop., Lotus corniculatus L., Anthyllis vulneraria L. and Plantago media L.) growing in controlled environment cabinets and exposed to either ambient or elevated CO₂ . This study revealed differences in that effect of CO₂ and the- control of leaf growth between the four species. Leaf extension rate (LER) increased significantly at night (average over 8 h) in elevated C0₂ for S. minor A. vulneraria and P. Media with a significant increase over the first 4h of darkness for S. minor, L, corniculatus and P. media, whilst for S. minor and P. media average day-time LER (over 16 h) also increased significantly in elevated CO₂ , as compared with ambient CO₂ . Water potential (ψpS), solute potential (ψpS), turgor pressure (P), yield turgor (Y) and the effective turgor for growth (Pe) were measured using psychrometers. Bollrte potentials of s, minor, A. vulneraria and P. media decreased significantly following exposure to elevated co₂ with a Significant reduction in ψpS, during the day in A. Vulneraria. Turgor pressure increased significantly in elevated CO₂ as compared with ambient CO₂ in A. vulneraria but there was no effect of elevated CO₂ on P in the uther species. effects of CO₂ on ψpS; or ψpS; were observed, Leaf cell wall extensibility (ø) increased significantly in leaves of V. minor, L. corniculatus and P. exposed to elevated Co₂ . whereas in A. vulneraria, there was no effect of CO² on extensibility. These results suggest that the mechanism by which elevated CO₂ promotes leaf growth differs between species arisce in S. minor, L. corniculatus, and p media. CO₂ through an influence on cell wall properties, whist in A vulneraria higher values of P explain that increased leaf growth in elevated CO₂ for this species.