ELEVATED CO2, WATER RELATIONS AND BIOPHYSICS OF LEAF EXTENSION IN 4 CHALK GRASSLAND HERBS

Diurnal measurements of leaf or leaflet extension, water relations and cell wall extensibility (phi) 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 the 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 CO2 for S. minor, A. vulneraria and P. media with a significant increase over the first 4 h of darkness for S. mines, 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 (Psi), solute potential (Psi(s)), turgor pressure (P), yield turgor (Y) and the effective turgor for growth (Pe) were measured using psychrometers. Solute potentials of S. minor, A. vulneraria acid P. media decreased significantly following exposure to elevated CO2 with a significant reduction in Psi(s) 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 other species. No effects of CO2 on Psi, Y or Pe were observed. Leaf cell wall extensibility (phi) increased significantly in leaves of S. minor, L. corniculatus and P. media 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 since in S. minor, L. corniculatus and P. media, CO2 promoted growth through an influence on cell wall properties, whilst in A. vulneraria, higher values of P explain the increased leaf growth in elevated CO2 for this species.