INCREASED ROOT-GROWTH IN ELEVATED CO2 - A BIOPHYSICAL ANALYSIS OF ROOT CELL ELONGATION

A biophysical analysis of root expansion was conducted in four chalk downland herbs (Sanguisorba minor Scop., Lotus corniculatus L., Anthyllis vulneraria L. and Plantago media L.) exposed to either ambient or elevated CO, in controlled environment cabinets. Measurements of fine (F) and extra-fine (EF) root extension rate (RER), water relations, and cell wall tensiometric extensibility revealed differences in the diurnal pattern of root growth between species. After 35 d of exposure to elevated CO2, RER of both F and EF roots increased significantly in darkness and on illumination for S. minor, whilst for A. vulneraria (EF roots only) and L. corniculatus a significant increase occurred at night whereas for P. media a significant increase occurred during the day. Cells measured in the zone of elongation were longer in all species exposed to elevated CO2. Water potential (Psi), solute potential (Psi(s)), turgor pressure (P), yield turgor (Y) and effective turgor (Pe) were measured by stress-relaxation of excised root tips placed in psychrometers. Solute potentials decreased significantly for all species following exposure to elevated CO2. In S. minor and L. corniculatus, P and Pe, respectively, were higher in elevated CO2. No significant effects of CO2 on Y were observed (not shown). Root cell wall tensiometric extensibility, measured as % plasticity, increased in all species exposed to elevated CO2. These results suggest that root growth is enhanced following increased cell expansion and that increased P and cell wall tensiometric extensibility are both important for root growth in elevated CO2.