Growth of Arabidopsis thaliana seedlings under water deficit studied by control of water potential in nutrient-agar media

We have characterized the growth responses of Arabidopsis thaliana seedling s to water deficit. To manipulate the water potential, we developed a metho d whereby the nutrient-agar medium could be supplemented with polyethylene glycol (PEG 8000); PEG was introduced into gelled media by diffusion, which produced media with water potential as low as -1.6 MPa. For dark-grown pla nts, hypocotyl growth had a hyperbolic dependence on water potential, and w as virtually stopped by -1 MPa. In contrast, primary root elongation was st imulated by moderate deficit and even at -1.6 MPa was not significantly les s than the control. That these results did not depend on a direct effect of PEG was attested by obtaining indistinguishable results when a dialysis me mbrane impermeable to PEG was placed between the medium and the seedlings. For light-grown seedlings, moderate deficit also stimulated primary root el ongation and severe deficit reduced elongation only partially. These change s in elongation were paralleled by changes in root system dry weight. At mo derate deficit, lateral root elongation and initiation were unaffected and at higher stress levels both were inhibited. Primary root diameter increase d steadily with time in well-watered controls and under water deficit incre ased transiently before stabilizing at a diameter that was inversely propor tional to the deficit. Along with stimulated primary root elongation, moder ate water deficit also stimulated the rate of cell production. Thus, A. tha liana responds to water deficit vigorously, which enhances its use as a mod el to uncover mechanisms underlying plant responses to water deficit.