Growth-induced water potentials originate from wall yielding during growth

Multicellular plants display growth-induced water potentials that generate tensions on water in the apoplast and move water into the growing cells. Th e potentials are sometimes assumed to arise from wall yielding, keeping the turgor pressure below what otherwise would occur. There has been no direct test of this theory, and therefore whole plants or growing regions of stem s (hypocotyls) of dark-grown soybean (Glycine max L. Merr.) seedlings were sealed in a pressure chamber, and wall yielding was decreased by applying e xternal pressure. In whole plants, external pressure had little effect beca use the plants and water supply were uniformly exposed to the pressure. If pressure was applied to the stem while the roots were outside! in water, st em elongation was markedly inhibited because the pressure raised the water potential of the growing region and decreased water entry, reducing wall yi elding. Further increasing the pressure prevented water entry completely an d measured the tensions in the apoplast in the same growing regions. Tensio ns were about 0.19 MPa at low external pressure, but diminished as wall yie lding was inhibited. At external pressures of about 0.63 MPa, wall yielding was abolished and tensions approached zero. There was a linear relation be tween wall yielding and tension, supporting the theory that wall yielding l owers the turgor thus causing most of the growth-induced water potential.