THE DIFFERENT EFFECTS OF PEG-6000 AND NACL ON LEAF DEVELOPMENT ARE ASSOCIATED WITH DIFFERENTIAL INHIBITION OF ROOT WATER TRANSPORT

The inhibitory effects of PEG on whole-plant growth can exceed the eff ects of other osmolytes such as NaCl, and this has been ascribed to to xic contaminants, or to reduced oxygen availability in PEG solutions. We investigated another possibility, namely that PEG has an additional inhibitory effect on root water transport which in turn affects leaf development. The effects on first-leaf growth of applications of PEG 6 000 or iso-osmotic NaCl to the roots were determined using hydroponica lly grown maize (Zea mays L,) seedlings. Leaf growth rates were inhibi ted within minutes of PEG application to the roots and remained inhibi ted for days. The inhibitory effects on growth of NaCl, and also of KC l and mannitol, were much smaller. The comparative effects of NaCl and PEG on root water transport were determined by assaying pressurized f low through excised roots, PEG induced a 7-fold greater inhibition of flow through live roots than NaCl. Killing of the roots by heat treatm ent, to reduce cell membrane resistances to solute penetration, nearly doubled the flow rate for roots in NaCl, but not for roots in PEG. We suggest that the greater viscosity of PEG solutions, as compared with NaCl, may be a primary factor contributing to the additional inhibiti on of water flow through live and killed roots. PEG did not have addit ional effects on leaf turgor but had a 3 times greater inhibitory effe ct than NaCl on the irreversible extensibility of the leaves and induc ed 16 times more leaf accumulation of the growth inhibitory stress hor mone abscisic acid (ABA). We conclude that greater inhibition of root water transport by PEG 6000, as compared with NaCl, leads to additiona l reductions in extensibility, additional ABA accumulation, and a grea ter inhibition of leaf growth.