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 effects of other osmolytes such as NaCl, and this has been ascribed to toxic 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 6000 or iso-osmotic NaCl to the roots were determined using hydroponically grown maize (Zea mays L,) seedlings. Leaf growth rates were inhibited within minutes of PEG application to the roots and remained inhibited for days. The inhibitory effects on growth of NaCl, and also of KCl and mannitol, were much smaller. The comparative effects of NaCl and PEG on root water transport were determined by assaying pressurized flow through excised roots, PEG induced a 7-fold greater inhibition of flow through live roots than NaCl. Killing of the roots by heat treatment, 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 inhibition of water flow through live and killed roots. PEG did not have additional effects on leaf turgor but had a 3 times greater inhibitory effect than NaCl on the irreversible extensibility of the leaves and induced 16 times more leaf accumulation of the growth inhibitory stress hormone abscisic acid (ABA). We conclude that greater inhibition of root water transport by PEG 6000, as compared with NaCl, leads to additional reductions in extensibility, additional ABA accumulation, and a greater inhibition of leaf growth.