O. Chazen et al., THE DIFFERENT EFFECTS OF PEG-6000 AND NACL ON LEAF DEVELOPMENT ARE ASSOCIATED WITH DIFFERENTIAL INHIBITION OF ROOT WATER TRANSPORT, Plant, cell and environment, 18(7), 1995, pp. 727-735
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.