Transient resistance to root growth inhibition by moderate water stress: A possible explanation

Abstract

Root growth is dependent on a continuous supply of water and organic solutes needed to maintain cell expansion. It might therefore seem obvious that water stress inhibits root growth. However, some reports indicate that root growth can be maintained or even stimulated during exposure to moderate water stress. This phenomenon was investigated in rice (Oryza sativa L) seedlings grown hydroponically in a controlled environment. Seedlings were exposed for fixed periods to defined levels of water stress induced by PEG 6000 at external water potentials ranging from 0 to — 0.6 MPa. Effects of water stress on rates of root and leaf elongation, as compared to controls without PEG, were determined for 2 cultivars:- Salumpikit, an upland rice and IR20, a paddy rice. Leaf elongation in Salumpikit was resistant to growth inhibition by external water potential of −0.1 and −0.2 MPa and declined progressively at the more negative water potentials. Root elongation in Salumpikit was progressively inhibited over the whole range of water potentials. However, in IR20, leaf elongation was progressively inhibited while root elongation was remarkably resistant to inhibition at external water potentials ranging from 0.1 to 0.4 MPa. Root elongation was inhibited at more severe levels of stress i.e. water potentials of −0.5 and −0.6 MPa. Maintenance of root growth in water-stressed rice was therefore cultivar dependent and only evident under mild to moderate levels of stress. Similar inverse relationships between the early effects of water stress on leaf growth and the inhibition, maintenance or stimulation of root growth were also observed in literature reports on maize and arabidopsis. The findings suggest that early leaf growth inhibition by mild to moderate water stress and a diversion to the roots of water and organic solutes normally utilized for leaf-growth, may facilitate a transient maintenance or stimulation of root growth in drying soils.