Root signalling and osmotic adjustment during intermittent soil drying sustain grain yield of field grown wheat

A field study was conducted to investigate the effect of intermittent soil drying on resulting non-hydraulic and hydraulic root signals, leaf gas exch ange, leaf growth, day of heading, leaf osmotic adjustment and yield of whe at grown in sand and loam soils in lysimeters. A 40-day-drought treatment w as imposed when the flag leaf started to emerge and was terminated close to maturity. Soil water content and soil water potential of various soil laye rs were measured using the neutron moderation method and tensiometers, resp ectively. Soil drying in the top soil layers induced increase in both xylem and bulk-leaf abscisic acid (ABA) content and reduced the stomatal conduct ance and leaf growth even before a measurable change in leaf water potentia l could be detected in droughted plants when compared with fully watered pl ants. Further, headings and flowering occurred 4 days earlier in the drough ted than in the well-watered plants before any loss in leaf water potential had occurred as compared with the fully watered plants. When more severe d rought reduced the leaf water status, further accumulation of leaf ABA occu rred and transpiration decreased in addition to gradual osmotic adjustment and senescence of older leaves. The osmotic adjustment sustained leaf turgo r pressure during soil drying. At severe drought, the osmotic adjustment at full turgor in the flag leaves was 0.85 MPa. In sand, the kernel dry weigh t increased and as a result similar grain yield was obtained in both the tr eatments. In loam which had more water available than sand, no significant reduction in the final yield was induced by the drought. It is concluded th at (1) non-hydraulic root signals caused early drought adaptation at mild w ater stress by reducing leaf growth and stomatal conductance and hastening of heading and flowering; (2) osmotic adjustment sustained turgor maintenan ce and hence the yield-forming processes during moderate and severe water s tress. (C) 1999 Elsevier Science B.V. All rights reserved.