M. Ali et al., Root signalling and osmotic adjustment during intermittent soil drying sustain grain yield of field grown wheat, FIELD CR RE, 62(1), 1999, pp. 35-52
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.