Date of Award:

5-1981

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Herman H. Wiebe

Committee

Herman H. Wiebe

Committee

M. M. Caldwell

Committee

R. J. Hanks

Committee

F. B. Salisbury

Committee

M. A. Walsh

Committee

R. W. Brown

Abstract

Hydraulic conductivity of single roots and whole root systems was studied for soil, sand and solution-grown cotton plants (Gossypium hirsutum L.). Conductivity varied widely depending on plant age and growing conditions. Total conductivity for soil-grown plants decreased from 357 x 10-6 µ1 s-1 bar-1 cm-1 one week after germination to 5 x 10-6 µ1 s-1 bar-1 cm-1 at boll set. There was a concomitant increase in total root length. Solution-grown plants had higher root conductivities than soil- or sand-grown plants. Root conductance was decreased by low temperatures, lack of oxygen and low nitrogen. Root resistance was flux dependent at all growth stages studies. Xylem development was studied and flux measurements were made at varying distances from the root apex. Appreciable radial and axial resistance was shown to exist in the terminal 6-8 cm of root.

Plants were drought acclimated during the vegetative stage by withholding water for one to three stress cycles. Root system conductance (flux) was appreciably lowered (5-78%) by water stress preconditioning. Reduced radial conductivity was the main factor in the reduced conductance, although reduced root growth was also a factor. Preliminary observations on suberization and electrolyte leakage support the interpretation that the acclimation change occurred in the radial pathway. The persistence of the acclimated state of decreased conductivity was shown to diminish with time after preconditioning from a 78% decrease after 7 days to a 50% decrease after 21 days, and a return to control levels within 35 days.

Other effects of drought acclimation were studied. Stomata remained open to lower values of water potential in preconditioned plants. Transpiration was decreased by water stress preconditioning. Both leaves and roots adjusted osmotically in response to drought acclimation, with roots exhibiting greater percentage osmotic adjustment than leaves. The magnitude of the adjustment and its persistence was related to the severity of the drought and the number of drought stress cycles. The exponential nature of the pressure-flux relationship of roots was unaffected by drought acclimation, although flux at all pressures was decreased. Root pressure was lowered by water stress preconditioning, as was the flow of water from roots to soil when the normal water potential gradient was reversed.

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