Doctoral Dissertations

Date of Award

12-1976

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

Russell J. Lewis

Committee Members

Gordon E. Hunt, John H. Reynolds, J. M. Stewart

Abstract

The mechanism of Al tolerance in plants was investigated in a series of greenhouse experiments which compared two varieties of snapbeans (Phaseolus vulgaris L.) that were differentially tolerant to Al. The more tolerant 'Dade' variety was compared to 'Romano' with respect to growth, mineral nutrition, pH, total acidity, Al fixation by root macerates, organic acid content, morphological effects of Al, and to location of Al and P in roots. To compare the two varieties with respect to growth and mineral nutrition, seedlings were grown in one-fifth strength Hoagland's nutrient solution (pH 4.8) at 0, 4, 8 and 12 ppm added Al for 10 days. The Al treatments reduced yields and the concentrations of P and Mg in roots and tops of both varieties. In both varieties, the Al treatments generally increased the K concentration in roots and tops. In the Romano variety Al decreased the Ca concentration in roots and tops. The Al-tolerant Dade variety had a significantly higher root weight than Romano at 12 ppm added Al. Comparison of the pH, titratable acidity, Al fixing capacity of root macerates and citric and malic acid concentrations revealed no major differences between varieties. Plants of both varieties treated for 12 days at 20 ppm Al at pH 4.6 exhibited typical symptoms of A toxicity. These included upward curling of roots; root discoloration; significant reductions in root and top growth; inhibition of lateral root growth; abnormally dark green leaves and gelatinous root tips. The Romano variety also showed purple coloration of stems and petioles. Anatomical changes induced by M included swelling of root tips; curling backward and detachment of root caps; disorganization of cells in the root cap and meristem and loss of cell contents of root cap cells. The relative degree of Al injury was greater in the susceptible Romano variety. Elemental distribution in Dade and Romano snapbean roots, treated for 12 days with 20 ppm added Al, was determined by staining with Mo and by energy dispersive analysis of X-rays generated in the scanning electron microscope. The results of both techniques revealed that young meristematic tissues of the root tip are the sites of Al action. The location of Al was identical to that for P, suggesting the precipitation of P by Al. High concentrations of Al and P were found on the root surface and within the root cap. There was no relationship between the distribution of Al and that of Ca, Mg or K. Aluminum was located in cell walls, cell contents and in nuclei of root cap and meristematic cells. It is suggested that Al disrupts the cell division process in the nucleus. The ability to maintain adequate levels of P in tops, and the capacity to exclude a major part of the Al from sensitive root tissues by regeneration of root cap cells probably account for the greater tolerance of the Dade variety to Al.

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