Date of Award:

5-1971

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Botany

Committee Chair(s)

Orson S. Cannon

Committee

Orson S. Cannon

Committee

Bryce N. Wadley

Committee

W. S. Boyle

Committee

David L. Mumford

Committee

Wade G. Dewey

Abstract

To determine possible unusual effects of resistant plants upon symptom expression of curly top disease in susceptible tomato plants (Lycopersicon esculentum Mill.), tops of susceptible plants were grafted to lower stems and roots of resistant plants and inoculated. The resistant plants used were wild relatives of L. esculentum: L. peruvianum var. dentatum Dun. and L. peruvianum var. humifusum C. H. Mul. Reciprocal grafts, in which tops of dentatum plants were grafted to lower stems and roots of L. esculentum, were also made and inoculated.

External symptoms in the grafted plant tops were observed and compared with those of nongrafted, inoculated control plants. Also, anatomical studies were made to determine the effect of the disease upon the phloem tissue of the scions. The diseased susceptible/resistant grafts exhibited normal symptoms for diseased susceptible plants including definite vein clearing, stunting, leaf contortion and stiffening, vein thickening, and epinasty. Typical phloem symptoms of hyperplasia and hypertrophy were also observed. These plants, however, did not develop typical yellowing. Instead, abnormally high concentrations of chlorophyll accumulated and the plants subsequently remained alive for an extended time following the death of susceptible control plants. All of the diseased susceptible/resistant grafts, however, eventually died.

The opposite situation was observed in diseased resistant/susceptible grafts. The resistant scions developed yellowing but remained resistant to the other external and anatomical symptoms. It was concluded that a factor responsible for chlorophyll maintenance was located in the roots of resistant plants and that this factor was distinct from that which conferred resistance to the other symptoms. Likewise, at least two separate mechanisms for the symptom complex of curly top were recognized: that causing chlorophyll degeneration and that causing external morphological and anatomical symptoms (including vein clearing).

Diseased L. esculentum plants were treated through various methods with the growth regulator kinetin (6-furfurylaminopurine) to determine any possible relation between this compound and yellowing. Also, the effects of excessive nitrogen in delaying chlorophyll degeneration were investigated. Neither of these factors was found to alter the normal yellowing symptom.

Seedlings of L. esculentum and dentatum in the cotyledon stage were exposed to viruliferous or nonviruliferous leafhoppers. Wilting and death was noted in both groups after approximately 24 hours, although this reaction was definitely severe among seedlings of the viruliferous-fed treatment. Macroscopic observation revealed inhibition of secondary root initiation and growth, and inhibition of primary root growth among seedlings of both treatments with viruliferous-fed seedlings being more severely affected. The virus was therefore shown to exert an additive, but not a qualitatively distinct effect in causing wilting which is not a generally recognized symptom of curly top.

Anatomical examination revealed complete death and degeneration of cytoplasm in cells of both emerged and nonemerged secondary root tips and of primary root tips in seedlings of both treatments. This degeneration, which was accompanied by some cortical collapse in mature root tissue, accounted for inhibition of root growth and probably for the wilting. No qualitative difference was noted on the anatomical level between treatments.

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