The effects of acute and chronic gamma irradiation on Lupinus albus L.—III. Chronic effects

Abstract

Lupinus albus L. plants were grown from seed and chronically irradiated at dose rates of 2.75, 8.5, 16, 25, and 33 rad/hr. Exposures were continuous until all plants flowered; if flowering was suppressed, irradiation was discontinued after all control plants flowered—a period of 8–11 weeks. A Plastochron Index for lupine provided a base-line developmental time scale for evaluating radiation effects. Chronic irradiation induced a lengthening of the control plastochron interval (3.56 days). Slopes of leaf growth curves were depressed and there was a statistically significant reduction in the number of leaves formed. Other gross morphological effects were: missing or fused leaflets, shortened internodes, blind shoots, multiple shoots, a short-lived stimulation of anthocyanin production in petioles and premature death without flowering at 33 and 25 rad/hr and 50% of the plants at 16 rad/hr. All plants irradiated at 2.75, 8.5, and the 50% at 16 rad/hr which did flower were similar to controls with respect to vegetative morphology. The number of leaves formed was associated with the 16 rad/hr threshold for flowering; plants at the higher dose rates formed less than 12 leaves and failed to flower. Plants at the two lowest dose rates produced more than 12 leaves and all flowered, but no fruits formed at any dose rate. Control plants averaged 14 leaves and all flowered and fruited. Irradiation resulted in a reduced stem diameter at the two lowest dose rates but stem diameter was increased at the three highest dose rates. An increase or decrease in stem diameter reflected the differential sensitivity of different stem tissues as measured by an increase or decrease in their radial increments. The pith was reduced at the two lowest dose rates, stimulated at 16 and 25 rad/hr and returned to control level at 33 rad/hr; the vascular component was reduced at all dose rates. The cortex showed a dose-rate-dependent stimulation. When increase in stem diameter was expressed as percentage change, the cortex showed the highest increase over controls, the vascular tissues were negative contributors, the pith showed a percentage increase at 16 and 25 rad/hr but a percentage decrease at 2.75 and 8.5 rad/hr. Within the vascular cylinder, which was decreased at all dose rates, there was a dose-dependent decrease in the amount of secondary xylem and an increase in the amount of secondary phloem. Derivatives of the irradiated vascular cambium were slow to differentiate. Analyses of microscopic sections indicated that there was a statistically significant increase in the number of phloem parenchyma cells and only a slight increase in the number of sieve tube and companion cells. The average increase in phloem area for all dose rates was calculated to be 2.23 × greater than controls, while the average increase in cell number was 1.64 × greater than controls. This apparent discrepancy was covered by an increase in cell size, chiefly in the phloem parenchyma cells becoming highly vacuolated.