Effects of X-Rays upon Growth, Development, and Oxidizing Enzymes of Helianthus annuus

Irradiation of soaked seeds of Helianthus annuus causes a slight decrease in percentage of germination. The average total growth of the plants is less, and the average time of blossoming is shortened, even with doses so light that there are no immediate visible effects. A checking effect, proportional to the doses, is evident during the first three weeks of growth after seeds have been irradiated with medium doses. On unfolding, the leaves present a peculiar pitted or mosaic appearance, which seems to be due to an unequal distribution of chlorophyll, when medium or heavy doses are given the seeds. Heavy dosage does not stop development of soaked seeds until the cotyledons have been partially or wholly pushed through the soil. The root tip becomes brown in color and no growing stem tip appears. 2. Fasciations of stems, leaves, and flowers are induced by irradiation of seeds and seedlings with medium doses. Stem fasciations commonly took the form of flattening and dichotomous splitting of the main stem, which showed the presence of many blossoming branches instead of a single flower head. Fasciation of the flower head was represented by fusion in the involucral region and by forking of the stalk below the involucre, with two or three distinct heads resulting. Many leaf abnormalities,-ligulate leaves, altered phyllotaxy, and incipient fasciation, which marked earlier periods of growth, often disappeared before maturity. 3. Catalase activity is depressed in young seedlings whose seeds have been exposed to medium doses of X-rays. With older seedlings which are beginning to show recovery, however, there is but slight reduction in activity noticed. 4. Depressed respiration is associated with stunted growth when this accompanies heavy irradiation. The amount of carbon dioxide liberated from irradiated seedlings is less than that from controls similarly treated. 5. Normal oxidase activity is found in seedlings whose growth has been inhibited by heavy doses of X-rays. 6. Cells of the radicle tip from seedlings whose growth has been inhibited by heavy irradiation show elongation, great vacuolation or entire absence of protoplasm, and the absence of nuclei from many cells. The average size of the cells in the meristematic region of the radicle tip of irradiated material is greatly increased over that of the control. Xylem is increased at expense of the pith, and there is a greater suberin development in the hypocotyl regions of mature plants from irradiated seeds than in those of the control. The increase in vascular tissue in irradiated material is apparent even in seedlings but nine days old.