INDUCTION OF ABERRATIONS IN MEIOTIC CHROMOSOMES OF WHEAT BY MEANS OF THYMIDINE-H$sup 3$
Embryos removed from wheat seeds which had taken up water for 4 hr were incubated in thymidine-d for 4 hr, and then grown on isotope-free medium for 1 hr. No activity was found in these embryos. Since 16 hr is required for visible germination at 25 deg C, it may be assumed that cell division with consequent incorporation of radiothymidine in DNA must begin sometime during the second half of the pregermination period. Localization of thymidine-d during the periods 1 to 5, 4 to 9, and 8 to 13 hr after germination appeared to be in about the same amounts and indicated that cell division was proceeding at an even rate over these periods. The primary root, both lateral roots, and coleoptile were the most active organs. The first foliage leaf was the next most active component. The high incidence of stunting and retardation in wheat seediings after germination of seeds in thymidine-d was due largely to radiation injury of the early root system. Injury to the seedling root system was also the commonest effect after embryo incubation in thymidine-d; effects in the epicotyl were slightly fewer. This would be expected in view of thymidine localization. During the period 24 to 48 hr after germination, the first lateral roots surpassed the primary root in uptake of thymidine-d as they enlongated and passed the primary in length. The coleoptile, which doubled in length during this period, had a thymidine-d content slightly less than in the earlier periods. Cell elongation was, therefore, the chief contributor to coleoptile growth during this period. Increase in lengih of the first foilage leaf during this period was also through cell elongation. Thymidine moved from the endosperm to the developing embryo. The low yields of chromosome aberrations in pollen mother cells of adult plants arising from embryos incubated in thymidine-d or from seedlings with epicotyls cut back and then immersed in radiothymidine are accounted for on the basis of the following considerations. There are about 200 cells in the growing point at 24, 48, or 72 hr postgermination. The uptake of thymidine-d in the growing point was limited because oniy a few cells were in division, and the number of cells in DNA synthesis at any one time would therefore be small. Thus, radioautographs of serial sections showed strong label in the growing point in oniy two of fifteen embryos given a 24-hr exposure to thymidine-d, even when coleoptiles were cut back and the embryo was submerged upside down to ensure contact with isotope. The growing point was surrounded by organs which contained many dividing cells. Pronounced label was found in coleoptile, first foliage leaf, second foliage leaf, meristematic area proximal to growing point, third foilage leaf, and bases of roots in radioautographs of serial sections. Although some seedlings died of radiation damage after embryo treatment with thymidine-d, many eventually recovered the normal phenotype. This suggests that enough undamaged or only partially damaged cells survived within the injured organs to proliferate and eventually produce normal structures. The incidence of aberrations in meiotic chromosomes increased when exposure time of the growing point to isotope was increased and isotope was replenished to ensure its availability to the relatively few cells of the growing point undergoing DNA synthesis at any one time. (auth)
- Research Organization:
- Univ. of Missouri, Columbia
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-18-001424
- OSTI ID:
- 4157583
- Journal Information:
- Radiation Res., Vol. Vol: 20; Other Information: Orig. Receipt Date: 31-DEC-64
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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Related Subjects
AGE
BIOCHEMISTRY
CEREALS
CHROMOSOMES
CYTOLOGY
DISTRIBUTION
DNA
EMBRYOS
GERMINATION
LABELLED COMPOUNDS
LEAVES
METABOLISM
MITOSIS
MUTATIONS
NUCLEIC ACIDS
PHYSIOLOGY
PLANTS
POLLEN
QUANTITY RATIO
RADIATION EFFECTS
RADIOACTIVITY
RADIOAUTOGRAPHY
RECOVERY
REPRODUCTION
ROOTS
SEEDS
TEMPERATURE
THYMIDINE
TISSUES
TRITIUM
WATER
WHEAT