INTRODUCTION aberration information. Only in this division can cells heterozygous for inversions and translocations be detected with conventional staining techniques, in addition to the dicentrics, fragments, and ring chromosomes visible in mitotic cells. Since the morphological effects of ionizing radiation such as reduced vegetative growth, flowering, seed set and germination, death of shoot apices, and anomalous flowers and leaves are caused by alterations in genetic material, it seems worthwhile to study radiation effects at the chromosomal level in the field despite the variables in natural habitats. There have been many Although there was the initial intent to study annual plant species, for the last three years there have not been sufficient annuals, due to unfavorable growing conditions, to fulfill this objective. investigations of radiation-induced chromosomal aberrations in plants In Area 11, in the laboratory (Sax, 1941, 1950; Giles and Conger, 1950; Sparrow, 1951), but there appear to be few references concerning low-level radiation effects over long time periods in the field. The work on chronic low-level effects has been primarily concerned with morphological damage encountered in areas of natural radiation (Osburn, 1961; Mericle and Mericle, 1965). Bailey and Luquire (1967), however, studied chromosome morphology in Tradescantia popula-~ tions from naturally-irradiated areas and found no significant differences attributable to radiation. We have not been able to find any reports of cytological effects from longterm chronic radiation from nuclear debris in the field. In addition to our investigation of this area where morphological changes have not been observed, we also report on two species from the Rock Valley experimentally-irradiated area of Nevada Test Site (NIS) where morphological effects have been noted on one of the species reported on here, i.e., Krameria parvifolia (Vollmer and Bamberg, 1975) to be discussed subsequently. The search for morphological and phenological effects resulting from radiation within areas contaminated with radioactive materials at NTS have been carried out almost from the beginning of testing of nuclear devices there. To date, evidence of radiation damage to vegetation at NTS has been found in the vicinities of nuclear cratering experiments, or in the fallout patterns of accidental venting of radioactive debris, Some effects on vegetation have also been noted adjacent to large gamma radiation sources set up in controlled experiments within NTS (Kaaz, et al., 1971; Rhoads, et al., 1969; Vollmer and Bamberg, 1975). Radioactively-contaminated areas where morphological or phenological effects have not been observed are also of interest from another point of view, particularly where, as in this case, there have been low-level doses for one to two decades. In some of these areas, the accumulated doses approach levels associated elsewhere with morphological effects. Site D, Area 11, NTS, one of our study sites is such an area. This site is contaminated by plutonium spread primarily by a non-nuclear high explosive, although there was possibly a small component of nuclear fission debris accompanying the initial distribution of plutonium. Plant species with relatively small numbers of large chromosomes are most suitable for cytological investigations. Of the nine shrub and seven annual species collected and examined, only three shrubs proved to be satisfactory for this kind of chromosome study; these included two Asteraceae: Machaeranthera tortifolia (Gray) Cronq. and Keck., (n=6), and Artemisia spinescens D.C. Eat. (n=9). The third belongs to the Krameriaceae, Krameria parvifolia, Benth., (n=6). Visual cytological methods require that cells to be examined must be collected when the chromosomes are condensed and individually distinct. The meiotic microsporocyte division fulfills this requirement and provides maximum 352 NTS, which was to have been the site of this annual species inves— tigation, there was one shrub species with flowers which occurred with a distribution adequate to allow collection of flower buds in areas for which there are radiation background measurements providing long-term dose estimates (Rhoads and Franks, 1975) and in areas which have "normal" background levels of radiation. This was Artemisia spinescens. The two other shrub species were collected in Rock Valley, NTS, where phenological changes have been noted in certain species subsequent to long-term chronic exposure to gamma radiation (Kaaz, Wallace, and Romney, 1971; Vollmer and Bamberg, 1975). The Rock Valley irradiated study site has been described in detail elsewhere (French, et al., 1974). In brief, the area has been irradiated since 1964 by a differentially shielded 17’Cs source at the center of a 340 m diameter Phenological studies of two shrub species within the area have circular plot. been reported: Ephedra nevadensias had fruit and stem production reduced after five years (Kaaz, et al., 1971). Reduction in fruit and leaf production, among other effects were noted in Krameria parvifolia (Vollmer and Bamberg, 1975) one of the subjects of this study. Although the Rock Valley facility cannot be considered as a low-level radiation area, as far as plants are concerned, it is a long-term chronic irradiation facility which serves in part as a basis for understanding the results of the area of our primary concern, Area 11. MATERIALS AND METHODS Flower buds were collected in April 1975 and from 31 March to 19 May 1976. The buds were kept in a fixative of 95% ethanol, chloroform and acetic acid (6:3:1) for 24 hours, then transferred to 70% ethanol for refrigerated storage until microscope slide preparation began. Twelve shrubs of Artemisia spinescens of a total population of about 25 shrubs at Site D, Area 11, and six of 22 shrubs sampled from about one mile south, just outside the enclosure gate provided anthers with pollen mother cells at the proper stages for chromosome analysis. Several other species were taken in the Rock Valley experimental area. Only two, Machaeranthera torttfolia and Krameria parvifolia, provided cells in the proper stages for analysis. Four- teen shrubs of M. tortifolia and 11 of K. parvifolia, from two radiation levels, a higher and a lower were useable. The radiation exposure rates are indicated on tables in Results. We were not successful in collecting buds of these species in areas with normal radiation backgrounds. 353