Rice seeds treated with 0·2–1·2 v/v % of ethyleneimine (EI) demonstrated increasingly delayed germination concomitant with increasing dose. At the time of germination, the release of storage products was slightly inhibited at lower doses and completely reduced at higher doses. With increasing time after germination the development of shoot length, content of reducing sugar and free amino acid, and synthesis of nucleic acid and protein in treated seeds, showed the same response pattern as the control, although at reduced levels in the treated seeds. Consequently, it is interpreted that the delay of germination is due to physiological dormancy, i.e. impaired release of dormancy which would normally yield active forms of enzymes.
Sprout inhibition of onions by gamma irradiation was found to be influenced by the physiological state of the bulbs at the time of irradiation, radiation dose and the storage temperature after irradiation. Sprouting was minimal during storage at ambient temperature (26 to 32°C) while exposure to low temperature (4 to 20°C) or widely fluctuating diurnal temperature (20°C min to 30°C max) accelerated it. Six to nine Krad inhibited sprouting during storage under the above conditions only if the bulbs were irradiated within a fortnight of harvest when they were in the dormant state. Twenty-five Krad caused a transient stimulation of sprouting regardless of the time of irradiation while 6 to 12 Krad only stimulated sprouting when the time interval between harvest and irradiation exceeded three weeks. Irradiation at all dose levels caused discoloration of the growth centre, the extent of which was minimal in bulbs exposed to low doses soon after harvest and in those stored at ambient temperature.
Criteria for predicting optimum selection procedure in mutation breeding of self-pollinating crops were discussed and formulated. Based on the mathematical formulae and some numerical computations, the efficiency of delayed selections was examined and the following derived as a general conclusion. Delayed selection in M3~M5 generations is of great practical merit when plants mutated at multiple loci are to be obtained. This method is considered promising even for the detection of single-locus mutants when difficulty in discrimination of the objective mutants is large, and/or when permissible population size per generation is small, probably below 10,000, as a rough estimate. It is, in general, thought that fifteen or more M2 plants should be derived per M1 spike or panicle. Small progeny size per M1 spike such as one, two or three as proposed by Yoshida et al. seems of limited utility.
After X-irradiation of dormant caryopses of the different radiosensitive winter wheat varieties “Bayro” and “Walthari” from five successive harvest-years, a negative correlation between phosphorus content of caryopses and their radiosensitivity was observed. To interpret the results two possibilities were discussed: 1. The amount of phosphorus content in developing embryos influences the size of chromosomes in the nuclei of the embryos and therefore also their radiosensitivity. 2. The intensity of physiological activities during the premitotic phase of germination, which depends on the phosphorus content, influences the duration of radiation-induced “mitotic delay” and therefore also the process of repair in this phase.
The possibility of using induced mutations for developing highly productive lines of Sorghum were examined herein. Mutant plants arising in the progeny of induced interchange heterozygotes of grain Sorghum variety I. S. 3566 were studied. The mutants were classified and considered under three different height classes of “2-Dwarf”, “1-Dwarf” and “0-Dwarf”. The “2-Dwarf” (tall) plants were observed to produce more grains than the “3-Dwarf ” (short) controls. Other vegetative and panicle characters in the apparently true-breeding mutant were also affected including increased numbers of leaves, nodes and productive tillers. The mutant plant produced red grains on compact panicles of reduced length compared to chalky white grains on the semicompact panicles of the untreated controls. An increase in grain weight of about 21 per cent was shown in selfed progenies studied through successive generations. The observed increase in yield can be traced to an increased number of grains produced on the main and tiller panicles.
Gamma irradiated medium induces the formation of buds in non-irradiated dark grown tobacco callus (Nicotiana tabacum Var. Wisconsin no. 38). Experiments were conducted to determine the component(s) of the medium that is effective in this radiation-induced organogenesis. Fractions of medium were irradiated singly and in combination, then combined with non-irradiated fractions to form the complete growth medium.
The effect of four straw and two 137cesium levels, with and without fertilization, on 137cesium uptake by wheat plants was studied using soils from the Blaine Lake and Waiteville soil associations of Saskatchewan (Canada).
In vitro culture of somatic cells of higher plants allows the plant breeder to apply the techniques of microbial genetics to specific breeding problems in crop plants. The basic techniques of callus and suspension culture, haploid plant culture from pollen, protoplast isolation, culture and fusion, and totipotency in plant cell cultures are reviewed. The genetical approaches which have application or potential application to plant breeding are also reviewed. These include selection of mutations in cell cultures, studying host-pathogen inter-relationships, transfer of genetic information into cells through uptake of native DNA, uptake of whole metaphase chromosomes or viral-induced transduction and interspecific hybridization through somatic cell fusion. The major difficulty to the application of many of these techniques to plant breeding programs is the inability to regenerate whole plants from somatic cells in some of the major crop plants. Some techniques such as mutant selection have been and are presently applicable in some crops, but others such as somatic cell fusion still require additional refinement before they will become generally applicable in a plant breeding situation. A survey is made of the present status of the culture methods possible in several important crop plants.
A recent study of the kinetics of developing microspores in half anther populations of Campelia zanonia has led to a better understanding of the fluctuations in compartmental indexes (e.g. the mitotic index) normally observed in plants grown under rigidly controlled environmental conditions.
Effects of gamma irradiation (10,000 Ci 137Cs source) for one growing season on biomass production of ground vegetation under northern Wisconsin aspen and maple-aspen-birch forests and on an abandoned logging road were evaluated during and one year after irradiation. No significant changes in production were determined during the irradiation year. One year later three distinct zones—semi-devastated, herbaceous, and original forest—developed along the radiation gradient. Biomass production under forest canopies decreased significantly in the semi-devastated zone, increased significantly in the herbaceous zone (primarily responding to additional light), and remained unchanged under the original forest. Logging road vegetation responded similarly but the changes were restricted within higher radiation doses. At comparable levels of radiation, production of species of the logging road vegetation was affected less than that of species under forest canopies. Such a trend was predictable from the generally smaller interphase chromosome volumes, ICV's, of the species on the logging road and from their ability to survive in severe habitats.
The effects of 2,4-D (10, 50 and 100 ppm) and gamma rays (10, 20 and 30 kR), both individually and in combination, were studied on the vegetative shoot apex of safflower. While 2,4-D had no effect on the shoot apex organization, gamma rays caused much injury. The extent of injury was, however, much less in the shoot tips treated with 10 ppm 2,4-D plus gamma ray exposures than in the apices treated with gamma rays alone. Occurrence of multinucleate cells and abnormally positioned tracheidal cells was observed in the apices treated with gamma rays and 2,4-D plus gamma ray combinations.
Dry seeds of uniform size of the rice variety T(N)1 were treated with DES alone and in combination with X-rays. The combined treatments affected more adversely the different growth metrics in the M1 generation and increased the frequency of appearance of chlorophyll and other mutations affecting various morphological characteristics in the M2 generation as compared to the treatment with DES or X-ray radiation alone. With respect to the spectrum of mutations induced by DES treatment, the frequency of chlorophyll mutations was noted to be relatively higher than that of other morphological mutations. The appearance of xantha mutations was noted to be the most frequent in the DES treatment in contrast to their rare appearance in irradiated material. The combined treatment had a considerable effect in altering the frequency and spectrum of morphological mutations as well as increasing the frequency of micromutations for grain yield and plant height.
Experiments were conducted to test the radioprotective effects of a naturally occurring reducing agent, ascorbic acid, on seeds (caryopses) of barley, Hordeum vulgare L. emend Lam. Seeds were soaked either before or after gamma or fission neutron irradiation in distilled water or ascorbic acid solutions ranging in concentration from 0·01 to 1·00 m. Results are reported as percentage germination, seedling height, seedling growth reduction, and (in one experiment) per cent of cells with chromosome aberrations. As evidenced by both reduced germination and seedling growth, ascorbic acid was toxic when seeds were soaked for 1 hr at ambient temperature prior to irradiation and then planted immediately. When seeds were soaked in ascorbic acid before irradiation and soaked after irradiation in air-bubbled water at 0 °C for 18 hr, the toxicity disappeared, and a protective effect (which increased with increasing ascorbic acid concentration) was observed for gamma and, to a lesser extent, for neutron irradiation. Additional studies suggested that the protective effect was related to reduced hydration of the embryos of seeds soaked in ascorbic 'acid. Also, no radioprotective effect was observed when seeds were presoaked for 2 or 16 hr in 0·01 Mascorbic acid solutions buffered at pH 3 or pH 7. A protective effect was observed for seeds of 1·5 per cent water content soaked after irradiation in an oxygen-bubbled ascorbic acid solution of 0·5 M but was not observed for seeds soaked in nitrogen-bubbled ascorbic acid. The protective effect against oxygen-dependent damage may be a result of interaction of ascorbic acid with radiation-induced free radicals.
The exposure of potatoes to gamma rays (10 krad) caused on induction of phenylalanine ammonia lyase (PAL) in the tubers. The induced activity was localized essentially in cortex tissue, mainly concentrated around the bud region. Whereas in parenchymatous tissue no induction was observed. The enhancement in PAL activity in buds was observed to be dependent on dose of irradiation and the duration of storage thereafter. The tubers exposed to a 10 krad dose showed maximal activation on storage at ambient conditions with 20 fold activity observed within three hours after irradiation. On further storage the activity declined and reached a constant level within a period of one week. The increase in PAL activity was partly due to activation of the inactive form of the enzyme (30–40%) and partly due to de nova synthesis of the protein (60–70%).
Strains of di-, tetra-, and hexaploid oats derived from untreated and EMS-treated seeds were used to estimate mutation rates for spontaneous and induced mutations at loci that affect quantitatively inherited traits. The oat strains were produced by using a dichotomous progeny scheme. The study was initiated with one plant from each of two cultivars for each ploidy level. From a first-generation plant, four seeds were chosen to produce second-generation plants. Two were sown without treatment, and the other two were treated with 0·04 M solution of ethyl methanesulfonate (EMS) for four hours. For the third and subsequent generations, two progenies were grown from each plant in the previous generation. Seeds in the treated line of descent were given EMS through generation 5. Both lines of descent were carried through six generations. The 125 strains (62 in the untreated line of descent, 62 in the treated line of descent, and one from the original plant) from one cultivar were sown in a field experiment and traits measured were grain yield, heading date, plant height, flag leaf length, number of spikelets per panicle, 100-seed weight, seed width, seed length, and protein percentage. A mutation was judged to have occurred when two criteria were met: (a) the observed value for a trait of a parent strain differed from comparable values in one or both of its progeny by an LSD unit and (b) the deviant value was inherited into later generations. Mutation rates were expressed as mutations per trait per 100 gametes. Genotypes differed with respect to the rates of occurrence of spontaneous and induced mutations. There was no obvious relationship between spontaneous and induced mutation rates that genotypes showed, but mutability was trait related. Traits most susceptible to mutation induction were grain yield, heading date, plant height, number of spikelets per panicle, and seed length. Spontaneous mutation rates for di-, tetra-, and hexaploid oats were 1·2, 0·5, and 0·3 mutations per trait per 100 gametes, respectively, whereas induced mutation rates were 1·2, 1·3, and 2·0 mutations per trait per 100 gemetes, respectively.
Metabolic disturbances resulting in growth retardation and death of rice irradiated with 60Co γ-rays at 10 to 40 kR dose levels were investigated. Seedling growth, release of storage products (reducing sugar and free amino acids) and incorporation of radioisotopic substances ([3H]-uridine, [3h]-thymidine and [3h]-leucine) remained almost uninhibited at doses up to 20 kR but were much reduced at 40 kR. Following the 40 kR treatment, shoot elongation was terminated at the 5th day of germination, corresponding to an interruption in the release of reducing sugar. Furthermore, the onset of active increase in [3h]-thymidine incorporation after germination was also delayed in the 40 kR irradiated samples.
Experiments were performed to find out whether the level of ploidy bestowed a species with any additional resistance against the lethal and mutagenic damage of alkylating agents. Seeds of 11 species of Solanum of the non-tuberous group formed the test system. Of these 2 were diploids (2 n = 24); 4 tetraploids (2 n = 48) and 5 hexaploids (2 n = 72). Seeds of all the species were exposed to aqueous solutions of ethyl mathanesulphonate; ethylenimine ; methyl nitrosoguanidine and 1,3-propanesultone and effect of these mutagens studied through M 1 and M 2 generations. A considerable difference in sensitivity was found even between different species of the same ploidy level and no positive relationship could be established between the level of ploidy and its sensitivity to alkylating mutagens. However, in general it has been observed that as the level of ploidy increased from diploid to hexaploid the tolerance to alkylating damage increased but not proportional to the level of ploidy. The increase in resistance varied from marginal to a highly significant level.
The effect of gamma radiation on dormant seeds of Avena fatua L. (wild oats) has been investigated. A. fatua seeds require a long after-ripening period for germination. Gibberellic acid and cold treatments are known to break seed dormancy. 10 kR of gamma radiation also produced an effect similar to these treatments. Shoot length and dry matter content of seedlings raised from 10 kR gamma-irradiated seeds were similar to those raised from control seeds. Higher radiation doses promoted seed germination but they also caused a reduction of seedling growth. Probable mechanisms by which gamma radiation may bring about these effects have been discussed.
