Rate of DNA replication in the DNA synthetic period of the barley chromosomes

The duration of DNA synthetic (S) period in Zea mays: A genetic control

The nuclear cycle among several diverse genetic stocks of Zea mays root meristem cells was compared and it was found that there were no significant differences among the nuclear cycle durations and its component phases. The durations of various periods of their mitotic cycles were studied by autoradiography of cells pulse-labelled with tritiated thymidine (3H-TdR). The total nuclear cycle was 10 to 11.5 hours and mitosis was 0.81 to 1.34 hours at 25°C. The S period is the longest interval (50% of the total time) of the nuclear cycle; of the rest of the cycle, G2 is longer than G1 or mitosis among all stocks. The constancy of the nuclear cycle among several stocks was adduced as evidence for strict genetic control of the cycle. Furthermore, it is demonstrated the DNA synthesis period is not dependent upon the amount of DNA present.

A modified Giemsa C-banding technique for Hordeum species

A Giemsa C-banding technique with a hot 1 N HCl hydrolysis step has been developed for barley chromosomes. This step makes it easy to obtain well separated C-banded chromosomes. To compare this technique with other C-banding techniques, chromosomes of H. vulgare cv. York were stained by both this technique and a modification of the technique of Kimber et al. (1976). With respect to centromeric and intercalary bands, both techniques produce a similar banding pattern, but telomeric bands observed by the modified technique of Kimber et al. (1976) were not detected by our procedure. This indicates that telomeric heterochromatin may be different chemically and/or structurally from the centromeric and intercalary heterochromatin and its appearance dependent upon the C-banding technique. The procedure described provides a relatively rapid technique for C-banding of barley chromosomes.

Cell kinetics, development of stomata and some effects of colchicine in barley

The developmental sequence of the formation of stomatal complexes in the leaf epidermis of barley was studied. Cell-kinetic parameters were obtained from two genotypes - "Early Bonus" and eceriferum-g, a mutant with an abnormal stomatal pattern. The distribution of mitotic frequencies as a function of position in the stomatal rows was analyzed at each stage of development leading to mature stomata. Regression curves obtained for each stage showed that the distributions were stage-specific. Thus, the mitotic frequencies presented similar values throughout the portion of the file where the first asymmetrical divisions take place, had a parabolic distribution for the stage of subsidiary formation, and showed a linear shape, with a negative slope, for the stage of guard-mother-cell divisions. The mutant genotype differed from the normal by having a faster rate of ordinary guard-mother-cell divisions as a function of position in the row. The higher level of subsidiary cell formation in the mutant was interpreted as a consequence of a displacement of the used markers, suggesting a precocious initiation of subsidiary-cell formation in eceriferum-g. Time estimations of the length of the cell cycles were obtained by cell-population studies after a pulse with colchicine. Eceriferum-g appeared to have slower cell cycles. The leaves treated with colchicine showed a shift in the elongation axis of the cells. Autoradiography after treatment with (3)H-thymidine showed ineorporation of the label in the portions of the row proximal to all three peaks of divisions indicating that all mitoses were preceded by the usual period of DNA synthesis. Labelling of lateral cells at a mature stage suggested DNA synthesis leading to endoploidy.

"Adaptation" and chromosome variability in excised roots of cereals

Heteroploidy, often coinciding with phenotypic or metabolic changes, was found in cereal root cultures. In some cases the changes were apparent after only 4 passages and affected the roots of bread wheat as well as those containing all or some of the wheat and rye genomes. It is unsafe to assume that cultured organised tissues are cytologically stable. The cause of the abnormality is not known but possible mechanisms are discussed.