PhD

An experiment was conducted at the Department of Agricultural
Botany, College of Agriculture, Vellayani during 1987-1989 for genetic
manipulations in sweet potato through gamma ray induced mutagenesis for
increased variability and to isolate out genotypes having wider adaptability
and better performance.
Stem cuttings of 8 to 10 cm length bearing two nodes each, taken from
fifteen sweet potato varieties were used for radiosensitivity analysis. Gamma
irradiation was done by a 60 Co gamma cell unit installed in the Radio Tracer
Laboratory of Kerala Agricultural University, Trichur. The material was
subjected to exposure of 2-10 kR at intervals of 2 kR. The chosen dose rate
was 0.162 MR/h.
The direct effect of doses on the material was assessed on the basis of
days to start sprouting, days to complete sprouting, sprouting percentage,
vine length, branch and tuber number and weight of tubers per vine. The
exposures above 4 kR caused lethality in the majority of the varieties and
hence comparative analysis for ratiosensitivity was assessed at the 2 kR level.
The gamma ray exposed population started sprouting late. The days
taken for completion of sprouting were also more in all the varieties. Gamma
rays, in addition, reduced the sprouting percentage. The percentage lethality
varied depending on variety. The vine length and number of branches per
vine also varied from variety to variety. They were found to be comparatively
less in treated population. The tuber number and weight of tubers per vine
were found to be significantly increased by gamma irradiation at 2 kR. Based
on the above observations the fifteen varieties were classified into three, viz.
low, medium and high radiation sensitive categories.
Induced
mutagenesis
was
done
in
continuation
with
the
radiosensitivity analysis using three varieties, each selected from the low,
medium and high radiation tolerant groups. The planting materials selected
for gamma irradiation included fresh cuttings, rooted cuttings and rooted
tubers which were exposed to radiation at a range of 500 – 2500 r, at 500 r
intervals. The dose rate was 0.162 MR/h. The irradiated materials along with
the control were planted on the subsequent day.
In vM1 generation the direct effect of gamma rays was assessed based
on days taken to start sprouting, days taken to complete sprouting, sprouting
percentage, lethality on the 30th day of planting and at harvest, vine length,
branch number per vine, fresh weight of vine, tuber number per vine, weight,
length, girth and volume of tuber and tuber yield per vine. From vM1 plants
3-4 noded cuttings were taken from the basal, middle and top portions for
raising vM2 generation. VM3 and vM4 generations were also raised in the
same manner. In vM2, vM3 and vM4 generations the yield parameters were
analysed in detail.
Classification of the phenotypes and frequency analysis
were also done. The salient findings of the experiment are the following:
There was a delay in sprount initiation and for completion of
sprouting caused by gamma ray exposure. A decrease in sprouting percentage
and an increase in lethality were noticed under higher levels of exposures.
Similarly a reduction in vine length and branch number per vine were found
at higher exposures. The fresh weight of vine was reduced and the tuber
number increased at higher exposures. There was an increase in mean tuber
weight, length, girth, volume and tuber yield per vine at higher exposures. All
the exposures and the different modes of treatment induced phenotypic
variants both in negative and positive directions. Positive variants were in
higher frequency in later generations. Irradiation of rooted cuttings was
found to be more economical or beneficial compared to fresh cuttings and
rooted tubers.
The study enabled to isolate out two promising types, one each from
S5 and Bhadrakalichuvala. These mutants outyielded the control and are
being multiplied by vine cuttings for farm trials in different agroecological
milieus of the State.