STUDIES ON THE CYTOLOGY AND EFFECT OF GAMMA RADIATION IN WINGED BEAN ; PSOPHOCARPUS TETRAGONOLOBUS (L.) D.C

Abstract

The winged bean Psophocarpus tetragonolobus (L) D.C. is a promising, high-protein crop recommended for cultivation in developing countries. In this study, some of the effects of gamma irradiation encountered in mutation breeding of this crop were determined.

It was found that oxygen and seed moisture content or irradiated seeds were very important components in mutation breeding of winged bean. The optimum dosage required for gamma irradiation of winged beans is about 20 krad and it increased by 25% when the seeds had 13% of moisture and above, and hydrated anaerobically after irradiation (25 krad).

Karyotype analysis of twelve varieties of winged beans collected from seven geographic regions showed variations in chromosome length and position of centromere as seen in certain varieties like SLS 40, SLS 100 and UPS 102. Based on the variation of chromosome morphology, these twelve varieties were grouped into three types.

All the twelve varieties of winged beans studied had a somatic chromosome number of 2n=18 except for the variety UPS 102, where it was 2n=18 or 20. This variety had an extra pair of very small chromosomes.

The duration of a cell cycle in the root cells of winged bean was found to be 8 ¤ 2 hours at 25 ¤ 1?C. Ionizing radiations at doses above 20 krad appeared to cause damaging effects on the cells. The radiosensitivity of the cells was proportional to doses as well as the atmospheric oxygen during hydration. An increase in bridges formed per cell at anaphase was seen as the dose increased and when the irradiated seeds were hydrated aerobically. These factors accounted for the slow mitotic divisions leading to poor germination and slow growth of seedlings.

Different segments of cotyledons, epicotyls and stems of controls and M1 seedlings (10-25 krad) were grown on MS medium supplemented with, indoleacetic acid (IAA) and benzyladenine (BA). The tissures from M? seedlings showed less callus formation, green pigment development and organogenesis when compared with the controls.

Explants from irradiated seedlings (with 1 krad) produced only callus on MS basal medium supplemented with both the hormones. The callus tissue dried up after about one month in culture and even after subculturing. The tissues treated with higher dosage i.e. 1.5 and 2 krad did not even form the callus.

Electrophoretic studies indicated seven major anodal peroxidase bands, five major cathodal bands and six esterase bands in all the nine varieties studied. The combination of the major bands of these enzyme systems along the minor bands can be used for identifying the varieties of Psophocarpus tetragonolobus. In the M1 plants (leaves), an increased number of bands was observed especially when the gamma dose range was increased from 15 to 25 krad. At 10 krad irradiation, the M1 generation showed no change in the isozyme patterns. But in the M2 generation, the bands differed from the M1. That is the new bands of M1 plants only appeared at random in M2 which suggested the possibility of segregation.

Irradiation of young bods with gamma rays of 50 krad to 400 krad did not increase the shelf-life of these pods.