Irradiation-induced variations in M2 populations of Eksotika papaya

Y.K. Chan, H.K. Lee and I. Rusna

Abstract

Mutation breeding of papaya by gamma irradiation was started at MARDI in August, 2000 as a contract research project with the International Atomic Energy Agency (IAEA). Irradiation of seed of ‘Eksotika’ variety was carried out at MINT using 525 Gy for dry seed and 42.5 Gy for pre-soaked seed previously found to be the most suitable dosages for mass irradiation. A total of 200 M1 plants from each dosage treatment were subsequently raised. M2 seed were obtained by self-pollinating M1 hermaphrodite trees and a population of 1,000 M2 plants was established for studies on variation of 13 traits. Another 1,920 M2 seedlings were raised, inoculated and screened for resistance to papaya ringspot virus disease. In the M2 populations, wide variability was recorded for many traits. At  the seedling stage, low irradiation of 42.5 Gy on pre-soaked seed produced a high number of M2 progenies that were shorter and more vigorous in leaf development than those irradiated at 525 Gy and the control seedlings. The distribution patterns of M2 progenies for nine quantitative traits showed great variation with ranges often exceeding the limits of the control population. There appears to be good prospects in improving Eksotika papaya using irradiation, especially in selecting for more dwarf trees, lower fruit bearing stature, higher total soluble solids in fruit and larger fruit. Several M2 mutants also showed very good resistance to malformed top disease caused by Cladosporium. However, no resistance to papaya ringspot virus disease was found in the 1,920 M2 seedlings that were inoculated and screened.

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