Abstract
Gamma irradiation mutagenesis is a technique commonly used to induce seedlessness in citrus fruits. We compared fruit quality traits of eight different seeded mandarin varieties within the Israeli citrus breeding collection with those of their corresponding gamma-irradiated low-seeded mutants. The mandarin varieties compared were: “Rishon” with “Kedem”; “Michal” with irradiated “Michal”; “Merav” with “Meravit”; “Vered” with “Vardit”; “Ora” with “Or”; “Murcott” with “Mor”; “Shani” with irradiated “Shani”; “King” with irradiated “King”. Mutational breeding by gamma irradiation reduced average fruit seed number by 70% - 92%. Furthermore, mutational breeding by gamma irradiation delayed ripening of the early-season mandarin varieties “Rishon” and “Michal” by 8 - 13 days, but did not delay ripening of most mid- and late-season varieties. Gamma-irradia- tion mutagenesis also reduced fruit weight of seven of the eight tested low-seeded varieties by 6 - 41 g per fruit. Gamma-irradiation mutagenesis had variable effects on biochemical composition and nutritional quality, in that some irradiated clones presented no changes whereas other had either higher or lower levels of juice total soluble solids, acidity, vitamin C and total antioxidant activity. Finally, sensory evaluations by a trained panel revealed that the flavor of most of the low-seeded, gamma-irradiated varieties was slightly or significantly preferable to that of unirradiated varieties.
Highlights
During the last decade, there has been a continuous decrease in consumption and global marketing of difficultto-peel citrus fruits, such as oranges and grapefruit, and this decrease has been accompanied by increased consumption of and demand for fresh, easy-to-peel mandarins [1]
Gamma irradiation at the doses used to induce seedlessness is a rather drastic procedure, which often results in induction of numerous recessive chromosomal aberrations and, it is very likely that it impacts the expression of many genes and thereby, in some cases, impairs fruit quality [3] [4]
Mutational breeding by gamma irradiation delayed ripening of the early-season mandarin varieties “Rishon” and “Michal”, harvested in October and November, by 8 and 13 days, respectively, but did not delay ripening of most mid- and late-season varieties (Table 1)
Summary
There has been a continuous decrease in consumption and global marketing of difficultto-peel citrus fruits, such as oranges and grapefruit, and this decrease has been accompanied by increased consumption of and demand for fresh, easy-to-peel mandarins [1]. Gamma-irradiation mutagenesis of bud wood has been the method most commonly used by citrus breeders worldwide to obtain seedless clones from commercial seeded varieties [3] [4]. No matter what method was used to create seedlessness, e.g. gamma irradiation or triploid breeding, it is known that seeds form an important source for production of certain plant growth regulators such as gibberellins, and, induction of seedlessness may indirectly interfere with the normal hormonal balance within the fruit which may, in turn, affect overall fruit quality [11]. It was reported that gibberellic acid produced by seeds of “Valencia” oranges was responsible for the regreening phenotype [12]. It was reported that seeded avocado fruit has a delayed-ripening phenotype as compared with seedless fruit—a phenomenon related to the effect of the avocado seed on the ethylene biosynthesis and response pathways [14]
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