Inducing mutations in Citrus spp.: Sensitivity of different sources of plant material to gamma radiation

Gamma radiation represents an alternative for improving genotypes for which breeding through hybridization involves many difficulties because of their complex reproductive biology, as in the case of citrus. In this study in vitro and ex vitro mutagenesis was induced by gamma radiation in several citrus species ('Alemow' and sour orange as citrus rootstocks, lemon cv. 'Fino 49' and 'Verna 51', mandarin cv. 'Nova' and lime cv. 'Bearss'). Three different sources of materials - seeds, budwoods and nodal segments - from in vitro explants, were tested. Seeds and budwoods were germinated or induced to sprout, and de novo regeneration was obtained from in vitro nodal segments from which preexisting buds were previously removed. Budwoods were tested in summer and winter in order to assess their capacity for mutation and further sprouted in different seasons. Seeds were seen to be more resistant to gamma radiation (LD50 of 127 Gy in 'Alernow and 156 Gy in sour orange) than budwoods (LD50 about 50 Gy for all cultivars) and nodal segments (LD50 about 25 Gy for both lemon cultivars), the last being the most sensitive material tested. Similar LD50 were found for budwoods collected in winter and summer for all the genotypes. All the tested plant material can be considered suitable for gamma irradiation, although budwood is the most widely available and tissue culture material needs the highest degree of expertise.