Comparison of two in vitro radiation-induced mutagenesis protocols for the Citrus rootstock Swingle citrumelo

In order to establish an efficient in vitro radiation-induced mutagenesis scheme for the rootstock Swingle citrumelo, two protocols were compared: Protocol 1 (seeds were irradiated and axillary shoots from the regenerated plantlets were propagated) and Protocol 2 (epicotyl segments were irradiated and the induced adventitious shoots were propagated). Radiosensitivity against [.sup.60]Co gamma rays, shoot induction and three root induction treatments (MS, MS + NAA and MS + IB A) were evaluated. The efficiency regarding the total number of plants and the time required until their acclimatization was compared. The mutated population size was estimated. Mutagenic doses (L[D.sub.30]) of 29 and 37 Gy were determined for seeds and epicotyl segments, respectively. Shoot induction was higher than 93 % in both protocols. Root induction was significantly higher for adventitious shoots on MS + NAA, due to a higher secondary root induction (92 %), which also enhanced their adaptation to substrate (91,8 vs. 70,4 %). Starting from 1000 seeds, 50 and 70 mutants were estimated from Protocols 1 and 2, respectively, considering the proposed L[D.sub.30] and 0,5 % frequency of induced mutations. It was demonstrated that Protocol 2: radiation-induced mutagenesis based on adventitious organogenesis was more efficient than Protocol 1, based on seed irradiation and axillary shoots propagation; since half the time (seven months) is required to obtain a higher number of mutants. It was mainly due to a high root induction potential and a better root architecture, which allowed for a higher plant ex vitro survival. Key words: induced mutation, radiosensitivity, micropropagation, Citrus