DEVELOPMENT OF PHOTOAUTOTROPHY AND PHOTOINHIBITION OF GARDENIA JASMINOIDES PLANTLETS DURING MICROPROPAGATION

This paper reports on the fast fluorescence responses of Gardenia jasm inoides Ellis plantlets, at two successive stages (shoot multiplicatio n and root induction) of culture in vitro. We test whether plantlets i n vitro suffer photoinhibition during culture and whether the degree o f photoautotrophy of these mixotrophic plantlets has any effect on the extent of photoinhibitory impairment. In this regard the effects of d ifferent sucrose levels in the medium and PPFD during growth on the de velopment of photoautotrophy and the extent of photoinhibition were ev aluated. Plantlets were grown under low, intermediate, and high (50, 1 00, and 300 mu mol m(-2) s(-1)) PPFD, and at 3 different sucrose conce ntrations (0.5, 1.5, and 3.0%, w/v) in the medium, during shoot multip lication. During root induction the same growth conditions were assaye d except for the high PPFD. The development of photoautotrophy was ass essed via the difference between the stable carbon isotope composition of sucrose used as heterotrophic carbon source and that of leaflets g rown in vitro. Plantlets from root induction showed more developed pho toautotrophy than those from shoot multiplication. For both stages the low-sucrose medium stimulated the photoautotrophy of plantlets irt vi tro. In addition, intermediate PPFD induced photoautotrophy during sho ot multiplication. For plantlets of both culture stages at the lowest PPFD no photoinhibition occurred irrespective of the sucrose concentra tion in media. However, during the shoot multiplication stage chloroph yll fluorescence measurements showed a decrease in F-v/F-m and in t(1/ 2) as growing PPFD increased, indicating photoinhibitory damage. The d ecline of F-v/F-m was caused mostly by an increase in F-o, indicating the inactivation of PSII reaction centers. However plantlets growing u nder low sucrose showed reduced susceptibility to photoinhibition. Dur ing root induction, only plantlets cultured with high sucrose showed a decrease in F-v/F-m as PPFD increased, although t(1/2) remained uncha nged. In this case, the decline of F-v/F-m was mostly due to a decreas e in F-m, which indicates increased photoprotection rather than occurr ence of photodamage. Therefore, growth in low-sucrose media had a prot ective effect on the resistance of PSII to light stress. In addition, plantlets were more resistant to photoinhibition during root induction than during shoot multiplication. Results suggest that increased phot oautotrophy of plantlets reduces susceptibility to photoinhibition dur ing gardenia culture in vitro.