CULTURE ON SUGAR MEDIUM ENHANCES PHOTOSYNTHETIC CAPACITY AND HIGH LIGHT RESISTANCE OF PLANTLETS GROWN IN-VITRO

The significance of photosynthetic photon flux (PPF) and sugar feeding for the production of plants in vitro is only poorly understood. Nico tiana tabacum L. plantlets were grown photoautotrophically and photomi xotrophically (3% sucrose) at two different PPFs (60 mu mol m(-2) s(-1 ) and 200 mu mol m(-2) s(-1)) to investigate the effect of these cultu re parameters on photosynthetic performance and growth. Photomixotroph ically-grown plantlets showed an increase in carbohydrate content, mai nly in glucose and Fructose. Plant growth, dry matter accumulation and total leaf area were higher under photomixotrophic than photoautotrop hic conditions. Not only biomass formation but also photosynthesis was positively affected by exogenous sucrose; the chlorophyll (Chi) conte nt and the light-saturated rate of photosynthetic oxygen evolution wer e higher in photomixotrophic plantlets. Photoinhibition occurred in pl antlets that were grown photoautotrophically at the higher PPF. It bec ame apparent as a. loss in Chi content and photochemical efficiency. P hotoinhibited plantlets showed a decrease in the D2/LHCII and CP47/LHC II ratios, suggesting a preferential loss of proteins from the photosy stem II (PSII) core. The increased content of xanthophyll cycle pigmen ts in photoinhibited plantlets indicated that also protective mechanis ms were activated. Photomixotrophic growth of the plantlets prevented the occurrence of photoinhibitory symptoms. Therefore, we conclude tha t culture on sugar medium increases not only the photosynthetic potent ial but also the high light resistance of plantlets grown in vitro.