PARENT SESSION
Posters P6C Photosynthesis, respiration and alternative electron sinks. Abstracts (660-680)

Degradation and conversion of starch is blocked in photosynthesizing leaves of Arabidopsis thaliana. Olav Keerberg^*,, Hiie Ivanova, Hille Keerberg, Tiit Pärnik,

ABSTRACT- Leaves of Arabidopsis thaliana were illuminated 75 min in the medium of ¹⁴CO₂ and transferred into nonradioactive medium where conversion of labeled starch and soluble photosynthates was followed in the dark and in the light at different concentrations of ¹²CO₂. No loss of labeled carbon in starch was detected during 3h illumination of leaves in the light at 360 L L^{-1 12}CO₂ while 40% of labeled starch was metabolized in the dark. The amount of prefixed carbon in soluble photosynthates decreased 28% in the light and 13% in the dark. 85 to 90% of the decrease in soluble fraction resulted from the changes in sucrose which radioactivity decreased 65% in the light and 30% in the dark. Label in glucose and fructose remained unchanged or increased in the light but decreased in the dark. When leaves were illuminated in CO₂-free air the changes in starch and soluble photosynthates were similar to those followed in the dark: the radioactivity of starch decreased by 50% while label in soluble photosynthates remained practically unchanged. The results suggest that degradation and conversion of accumulated in chloroplasts starch were blocked in photosynthesizing leaves in the light. The only substrates of respiratory and transport processes in the light were primary photosynthates and stored soluble photosynthates, predominantly sucrose. Suppression of starch degradation was found to be induced at the irradiances below 60 mol m^-2 s^-1 PPFD. In a mutant of A. thaliana deficient in chloroplast phosphoglucomutase a slight increase of starch radioactivity in nonradioactive medium was detected in the light as well in the dark, the result pointing to the possibility that in this mutant stored soluble photosynthates can be used for the synthesis of starch. It is concluded that suppression of starch degragation in the light is not brought about by direct effects of light on the activity or expression of starch-degrading enzymes but is mediated by the reactions of photosynthetic carbon metabolism.

KEY WORDS: starch degradation, photosynthetic carbon metabolism, arabidopsis, suppression by light