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

Photorespiration and respiration in leaves of various C₃-species under low and high light. Tiit Pärnik^*,, Hiie Ivanova, Olav Keerberg,

ABSTRACT- Components of CO₂ exchange in leaves of C₃ species with different rates of starch synthesis were measured under low and high irradiances, at 120 and 750 mol m^-2 s^-1 PPFD, respectively. In Solanum tuberosum, Helianthus annuus, Plantago lanceolata and Arabidopsis thaliana the rate of starch synthesis was higher than 40% true photosynthesis ("high starch" - HS plants) while in leaves of Triticum aestivum, Secale cereale and Poa trivialis it was less than 10% true photosynthesis ("low starch" - LS plants). The rates of photorespiratory decarboxylations of primary (Rpp) and stored (Rps) photosynthates and of respiratory decarboxylations of primary (Rrp) and stored (Rrs) photosynthates in the light were measured by means of a radiogasometric method. Respiration in the dark (Rd) was measured 40 min after switching off the light. Photorespiration (Rp=Rpp+Rps) was the predominating component of decarboxylation reactions in leaves under high light. At low light photorespiratory (Rp) and respiratory (Rr=Rrp+Rrs) CO₂ fluxes were approximately equal. Rd in HS plants was 1.5-2 times higher than in LS plants. The rate of respiratory decarboxylation of stored photosynthates in the dark (Rd) was significantly higher than in the light (Rrs) indicating that the process was inhibited by light. Under low irradiance the inhibition was less pronounced and was partially compensated by the respiration of primary photosynthates (Rrp) in LS plants. In LS leaves Rr did not depend on light conditions but in HS leaves Rr was at low light two times higher than at high light. No significant differences between LS and HS plants were detected in Rr under high light while under low light Rr of HS plants was 1.4 times higher than in LS plants. Contribution of stored photosynthates to total decarboxylation was 10% at high light and 40% at low light. Due to lower Rd of LS plants the ratio Rs/Rd in LS plants was two times higher than in HS plants. On the basis of the measured rates of internal CO₂ fluxes the specificity of Rubisco in vivo was calculated. No significant differences in mean values of the specifity were found between HS and LS plants. The possible role of products of starch degradation as substrates of photorespiration and respiration will be discussed.

KEY WORDS: respiration, rate of starch synthesis, photorespiration, irradiance