OXYGEN-SENSITIVE DIFFERENCES IN THE RELATIONSHIP BETWEEN PHOTOSYNTHETIC ELECTRON-TRANSPORT AND CO2 ASSIMILATION IN C-3 AND C-4 PLANTS DURING STATE TRANSITIONS

Wheat (C-3) and maize (C-4) leaves were exposed to light treatments th at were limiting for CO2 assimilation and which excite preferentially photosystem I (PSI) or photosystem II (PSII) and induce State 1 or Sta te 2, respectively. In order to examine the relationships between line ar electron transport and CO2 in leaves during State transitions, simu ltaneous measurements of CO2 assimilation, chlorophyll fluorescence ri nd absorbance at 820 nm were used to estimate the quantum efficiencies of CO2 assimilation and PSII and PSI photochemistry. In wheat leaves with photorespiratory activity, no significant change in quantum effic iency of CO2 assimilation was observed during State transitions. This was not the case when photorespiration was inhibited with either 2% O- 2 or 1000 ppm CO2 and transition from State 2 to State 1 was accompani ed by a large decrease (c. 20%) in the quantum efficiency of CO2 assim ilation which was not associated with a decrease in the quantum effici ency of electron transport through PSII. Photorespiration appears to b uffer the quantum efficiency of CO2 assimilation from changes associat ed with decreases in the rate of CO2 fixation resulting from imbalance s in PPFD absorption by PSI and PSII. When maize leaves were subjected to similar State transitions, no significant change in the quantum ef ficiency of CO2 assimilation was observed on transition from State 2 t o State 1, but on switching back to State 2 a very large decrease (c. 40%) was observed. This decrease could be prevented if leaves were mai ntained in either 2% O-2 or 593 ppm CO2. The possible occurrence of ph otorespiration in maize leaves on transition from State 1 to State 2, which could result from an inhibition of the CO2 concentrating mechani sm, cannot account for the decrease in the quantum efficiency of CO2 a ssimilation since the relationship between PSII electron transport and CO2 assimilation remained similar throughout the State transitions. A lso changes in the phosphorylation status of the light-harvesting chlo rophyll a/b protein associated with PSII cannot be implicated in this phenomenon.