Kenji Hirao

Estimation of Photorespiration Rate by Simultaneous Measurements of CO2, Gas Exchange Rate, and Chlorophyll Fluorescence Quenching in the C3 Plant Vigna Radiata (L.) Wilczek and the C4 Plant Amaranthus Mongostanus L.

So far the photorespiration rate (RP) in a leaf has been determined as the difference between the net photosynthetic rates (PN) measured in 21 % O2 air (PN21%) and 3 % O2 air (PN3%). In the C3 plant Vigna radiata and the C4 plant Amaranthus mongostanus L., PN and chlorophyll fluorescence quenching in leaves were monitored simultaneously. RP of leaves in situ was estimated as termed RPE from the electron transport rates through photosystem 2 (PS2), and compared with RPO (PN3% − PN21%). In V. radiataRPO was 11.9 µmol(CO2) m−2 s−1 and the ratio of RPO to PN21% was 42.2 %, whereas the ratio of RPE to PN21% was 25.7 %. This suggests that RPO may be over-estimated for the real RP in normal air. In A. mongostanus, PN was almost not changed with a decrease in O2 concentration from 21 to 3 %, whereas the quantum yield of PS2 was evidently affected by the change in O2 concentration. This fact shows the presence of photorespiration in this C4 species, where RPE was equivalent to 3.8 % of PN21%.

Concurrent Monitoring of Oxygen Evolution and Chlorophyll Fluorescence in Mungbean Leaves with a Liquid-Phase Oxygen Electrode

In the liquid-phase oxygen electrode, photosynthetic rate is determined by the rate of oxygen evolution from sliced leaf pieces stirred in CO2-saturated reaction solution. The slicing and stirring treatments promote CO2 absorption of leaf pieces, but there is some difficulty in determining fluorescence emittance from the moving objectives. The barrier of CO2 uptake into leaf was removed by peeling its epidermis, and oxygen evolution rates from peeled leaves were readily measured (Yatomi, M. et al. 1992). A liquid-phase oxygen electrode was partly improved to make concurrent measurements of oxygen evolution rate and chlorophyll fluorescence quenching. In this study, we used this tool and observe the responses of gross oxygen evolution rate (Og), the quantum yield of PSII ( e) and photorespiration rate (Pr) to the changes of photosynthetic environment.