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 (R _P) in a leaf has been determined as the difference between the net photosynthetic rates (P _N) measured in 21 % O₂ air (P _N21%) and 3 % O₂ air (P _N3%). In the C₃ plant Vigna radiata and the C₄ plant Amaranthus mongostanus L., P _N and chlorophyll fluorescence quenching in leaves were monitored simultaneously. R _P of leaves in situ was estimated as termed R _PE from the electron transport rates through photosystem 2 (PS2), and compared with R _PO (P _N3% – P _N21%). In V. radiata R _PO was 11.9 µmol(CO₂) m^–2 s^–1 and the ratio of R _PO to P _N21% was 42.2 %, whereas the ratio of R _PE to P _N21% was 25.7 %. This suggests that R _PO may be over-estimated for the real R _P in normal air. In A. mongostanus, P _N 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 R _PE was equivalent to 3.8 % of P _N21%.