Photorespiration Rate in Mungbean (Vigna radiata (L.) Wilczek) Leaves - A further estimation from the quantum yield at photosystem II

Photorespiration rates (PR) in peeled (PL, without epidermis) and nonpeeled (NL) leaves of mungbean (Vigna radiata (L.) Wilczek) were estimated from electron transport situations at photosystem II using the chlorophyll fluorescence quenching diagnosis. The number (k₁) of electron equivalents required to produce 9 mol adenosine 5′-triphosphate (ATP) for fixing 1 mol CO₂ was experimentally determined 4.62. Also the number (k₂) of electrons required for releasing 1 mol CO₂ in photorespiration was estimated 7.44 or 9.49 on the assumption that 14.5 or 18.5 mol ATP was consumed for 1 mol CO₂ releasing, respectively. The simultaneous equations were constructed with the parameters of k₁, k₂ and related elements to calculate PR, and the mutual relationships in energy balance among PR, gross photosynthetic rate (Pg), total photosynthetic rate (TC, TC = PR+Pg) and PR ratio (PR/TC) were discussed. The value of PR, 1.75 or 2.05 μmol m^-2 s^-1, in PL was estimated corresponding to k₂ = 7.44 or 9.49 in a light intensity of 300 μmol m^-2 s^-1 PPFD, and PR/TC was 18.2 or 20.6%, respectively. The estimation by chlorophyll fluorescence quenching diagnosis may give a more reasonable value of PR as compared with the PR obtained by the subtraction method (PR = Pg measured in 2% [O₂] air -Pg measured in 2% [O₂] air).