Effect of Hydroxylamine Derivatives on Photorespiration in the Tobacco Aurea Mutant Nicotiana tabacum Su/su

Abstract

O-(p-nitrophenyl)-hydroxylamine affects the size of the Warburg effect measured in leaves of the tobacco aurea mutant Su/su. If leaves are treated with a 10-5 м aqueous solution of the hydroxylamine derivative the Warburg effect which is usually very high in this mutant, is practically reduced to zero. Thus, if in the control (the untreated leaf) the Warburg effect consists in an enhancement of CO2-fixation from 270 μmol CO2 fixed mg·chlorophyll-1·h-1 under 21% oxygen partial pressure to 530 μmol CO2 fixed · mg Chl-1 · h-1 under 3% oxygen, the treated sample yields the maximal rate of photosynthesis already under 21% oxygen partial pressure. At concentrations around 10-4 м the compound is toxic to the plants and inhibits CO2 fixation substantially. A second hydroxylamine derivative used in this study has an additional methyl group in the ortho position to the -O-NH2 group. This compound is already toxic to the plant at concentrations around 10-5 м at which O-(p-nitrophenyl)-hydroxylamine optimally enhances CO2 fixation at normal oxygen partial pressure. From our studies it appears that O-(p-nitrophenyl)-hy- droxylamine binds to ribulose-1.5-bisphosphate carboxylase inducing a conformational change of the enzyme. This is concluded from the observation that a monospecific antiserum to ribulose-1.5- bisphosphate carboxylase is exhausted at considerably higher enzyme concentrations when the enzyme has been treated with the hydroxylamine derivative prior to the antiserum addition. The observation is interpreted as being due to a conformational change induced by the compound, leaving less antigenic determinants accessible in the enzyme surface, when compared to the untreated control. The inhibition of photosynthetic CO2 fixation at high concentrations of both hydroxylamine derivative studied is due to the inhibition of photosynthetic electron transport on the donor side of photosystem II. a phenomenon which is known already for unsubstituted hydroxylamine from the literature. The inhibition of electron transport on the donor side is deduced from an analysis of fluorescence rise kinetics at room temperature.