Light-induced carbon dioxide and oxygen uptake in plant leaves measured bythe photoacoustic method

Leaf discs, enclosed in a photoacoustic (PA) chamber, generate two types of PA gas-uptake signals under certain conditions. Type I is manifested by a severe signal decrease that develops slowly under very low-light intensity and often reaches negative values. It is partially reversed by low-intensit y far-red light. Type II occurs transiently in modulated far-red light. It is manifested by a rapid and dramatic decrease of the PA signal, upon the a ddition of short-wave background light, which is subsequently reversed. It differs from type-I uptake in that it occurs at much higher total light int ensities. A thorough study, including modulation frequency and atmospheric composition dependencies, indicates different mechanisms for the two types of uptakes. Type-I uptake results from CO2 accumulation in the PA cell by l eaf respiration and reflects modulations in CO2 solubilization. Type-II upt ake likely reflects oxygen photoreduction in photosystem I, occurring prior to the activation of photosynthesis (i.e. during photosynthesis induction) . This is supported by the complete suppression of type-H uptake when O-2 w as removed. Also, type-Il uptake was only mildly sensitive to CO2 eliminati on, whereas type-I uptake was totally dependent on the presence of CO2 Type -II uptake consists usually of two uptake waves. Fluorescence transients me asured in parallel give further support to the reality and interpretation o f these two uptake waves. PA could thus provide a unique opportunity to mon itor oxygen photoreduction in vivo with high sensitivity and time resolutio n.