Studies on photoacoustic uptake signals in tobacco leaves under high carbon dioxide levels

Abstract

Photoacoustic signals were measured in expanded tobacco leaves, exposed to a controlled atmosphere by being only partly enclosed within the photoacoustic cell. It was aimed to corroborate the conjecture of Reising and Schreiber (Photosynthesis Research 42: 65-73, 1994) that under exceptionally high CO2 levels (ca. 1–5%) the photobaric uptake contribution reflects CO2 uptake induced by light dependent stromal alkalinization. This is shown here by: (1) the shallower damping of the uptake signal vs. the modulation frequency, compared to a normal oxygen evolution signal; (2) the partial inhibition of the uptake signal under 5% CO2 by nigericin; (3) the complete absence of uptake signals under 5% CO2 in a carbonic-anhydrase-deficient mutant, which gave rather a normal oxygen evolution signal. The photoacoustic signals from the wild type and the transgenic tobacco in air could not be distinguished, indicating that the CO2 uptake signal is negligible under this condition. Uptake photobaric signals were also measured in modulated far-red light (ca. 715–750 nm), following addition of white background light (in light limiting intensity). In normal tobacco under 5% CO2, the background light induced an uptake transient, lasting about a minute, then declining to a low steady level. Significantly smaller transients were obtained under normal air, and in the carbonic-anhydrase deficient mutant also under 5% CO2. Extrapolation to zero frequency of the signal damping vs. modulation frequency, in both tobacco genotypes, suggests however similar magnitudes of the uptake transients. On the other hand, no proportional steady-state uptake was observed for the last two cases. Presumably, the steady uptake under 5% CO2 in modulated far-red light reflects CO2 solubilization, while it is an open question whether the transient could be partly contributed also by oxygen photoreduction by PS I (Mehler reaction). It is reasoned that, under conditions of low light, the respiratory activity results in accumulation of CO2 in the photoacoustic cell, which is sufficient to induce an uptake phenomenon, giving a more satisfactory interpretation for the so-called 'low light state' [Cananni and Malkin (1984) Biochim Biophys Acta 766: 525–532].