Article révisé par les pairs
Résumé : A slow water stress over several days was imposed on tobacco plants (Nicotiana tabacum L. var. Xanthi) by withholding water from the soil. Photosynthesis was measured in leaves from those water‐stressed plants by the photoacoustic method. Slow drought induced marked changes in the photoacoustic signals, which were largely similar to those observed previously in leaves subjected to rapid desiccation in air (over 3–4 h), reflecting two simultaneous changes: 1) Modification of the heat and oxygen diffusion characteristics of the leaves due to changes in their anatomical structure [shown by the change in the slope of the plot of the oxygen (AOX) to photothermal signal (APT) ratio vs the square root of the modulation frequency]; 2) Inhibition of gross photosynthesis measured by the extrapolation of the AOX/APT ratio to zero frequency. However, in contrast to rapid water stress in detached leaves, where it was shown that mainly the oxidizing side of photosystem II (PS II) was damaged, we found a slower and more complex phenomenology having largely biphasic kinetics. During the first 6 days, there was a strong reduction in the photochemical energy storage, but the inhibition of oxygen evolution was relatively mild. The Emerson enhancement in state 1 dropped considerably, indicating lowering of the apparent absorption cross‐section of PS II. Fluorescence measurements suggest that PS II reaction center itseIf may be the primary site of the damage. PS I activity, judged by cytochrome f photooxidation, remained largely intact. The subsequent days were associated with a further spectacular decrease in the oxygen evolution quantum yield with both photosystems damaged. The photochemical energy storage continued to decrease further. The Emerson enhancement ratio of the remaining activities in both State 1 and 2 showed a marked increase, indicating the reestablishment of a strong imbalance in the distribution of excitation energy within the photochemical apparatus in favor of PS II. All the photoacoustic changes observed in response to drought were completely reversible within 2–3 days upon rewatering of the soil. Copyright © 1987, Wiley Blackwell. All rights reserved