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Bioenergetic properties of thylakoids from plants submitted to a water stress stress (watering stopped for 6-15 days) have been measured in two lupin genotypes characterized as resistant or susceptible to drought. This energy coupling was assessed by flow-force relationships relating the phosphorylation rate to the magnitude of the proton gradient % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakabbaaa6daaahjxzL5gapeqa% aiabgs5aenaaxacabaGaeqiVd0galeqabaGaaiOFaaaakmaaBaaale% aacaWGibWaaWbaaWqabeaacqGHRaWkaaaaleqaaaaa!4D55!\[\Delta \mathop \mu \limits^\~ _{H^ + } \]. The fluorescent probe 9-aminoacridine was used to express, as a ΔpH, the whole % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqef0uAJj3BZ9Mz0bYu% H52CGmvzYLMzaerbd9wDYLwzYbItLDharqqr1ngBPrgifHhDYfgasa% acOqpw0xe9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% GabaqaaiGacaGaamqadaabaeaafiaakabbaaa6daaahjxzL5gapeqa% aiabgs5aenaaxacabaGaeqiVd0galeqabaGaaiOFaaaakmaaBaaale% aacaWGibWaaWbaaWqabeaacqGHRaWkaaaaleqaaaaa!4D55!\[\Delta \mathop \mu \limits^\~ _{H^ + } \] by calibrating fluorescence quenching against the phosphate potential ΔGp in 'state 4', i.e., when ATP synthesis is strictly balanced by its hydrolysis. This calibration procedure was shown to be unaffected by treatments. At equal energization (iso-ΔpH), ATP synthesis was halved by a medium stress and disappeared for a more severe stress, whereas ΔpH at equal energy input (light) declined only under a severe drought. For an identical ΔpH, PS 1-driven phosphorylation is always more efficient than PS 2, both in control and stressed plants. Thus, uncoupling is not the cause of the phosphorylation decline; moreover, retention of a 'micro-chemiosmotic' type of coupling implies that the distribution of photosystems and ATPases is unchanged. Parallel to these functional alterations, the lipid content of thylakoids dramatically dropped. As galactolipids fell strongly, neutral lipids rose slightly. Fatty acids decreased then increased with stress, yet phosphorylation did not recover in the latter case and membrane permeability to protons remained unaffected. Overall, these observations suggest a preserved thylakoid structure and this was indeed observed on electron micrographs, even for a severe stress. Therefore, the membrane integrity is probably preserved more by the protein network than by the lipid matrix and the loss of the phosphorylating activity mainly reflects a loss of ATPases or at least their inactivation, possibly due to their altered lipid environment. Finally, from the bioenergetic point of view, the susceptible genotype was unexpectedly less affected by drought than the resistant.

In inside-out vesicles, a large difference is observed in oxygen yield activity between pH 6.5 and 7.6 which is in contrast to right-side-out vesicles. The light saturation behaviour of the transition Sz + S1 also shows a pH dependence. In inside-out thylakoids, at pH 6.5, the light saturation curve of the S2 + SJ transitions exhibits 2 different light saturation processes whereas at pH 7.6 it resembles the light saturation behaviour of the other S-state transitions. This result is explained by the existence of a second donor in the S2 state (diffe~nt from the main donor), which cont~but~ to about 30% of the Sz + S3 transitions at pH 6.5. This second donor needs to be protonated and thus, is not active at pH 7.6 in inside-out thylakoids.pH effect Oxygen evolution Photosystem II Inside-out thylakoid Secondary donor

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