ENERGY COUPLING, MEMBRANE-LIPIDS AND STRUCTURE OF THYLAKOIDS OF LUPIN PLANTS SUBMITTED TO WATER-STRESS

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 DELTA-mu(H+). The fluorescent probe 9-aminoacridine was used to express, as a DELTA-pH, the whole DELTA-mu(H+) by calibrating fluorescence quenching against the phosphate potential DELTA-Gp 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-DELTA-pH), ATP synthesis was halved by a medium stress and disappeared for a more severe stress, whereas DELTA-pH at equal energy input (light) declined only under a severe drought. For an identical DELTA-pH, 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.