Chloroplast Response to Low Leaf Water Potentials: III. Differing Inhibition of Electron Transport and Photophosphorylation.

Cyclic and noncyclic photophosphorylation and electron transport by photosystem 1, photosystem 2, and from water to methyl viologen ("whole chain") were studied in chloroplasts isolated from sunflower (Helianthus annus L. var Russian Mammoth) leaves that had been desiccated to varying degrees. Electron transport showed considerable inhibition at leaf water potentials of -9 bars when the chloroplasts were exposed to an uncoupler in vitro, and it continued to decline in activity as leaf water potentials decreased. Electron transport by photosystem 2 and coupled electron transport by photosystem 1 and the whole chain were unaffected at leaf water potentials of -10 to -11 bars but became progressively inhibited between leaf water potentials of -11 and -17 bars. A low, stable activity remained at leaf water potentials below -17 bars. In contrast, both types of photophosphorylation were unaffected by leaf water potentials of -10 to -11 bars, but then ultimately became zero at leaf water potentials of -17 bars. Although the chloroplasts isolated from the desiccated leaves were coupled at leaf water potentials of -11 to -12 bars, they became progressively uncoupled as leaf water potentials decreased to -17 bars. Abscisic acid and ribonuclease had no effect on chloroplast photophosphorylation. The results are generally consistent with the idea that chloroplast activity begins to decrease at the same leaf water potentials that cause stomatal closure in sunflower leaves and that chloroplast electron transport begins to limit photosynthesis at leaf water potentials below about -11 bars. However, it suggests that, during severe desiccation, the limitation may shift from electron transport to photophosphorylation.