Nitrogen Fixation Control under Drought Stress. Localized or Systemic?

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Abstract
Upon the transition of dark-adapted plants to low light, the energy-dependent quenching (qE) of excitation energy is only transiently induced due to the only transient generation of the transthylakoid pH gradient. We investigated the transient qE (qETR) in different Arabidopsis (Arabidopsis thaliana) mutants. In dark-adapted plants, qETR was absent in the npq4 mutant (deficient in the PsbS protein) and the pgr1 mutant (restricted in lumen acidification). In comparison with wild-type plants, qETR was reduced in the zeaxanthin (Zx)-deficient npq1 mutant and increased in the Zx-accumulating npq2 mutant. After preillumination of plants (to allow the synthesis of large amounts of Zx), the formation and relaxation of qETR was accelerated in all plants (except for npq4) in comparison with the respective dark-adapted plants. The extent of qETR, however, was unchanged in npq1 and npq4, decreased in npq2, but increased in wild-type and pgr1 plants. Even in presence of high levels of Zx, qETR in pgr1 mutants was still lower than that in wild-type plants. In the presence of the uncoupler nigericin, qETR was completely abolished in all genotypes. Thus, the transient qETR shows essentially the same characteristics as the steady-state qE; it is strictly dependent on the PsbS protein and a low lumen pH, but the extent of qETR is largely modulated by Zx. These results indicate that qETR does not represent a different quenching mechanism in comparison with the steady-state qE, but simply reflects the response of qE to the dynamics of the lumen pH during light activation of photosynthesis.