Preferential damaging effects of limited magnesium bioavailability on photosystem I in Sulla carnosa plants

Main conclusion Magnesium deficiency preferentially inhibits photosystem I rather than photosystem II in Sulla carnosa plants. The effects of magnesium (Mg2+) deficiency on growth, photosynthetic performance, pigment and polypeptide composition of chloroplast membranes were studied in the halophyte Sulla carnosa (Desf.), an annual legume endemic to Tunisia and Algeria. The results demonstrate a gradual decrease in biomass production with decreasing Mg2+ availability in the growth medium. The increase of Mg2+ deficiency was also associated with a decline of the net CO2 assimilation (Pn) in fully expanded leaves, a decrease in the amount of photosynthetic pigments, and an increase in the lipid peroxidation in plants exposed to decreased Mg2+ concentrations. Interestingly, while CO2 assimilation already was affected at Mg2+ concentrations below 1.5 mM, the photochemical efficiency of photosystem II (PSII) declined only in the absence of Mg2+. In contrast, plants of S. carnosa grown in Mg2+-deficient conditions exhibited a significant decrease in photosystem I (PSI) photochemistry in vivo at much higher Mg2+ levels compared to PSII photochemical activity. The inhibitory effect of Mg2+ deficiency on PSI photochemistry strongly correlated with significantly lower relative abundance of PSI-related chlorophyll–protein complexes and lower amounts of PSI-associated polypeptides, PsaA, PsaB, and Lhca proteins within the same range of Mg2+ concentrations. These observations were associated with a higher intersystem electron pool size, restricted linear electron transport and a lower rate of reduction of P700+ in the dark indicating restricted capacity for PSI cyclic electron transfer in plants exposed to Mg2+-deficient conditions compared to controls. These results clearly indicate that PSI, rather than PSII is preferentially targeted and damaged under Mg2+-deficiency conditions.