Minimum magnesium concentrations for photosynthetic efficiency in wheat and sunflower seedlings

Photosynthetic processes in the chloroplast depend on the abundance of magnesium (Mg) in relatively high amounts; hence chloroplasts might react more sensitive to Mg-deficiency than other physiological processes within other organelles. Most authors suggest a critical Mg concentration to be 1.5 mg g^-1 DM for biomass and yield formation. However, it is not yet elucidated whether this value also applies to photosynthetic processes. The present study focused on the response of photosynthetic processes to different Mg tissue concentrations. Wheat (Triticum aestivum) and sunflower (Helianthus annuus) plants were grown hydroponically for 10 days with 8 different levels of Mg supply (1.0, 0.5, 0.25, 0.1, 0.075, 0.05, 0.025, 0.01 mM Mg). Specific leaf mass, SPAD values, assimilation rate, F_v/F_m, electron transport rate and photochemical and non-photochemical quenching parameters were determined on youngest mature leaves. Tissue Mg concentrations decreased with lowering Mg supply to lowest concentrations of 0.7 mg g^-1 DM in wheat leaves, but photosynthetic capacity was not affected. In sunflower leaves, lowest Mg concentrations of 0.56 mg g^-1 DM were achieved and a diminished photosynthetic capacity was observed. The study shows that a Mg tissue concentration of 1.5 mg g^-1 DM did not induce a negative effect on the photosynthetic capacity of wheat and sunflower leaves under our experimental conditions and hence, the critical Mg concentration for photosynthetic processes might be lower than for biomass and yield formation.