Effects of potassium supply on growth, gas exchange, phenolic composition, and related antioxidant properties in the forage legume Sulla carnosa

The effects of potassium (K+) deficiency were investigated in Sulla carnosa plants. Plants were grown hydroponically for one month in K+-sufficient (6 mM K+, Control) and K+-deficient (60 μm K+) solutions inside the greenhouse in Biotechnology Center of Borj Cedria, Tunisia. Growth, water status, pigment contents, photosynthetic gas exchange, photosystem II (PSII) photochemistry and leaf principal secondary metabolites (polyphenols, flavonoids, and condensed tannins), and their antioxidant properties (DPPH (1.1-diphenyl-2-picrylhydrazyl) scavenging capacity, ferric reducing power, chelating effect on ferrous ions, and β-carotene bleaching test) were determined. Growth of vegetative organs was decreased by some 50% by K+ deficiency with stems more affected (−68%) than roots (−42%) and leaves (−45%). Water content decreased in the three vegetative organs. Photosynthetic gas exchange and pigment contents were affected by low K+ conditions. In contrast to condensed tannins which remained constant, total polyphenols and flavonoids contents increased under K+ deficiency (by 62.7 and 14.5%, respectively). Furthermore, total antioxidant activity increased by 33.5% compared to control plants. Except for β-carotene bleaching test that increased, DPPH scavenging capacity, ferric reducing antioxidant power, and chelating effect on ferrous ions decreased owing to K+ deficiency. An increased and/or de novo synthesis of individual polyphenols was also observed by RP-HPLC analysis. As a whole, these data suggest that S. carnosa was able to modulate the metabolism of secondary metabolites and their antioxidant activity under conditions favouring reactive oxygen species (ROS) production in order to minimize the deleterious effects of these oxygen species.