Magnesium deficiency decreases biomass water-use efficiency and increases leaf water-use efficiency and oxidative stress in barley plants

AIMS: In water-scarce agro-environments a clear understanding of how plant nutrients like magnesium (Mg) affect plant traits related to water-use efficiency (WUE) is of great importance. Magnesium plays a crucial role in photosynthesis and is thus a major determinant of biomass formation. This study investigated the effect of Mg deficiency on leaf and whole plant water-use efficiency, δ¹³C composition, hydrogen peroxide (H₂O₂) production and the activity of key enzymes involved in ROS scavenging in barley. METHODS: Barley (Hordeum vulgare) was grown in hydroponic culture under three different levels of Mg supply (0.01, 0.1, 0.4 mM Mg). WUE was determined on the leaf-level (leaf-WUE), the biomass-level (biomass-WUE) and via carbon isotope discrimination (δ¹³C). Additionally, concentrations of Mg, chlorophyll and H₂O₂, and the activities of three antioxidative enzymes (ascorbate peroxidase, glutathione reductase and superoxide dismutase) in youngest fully expanded leaves were analyzed. RESULTS: Dry matter production was significantly decreased (by 34 % compared to control) in Mg deficient barley plants. Mg deficiency also markedly reduced leaf Mg concentrations and chlorophyll concentrations, but increased H₂O₂ concentrations (up to 55 % compared to control) and the activity of antioxidative enzymes. Severe Mg deficiency decreased biomass-WUE by 20 %, which was not reflected regarding leaf-WUE. In line with leaf-WUE data, discrimination against ¹³C (indicating time-integrated WUE) was significantly reduced under Mg deficiency. CONCLUSIONS: Mg deficiency increased oxidative stress indicating impairment in carbon gain and decreased biomass-WUE. Our study suggests that biomass-WUE was not primarily affected by photosynthesis-related processes, but might be dependent on effects of Mg on night-time transpiration, respiration or root exudation.