Magnesium fertilization to confer drought and salinity stress resistance in faba bean (Vicia faba L.)

The essential plant nutrient Mg²⁺ is of fundamental importance for growth and development of plants.  This work aimed at providing new insights into the physiological, anatomical, and ion-homeostatic mechanisms by which magnesium nutrition particularly foliar application ameliorate abiotic stress tolerance in faba bean (Vicfa faba L.), a legumes crop of global agricultural significance, which are highly susceptible to drought and salinity. In addition, the comparative importance of magnesium and potassium in ion homeostasis under salinity stress was explored and the effectiveness of foliar Mg²⁺ application as part of a short-term interventional measure was tested.

Mg²⁺ deficiency during drought stress causes excessive starch accumulation in the chloroplasts and disorganisation of the thylakoids and enlarged plastoglobuli, as well as impaired photoassimilate export. Foliar application of Mg²⁺ partially improved these effects by maintenance of chlorophyll concentration, photosynthetic rate, and chloroplast integrity providing the first ultrastructural evidence for foliar Mg²⁺ mediated protection against drought. 

Salinity stress coupled with Mg²⁺ deficiency has adverse effects on plant growth and ion homeostasis compared to K⁺ deficiency. There were dramatically increased Na⁺/Mg²⁺ ratios in magnesium deficient plants under salinity compared with their normal plants, although K⁺ deficient plants did not cause similar disruptions. This knowledge resulted in the Na⁺/Mg²⁺ ratio being introduced as a new physiological trait for resistance to salt. Elevated K⁺/Mg²⁺ under both saline and non-saline conditions confirmed potassium's antagonistic effect on magnesium uptake.  Effectiveness of foliar application of MgSO₄ which was found to be very efficient in overcoming physiological constraints resulting from severe Mg²⁺ deficit under conditions of salinity stress such as impaired photosynthesis and transpiration rate, dysfunction of stomatal operation, impaired ion homeostasis, and lower overall growth. Foliar Mg²⁺ was found to reduce accumulation of leaf Na⁺, decrease the toxic Na⁺/Mg²⁺ ratio, reestablish synthesis of malate by re-establishing the function of Mg²⁺ dependent enzymes and normalise stomatal regulation.