Effect of exogenous application of salt stress and glutamic acid on lettuce (Lactuca sativa L.)

Salinity is a serious environmental issue which can negatively affect crop growth and productivity worldwide. Lettuce is generally considered as a salt-sensitive crop; however, different cultivars may have different adaptive mechanisms to this environmental stress. The application of biostimulants has proven to be a strategic strategy to improve plant responses to abiotic stresses and to foster resilience of crops during cultivation. This study intended to explore the physiological mechanisms underlying Romaine lettuce plant responses to salt stress, also in combination with the exogenous application of glutamic acid. The glutamic acid treatment was applied as foliar spray for the first time before salt exposure, followed by three applications during the stress. To understand the effect of salinity and glutamic acid treatment, different physiological and molecular analytical determinations were performed. High salinity induced a general stimulation of PSII and chlorophyll content. In particular, the performance index (+102%) and the number of reaction centres per cross section (+75,7%) increased, whereas the energy dissipation as heat per reaction centres (-32,1%) and the net rate of the centres’ closure (Mo) (-39.4%) decreased. Moreover, a reduction of yield (-26,5%) was observed in plants grown under high salinity. The concentration of proline was stimulated by salinity whereas ABA levels were reduced. The analyses of the genes encoding for ROS scavenging enzymes showed a general downregulation in response to salinity with the only exception of LsSOD. The application of the glutamic acid did not show a clear effect of the amino acid on lettuce plants, regardless the different growing conditions.