Salt stress obstructs plant’s growth by affecting metabolic processes, ion homeostasis and over-production of reactive oxygen species. In this regard silicon (Si) has been known to augment a plant’s antioxidant defense system to combat adverse effects of salinity stress. In order to quantify the Si-mediated salinity tolerance, we studied the role of Si (200 ppm) applied through rooting media on antioxidant battery system of barley genotypes; B-10008 (salt-tolerant) and B-14011 (salt-sensitive) subjected to salt stress (200 mM NaCl). A significant decline in the accumulation of shoot (35–74%) and root (30–85%) biomass was observed under salinity stress, while Si application through rooting media enhancing biomass accumulation of shoots (33–49%) and root (32–37%) under salinity stress. The over-accumulation reactive oxygen species i.e., hydrogen peroxide (H₂O₂) is an inevitable process resulting into lipid peroxidation, which was evident by enhanced malondialdehyde levels (13–67%) under salinity stress. These events activated a defense system, which was marked by higher levels of total soluble proteins and uplifted activities of antioxidants enzymatic (SOD, POD, CAT, GR and APX) and non-enzymatic (α-tocopherol, total phenolics, AsA, total glutathione, GSH, GSSG and proline) in roots and leaves under salinity stress. The Si application through rooting media further strengthened the salt stressed barley plant’s defense system by up-regulating the activities of enzymatic and non-enzymatic antioxidant in order to mitigate excessive H₂O₂ efficiently. The results revealed that although salt-tolerant genotype (B-10008) was best adopted to tolerate salt stress, comparably the response of salt-sensitive genotype (B-14011) was more prominent (accumulation of antioxidant) after application of Si through rooting media under salinity stress.

The presence of significant amount of heavy metals in rivers and canals due to mixing of untreated industrial effluents is a common phenomenon, especially in developing countries. The agricultural crops are influenced by the presence of various pollutants in the sewage, being applied for irrigation purpose. The effluents containing copper affect the growth and development of crop species, thereby, ought to be mitigated by foliar spray of osmoprotectants, e.g. proline. A pot culture experiment was conducted at the University of Agriculture, Faisalabad-Pakistan during the crop season 2015–2016. The treatments consisted of (a) three wheat varieties (Punjab-96, MH-97, FSD-83), (b) two levels of copper (0, 400 µM) applied through rooting medium, and (c) two levels of proline (0, 80 mM) applied through foliar application. The treatments were arranged in a completely randomized design with four replications. The results showed that application of 400 µM copper caused a reduction in biomass accumulation, chlorophyll (‘a’ and ‘b’) contents, and eventually yield (100-grain weight). There were also significant decreases in gas exchange parameters (stomatal conductance, internal CO₂ concentration), photosynthetic rate, water-use-efficiency, and transpiration rate in response to copper stress. Metal toxicity caused the maximum reduction in productivity of PSII, electron transport rate, and photochemical quenching, while higher values of non-photochemical quenching were recorded in the wheat varieties. The activities of antioxidant enzymes (superoxide dismutase, catalase, peroxidase), as well as quantities of proline, protein and calcium contents were accelerated in response to copper stress. The uptake of calcium, magnesium, and potassium constituents by plants was reduced, while assimilation of calcium was increased in plants under copper stress. However, the occurrence of negative effects on these parameters due to copper stress was mitigated by foliar spray of proline at the rate of 80 mM solution. The exogenous application of proline at the rate of 80 mM resulted in the reduction of generation of reactive oxygen species and enhanced accumulation of proline and protein contents in wheat varieties under copper stress environment.