The accumulation of cadmium (Cd) in leaves reduces photosynthetic capacity by degrading photosynthetic pigments, reducing photosystem II activity, and producing reactive oxygen species (ROS). Though it was demonstrated that the application of Methyl Jasmonate (MeJA) induces heavy metal (HM) stress tolerance in plants, its role in adjusting redox balance and photosynthetic machinery is unclear. In this study, the role of MeJA in modulating photosystem II (PSII) activity and antioxidant defense system was investigated to reduce the toxic effects of Cd on the growth of pea (Pisum sativum L.) cultivars. One-week-old seedlings of three pea varieties were subjected to Cd stress (0, 50, 100 μm), and MeJA (0, 1, 5, 10 μm) was applied as a foliar spray for 2 weeks. Cadmium stress reduced the growth of all three pea varieties. Cadmium stress decreased photosynthetic pigments [Chl a (58.15%), Chl b (48.97%), total Chl (51.9%) and carotenoids (44.01%)] and efficiency of photosystem II [Fv/Fm (19.52%) and Y(II; 67.67%)], while it substantially increased Cd accumulation along with an increase in ROS (79.09%) and lipid peroxidation (129.28%). However, such adverse effects of Cd stress varied in different pea varieties. Exogenous application of MeJA increased the activity of a battery of antioxidant enzymes [superoxide dismutase (33.68%), peroxidase (29.75%), and catalase (38.86%)], improved photosynthetic pigments and PSII efficiency. This led to improved growth of pea varieties under Cd stress, such as increased fresh and dry weights of shoots and roots. In addition, improvement in root biomass by MeJA was more significant than that of shoot biomass. Thus, the mitigating effect of MeJA was attributed to its role in cellular redox balance and photosynthetic machinery of pea plants when exposed to Cd stress.

The individual and combined effects of micronutrients and hormones on freshwater-borne microalgae growth were investigated for biofuel feedstock in this experimental study. Five algal strains of Chlorella sp., Chlorococcum sp., Phormidium sp., Chlorella sp. and Cosmarium sp., AZH, AZS, ZAA1, ZAA2, and ZAA3, respectively, have been investigated. These strains were treated using different concentrations of micronutrients (iron chloride, manganese chloride, and sodium molybdenum oxide) and hormone (salicylic acid). The different treatments’ growth effects were as follows: iron chloride > sodium molybdenum oxide ˃ manganese chloride > salicylic acid. The order of the increases in the number of microalgal strain cells achieved by the application of the micronutrients and hormone was AZH > AZS > ZAA3 > ZAA2 > ZAA1. The combined treatments produced higher growth rates than the individual treatments, with the order of their effects being micronutrients + hormone > all three micronutrients > hormone alone. The increase in the number of microalgal strain cells under combined treatment was ZAA3 > AZH > AZS > ZAA2 and assessed using one-way ANOVA.

Due to global climate change, heat stress is a critical environmental factor for adversely affecting plant growth and productivity. It induces changes in cellular machinery and antioxidant enzymes’ functions in plants. The role of phytohormones especially salicylic acid (SA) is well documented to alleviate the heat stress-induced adverse effects in plants. Here, we investigated the ameliorative role of SA in heat-stressed chili (Capsicum frutescens L.) plants. For this purpose, four-week-old chili plants of three local cultivars (Moro, Tilhari and Ren-02) pretreated with 1 mM and 2 mM SA were subjected to heat stress (42 oC) for 72 h. Our results revealed that heat stress significantly reduced plant height, shoot biomass, chlorophyll contents and relative water contents, while it increased membrane peroxidation and hydrogen peroxide in all three chili cultivars. However, foliar spray with both concentrations of SA (particularly 2 mM) showed prominent recovery effects in high temperature exposed chili cultivars (Moro and Ren-02) with significantly improved biomass production, photosynthetic pigments, relative water contents, and catalase and peroxidase activities compared to their respective controls. Moreover, the foliar spray of 2 mM SA significantly reduced hydrogen peroxide and malonaldehyde contents under the heat stress conditions. In conclusion, SA significantly improved heat tolerance of chili plants by decreasing oxidative stress via increasing antioxidant activity and membrane stability.

Due to climate change, heat stress is becoming one of the most disastrous abiotic stresses and responsible for huge crop losses in different regions of the world. Adverse effects of heat stress can be encountered by the use of phytohormones and salicylic acid (SA) is one of those hormones having capability to protect plants from heat stress. Current research work was planned to explore the defensive role of exogenously applied SA in barley cultivars under high temperature. For this purpose, two weeks old plants of four barley cultivars (Jau-87, B-10007, B-14003 and B-14037) were foliar sprayed with SA (2 mM and 5 mM) for 24 hours and then were exposed to heat stress (42 °C). Our results showed that heat exposure made a prominent decrease in chlorophyll contents, carotenoids, relative water contents, antioxidant enzymes activities (superoxide dismutase, peroxidase and catalase) and anthocyanin activity while increased lipid peroxidation (malondialdehyde), hydrogen peroxide (H2O2) contents and proline accumulation. Foliar application of SA enhanced plants photosynthetic pigments (chlorophyll a, b and total), RWC, proline, and anthocyanin under control and stressed conditions. SA pre-treatment under heat stress reduced H2O2 and MDA levels whereas activities of reactive oxygen species (ROS) scavenging enzymes were elevated by all tested SA concentrations. These results suggest that SA can efficiently remove H2O2 by increasing activity of above mentioned enzymes hence reducing oxidative stress and malondialdehyde (MDA) contents. Proline, an important amino acid that acts as an anti-oxidative system in plants also highly accumulated in SA co-treated heat-stressed plants. Our results also showed that 2mM SA is more effective in protecting barley plants under heat stress as compared to 5mM SA. This concluded that defensive nature of SA regulates different physiological and biochemical processes to induce heat tolerance in barley.