Climate change has massive consequences on non-living factors in the environment, resulting in irregular precipitation, fluctuating atmospheric temperature, and variations in humidity. These changes cause biotic and abiotic stresses; plants must have defense mechanisms to survive. Therefore, plants divert some synthesized energy towards producing numerous plant secondary metabolites (PSMs), viz., flavonoids, alkaloids, and essential oils. These compounds act as protections for the plants, helping them to survive under stressful conditions. Medicinal and aromatic plants (MAPs) are sessile organisms that are not immune to harmful consequences of various abiotic stresses in which the PSMs have an important role in acting against the adverse effects. In this regard, the MAPs have a coherent defense mechanism for abiotic stresses. The secondary metabolites produced by these plants are useful as medicines and aromatic products for humans. However, not all stresses produce high secondary metabolites, as their production is highly specific to certain stresses. This review provides a comprehensive understanding of secondary metabolite production under various stressful conditions, including extreme temperature, drought, water logging, salinity, harmful radiation, elevated levels of ozone and CO2, heavy metals, and agrochemicals on MAPs. Additionally, the production of these compounds can be modified by subjecting plants to various stressors. Many authors have reported on PSMs in MAPs, which need to be well documented and exploited for humankind.

Drought stress severely limits maize germination and early seedling growth by reducing germination percentage, seedling vigor, water uptake, and biomass, while prolonging mean germination time. This study evaluated the ameliorative effects of seed priming with gibberellic acid (GA ₃ ) and melatonin on 10 maize genotypes (CLO 2450, DHM 117, SML 1, VQL 1, SKM 1, SML 2, LM 13, SML 163, SML 165, and SML 28) under 10% PEG-induced osmotic stress. Germination traits, water relations, biomass accumulation, and biochemical parameters, including α-amylase activity, total soluble sugar, proline, total phenolic content, soluble protein, lipid peroxidation, and electrolyte leakage, were assessed at 1, 7, and 14 days. Drought stress markedly suppressed enzymatic activity, carbohydrate mobilization, osmolyte accumulation, and protein content, while increasing oxidative damage and membrane leakage. GA ₃ priming (100 ppm) partially restored germination and metabolic traits, whereas higher concentrations showed diminished recovery. Melatonin priming (200 µM) consistently improved germination (>92%), seedling length (~19–20 cm), water uptake (~88%–90%), biomass, and antioxidant defenses, while limiting lipid peroxidation and electrolyte leakage. Violin plot and heatmap analyses confirmed that melatonin treatments closely resembled control performance, whereas GA ₃ provided intermediate recovery.

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Climate change poses a significant threat to agricultural sustainability. As the frequency of heavy rainfall has increased globally, waterlogging has become a pressing global issue that has a significant impact on the growth and development of oilseed crops. Due to decreased aerobic respiration in the rhizosphere, various physiological processes, including metabolic reactions, hormone production, and signaling cascades, are adversely impacted by waterlogging. These physiological changes impair reproductive health, resulting in decreased oilseed crop yields. In response to waterlogging, the most common resistance mechanisms developed by crop plants are development of aerenchyma, adventitious roots, and radial oxygen loss barrier. Consequently, the identification and selection of parents with resistance mechanisms, as well as their incorporation into breeding programmes, are essential for sustaining crop production. Thus, a better understanding of the physiological and biochemical mechanisms during waterlogging followed by identification of underlying key regulatory molecules would greatly facilitate the oilseed breeding programs. This review systematically summarizes the response of crop plants to waterlogging through adaptations and the strategies for introduction of waterlogging resistance in oilseed crops.

Plants have evolved both passive and active defence mechanisms to withstand insect herbivory. Passive defences include physical barriers and toxic substances on the plant's surface, while active defences are triggered by herbivore-associated signals. Recent studies have shown that defence mechanisms, including volatile emission, molecular pattern recognition, changes in Ca2+ levels, shifts in plasma membrane potential, NADPH oxidase mobilization and oxygen radical formation, are triggered by interplant and intraplant signalling. Plants detect specific elicitors produced by insects during infestation, enabling them to recognize and respond to herbivory. Infested plants emit chemical signals that lead to the production of volatile terpenoids, attracting host-seeking insects. Plants deploy a wide array of defensive compounds, including cyanogenic glucosides, glucosinolates (GSLs), phenolics, alkaloids, proteinase inhibitors (PIs) and saponins, which deter feeding and impair insect digestion. Morphological features such as trichomes and thorns provide additional protection. Plant hormones such as ethylene (ET), jasmonic acid (JA) and salicylic acid (SA) mediate plant immunity. Calcium (Ca2+) is crucial for regulating cellular processes and plant defence. Oxygen radical, especially hydrogen peroxide (H2O2), are critical for disease resistance. Nitric oxide (NO) influences H2O2 production, modulates the redox status and activates defence genes, thereby enhancing plant resilience against herbivory through various signalling pathways. By detailing the roles of various signalling molecules, hormones and defence compounds, the article aims to enhance understanding of plant defence strategies and the intricate signalling networks that underpin these responses.