Abiotic stressors, such as drought, heavy metals, and high salinity, are causing huge crop losses worldwide. These abiotic stressors are expected to become more extreme, less predictable, and more widespread in the near future. With the rapidly growing human population and changing global climate conditions, it is critical to prevent global crop losses to meet the increasing demand for food and other crop products. The reactive gaseous signaling molecule nitric oxide (NO) is involved in numerous plant developmental processes as well as plant responses to various abiotic stresses through its interactions with various molecules. Together, these interactions lead to the homeostasis of reactive oxygen species (ROS), proline and glutathione biosynthesis, post-translational modifications such as S-nitrosylation, and modulation of gene and protein expression. Exogenous application of various NO donors positively mitigates the negative effects of various abiotic stressors. In view of the multidimensional role of this signaling molecule, research over the past decade has investigated its potential in alleviating the deleterious effects of various abiotic stressors, particularly in ROS homeostasis. In this review, we highlight the recent molecular and physiological advances that provide insights into the functional role of NO in mediating various abiotic stress responses in plants.

Triacontanol (TRIA) is a natural plant growth regulator found in epicuticular waxes. It is used to enhance the crop production in millions of hectares, particularly in Asia. Quite a number of researchers have reported the TRIA-mediated improvement in growth, yield, photosynthesis, protein synthesis, uptake of water and nutrients, nitrogen-fixation, enzymes activities and contents of free amino acids, reducing sugars, soluble protein, and active constituents of essential oil in various crops. Expectedly, TRIA enhances the physiological efficiency of the cells and, thus, exploits the genetic potential of plant to a large extent. In fact, TRIA increased free amino acids, reducing sugars, and soluble protein of rice (Oryza sativa L.) and maize (Zea mays L.) within 5 min. TRIA elicited the appearance of L(+)-adenosine within 1 min in the roots of plants, the shoots of which were sprayed with nanomolar concentrations of TRIA. TRIA and octacosanol (OCTA), the primary alcohols, are ubiquitous in the environment. OCTA was reported to inhibit the activity of TRIA in the seedlings of rice (Oryza sativa L.) at equimolar concentrations; and both TRIA and OCTA elicited a second messenger, known as OCTAM and triacontanol second messenger (TRIM), respectively. TRIA rapidly increases the ratio of L(+)- to D(–)-adenosine, probably at the tonoplast. However, it is to be resolved as to how TRIA elicits L(+)-adenosine and what is the source of L(+)-adenosine in plants. Based on known metabolic processes, de novo synthesis of L(+)-adenosine is unlikely, because of the rapidity of the response. TRIA-mediated increase in dry matter production could influence the inter-relationship between primary and secondary metabolism, leading to increased biosynthesis of secondary products. Various studies present strong evidences that application of TRIA applied either to the root medium or to leaves enhanced the growth and yield of vegetables and other crops, including agronomic and horticultural crops as well as medicinal and aromatic crop plants under normal and adverse conditions. However, further investigations are required to elucidate the possible role of TRIA on plant growth regulation, physiological activities and secondary metabolite biosynthesis regarding medicinal and aromatic plants subjected to abiotic stress. The present review covers the pivotal role of TRIA in plant growth and development, its mode of action and its significance in improving the crop productivity and quality of agricultural crops.

Artemisia annua L. is an aromatic-antibacterial herb that destroys malarial parasites, lowers fevers and checks bleeding, and of which the secondary compound of interest is artemisinin. Enhanced production of the artemisinin content in the whole plant is highly desirable. Keeping in mind, the importance of this valuable antimalarial plant, field experiments were conducted to investigate the effects of triacontanol alone and in combination with gibberellic acid on growth attributes, photosynthesis, enzymatic activities, essential oil and artemisinin content and yield of Artemisia. The results indicate that combination of triacontanol and gibberellic acid (1.5 mg l⁻¹+75 mg l⁻¹) significantly increased activities of nitrate reductase and carbonic anhydrase by 25.9% and 21.5%, and net photosynthetic rate, stomatal conductance and internal CO₂ by 25.4%, 14.1% and 15.4% higher, respectively, when compared to unsprayed plants. This combined treatment also significantly enhanced artemisinin content and yield (29% and 61% higher values).