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Plant growth-promoting activity of radiation-processed chitosan has been widely investigated by several researchers. Taking a step further, different fractions of irradiated chitosan (ICH), separated by column chromatography, were assessed for their effect on growth, physiology, and yield attributes of Java citronella (Cymbopogon winterianus Jowitt), conducting a pot experiment in the net-house conditions. Structural characterization of ICH was carried out using scanning electron microscopy (SEM), ultraviolet-visible (UV-Vis) spectroscopy, and Fourier-transform infrared (FT-IR) spectroscopy. Among various fractions, Fraction 2 of irradiated chitosan (ICH-F2) proved the best for almost all the parameters studied including the growth biomarkers, photosynthesis-allied parameters, enzymatic activities, and yield attributes. Interestingly, the ICH-F2 fraction contained a higher amount of carbonyl group as compared to other fractions. Perhaps, this specialty made the ICH-F2 unique and a potent growth promoter. As compared to the control, ICH-F2 enhanced the chlorophyll content (38.6%), and the activities of carbonic anhydrase (27.7%), nitrate reductase (21.5%), and DXR, i.e., deoxy- d -xylulose-5-phosphate reductoisomerase (45.7%). Application of ICH-F2 also enhanced the essential oil (EO) content (35.4%) and EO yield (120.1%) of Java citronella significantly. GC-MS analysis of EO revealed a significant increase in the contents of major aroma-compounds, viz. citronellal (18.4%) and geraniol (20.5%) simultaneous with a concurrent decrease in citronellol (24.6%) and geranyl acetate (19.3%) in comparison to the control.

Salinity is one of the major challenges that the current agricultural system is facing. High salt concentration inhibits the uptake of water by plants and interrupts the cellular ions equilibrium resulting in osmotic stress, ion toxicity, and production of reactive oxygen species (ROS). It inhibits photosynthesis, disrupts the metabolic balance, and damages cellular structures, and ultimately results in the reduction of crop yield. It constrains the activity of the photosystem II complex and induces the loss of chlorophyll pigments. The indepth knowledge of the cellular and molecular mechanisms involved in plant tolerance to salinity can help to identify the candidate genes/proteins that mitigate crop losses due to high salt conditions in the soil. To minimize the salinity-induced damage plants possess some innate defense systems such as synthesis of osmolytes to maintain cell turgor, regulation of ion homeostasis to cater to ions imbalance in the cell, activation of the antioxidant system to limit excessive ROS generation, and maintain the cellular redox balance. At the molecular level, salt stress stimulates various transcriptions factors (AP2/ERF, bZIP, NAC, MYB, WRKY) and regulates the gene expression of both enzymatic (superoxide dismutase, peroxidase, catalase, ascorbic peroxidase, etc.) and nonenzymatic components (ascorbic acid, glycine betaine, tocopherols, flavonoids) of the antioxidant machinery, ion transporters (High-affinity K+ transporters, Na+/H+ exchanger, salt overly sensitive, etc.), photosynthesis-related proteins (OEE, cytochrome b6f, CAB, RuBisCO, etc.), proteins concerned with carbohydrate and energy metabolism (ATP synthase, GAPDH, citrate synthase, malate dehydrogenase, etc.) and signaling proteins (14-3-3-like proteins, annexin, calreticulin, etc.) to reduce the adverse effects induced by salinity especially in halophytes. In this chapter, recent advances on the deleterious effect of salinity on the plant processes and the various tolerance mechanisms used by plants are discussed. Besides, the latest integrative analysis of transcriptomics and proteomics, studies of plants under salinity stress were further taken into account to strengthen the understanding of plant response to salinity.

Phytoremediation is an ecologically and economically feasible technique to remove heavy metal from soil. The aim of the study was to examine cadmium (Cd) toxicity and phytoremediation aptitude of Brassica juncea. In the present study, plants survived when exposed to different levels of Cd (0, 25, 50 and 100 mg/kg soil) and accumulated a large amount of Cd in its root and shoot. Translocation factor (TF) of Cd from root to shoot was > 1 at both 45 and 60‐day stage of growth suggesting that B. juncea is a hyperaccumulator and strong candidate for phytoextraction of Cd. Alongside, Cd impaired photolysis of water, PSII activity, nutrient uptake, photosynthesis and sugar accumulation in the plant. Cd-generated oxidative stress restricts the growth of B. juncea. The toxic effect of Cd was more pronounced at 45‐day stage of growth signifying the drifting of plant towards acquirement of exclusion strategy.

The use of nanotechnology in agriculture is increasing at a phenomenal rate. It is, therefore, necessary to appreciate and elucidate the role of nanoparticles (NPs) in plant growth and development. Silicon is regarded as a ‘quasi-essential’ element for plants and regulates a range of physiological processes including germination, vegetative growth, photosynthesis and stress tolerance. It is, therefore, of importance to assess the effects of silicon nanoparticles (SNPs) on these physiological processes, as SNPs are considered more efficient than their bulk particles due to their small size and high surface area and reactivity. The present chapter deals with the role of SNPs in plant growth, photosynthesis and stress tolerance. Additionally, potential toxic effects of NPs are presented.

BRs are polyhydroxylated sterol derivatives, classified as phytohormones. Plants of Brassica juncea var. Varuna were grown in pots and an aqueous solution (10−8 M) of two brassinosteroid isomers 28-homobrassinolide (HBL) and 24-epibrassinolide (EBL) of same concentration (10−8 M) was applied to their leaves. The treatment up-regulated the photosynthetic machinery directly by enhancing water splitting activity, photochemical quenching, non-photochemical quenching, maximum PSII efficiency, actual PSII efficiency, electron transport rate, stomatal movement, stomatal conductance, internal CO2 concentration, transpiration rate, net photosynthetic rate and carbohydrate synthesis. Moreover, the level of biochemical enzymes (carbonic anhydrase and nitrate reductase), reactive oxygen species (superoxide and hydrogen peroxide) generation, antioxidant enzyme activity and mineral status (C, N, Mg, P, S, K), which indirectly influence the rate of photosynthesis, also improved in the treated plants. Out of the two BR analogues tested, EBL excelled in its effects over HBL.

Oligomers derived through irradiation of marine polysaccharides have generated a lot of interest of plant biologists as the application of these molecules has yielded positive results regarding various plant processes. To comprehend the previously established growth-promoting activity of irradiated chitosan (ICH) and to gain insight of the structure-property relationship, gamma rays induced structural changes were analyzed using techniques such as Fourier Transform Infrared (FT-IR) spectroscopy, Ultraviolet-visible (UV–Vis) spectroscopy, 13C-Nuclear Magnetic Resonance (NMR) spectroscopy and Scanning Electron Microscopy (SEM). Moreover, to study the bioactivity of ICH samples a pot experiment was conducted on citronella grass (Cymbopogon winterianus) to access its response to foliar application of various levels (40, 60, 80 and 100 mg L−1) of ICH in terms of growth, physiological attributes and essential oil (EO) production. The application of ICH at 80 mg L−1(ICH-80) resulted in the maximum values of most of the attributes studied. Due to this treatment, the maximum improvement in the content (29.58%) and yield (90.81%) of EO in Cymbopogon winterianus were achieved. Gas chromatography-mass spectrometry (GC–MS) analysis revealed that ICH-80 also increased the content of citronellal (14.81%) and geraniol (18.15%) of the EO as compared to the control.

Salinity is one of the most prominent environmental stress found in the cultivated crops worldwide because many of the crops are susceptible to soil salinization resulting from the accumulation of salts in the soil. Salinity alters the physiology and metabolism of plants by a decrease in the rate of photosynthesis, respiration, protein synthesis, and lipid metabolism that lead to a reduction in yield of many crops. To overcome this problem, plants grown in saline conditions are engineered with plant growth-promoting rhizobacteria (PGPR agriculturally important bacteria) that inhabited in the rhizosphere of the plant. Globally, about 20% of cultivable land, as well as 50% of cropland, is under salinity stress according to the United Nations Environment Programme (UNEP). The beneficial effects of PGPR in alleviating salt stress involve boosting key physiological and biochemical pathways, viz., water and nutrient uptake, photosynthetic machinery, ion homeostasis, regulation of osmotic balance, regulation of redox status, capacity, regulation of endogenous phytohormone level, and availability of volatile organic compounds for plants. Therefore, it is recommended that the application of PGPR is an effective means to combat salinity stress in agricultural fields, thereby enhancing world crop productivity. The main emphasis of the chapter is to evaluate the salinity tolerance mechanisms exhibited by PGPR.