Research has shown that chitosan induces plant stress tolerance and protection, but few studies have explored chemical modifications of chitosan and their effects on plants under water stress. Chitosan and its derivatives were applied (isolated or in mixture) to maize hybrids sensitive to water deficit under greenhouse conditions through foliar spraying at the pre-flowering stage. After the application, water deficit was induced for 15 days. Analyses of leaves and biochemical gas exchange in the ear leaf were performed on the first and fifteenth days of the stress period. Production attributes were also analysed at the end of the experiment. In general, the application of the two chitosan derivatives or their mixture potentiated the activities of the antioxidant enzymes superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and guaiacol peroxidase at the beginning of the stress period, in addition to reducing lipid peroxidation (malonaldehyde content) and increasing gas exchange and proline contents at the end of the stress period. The derivatives also increased the content of phenolic compounds and the activity of enzymes involved in their production (phenylalanine ammonia lyase and tyrosine ammonia lyase). Dehydroascorbate reductase and compounds such as total soluble sugars, total amino acids, starch, grain yield and harvest index increased for both the derivatives and chitosan. However, the mixture of derivatives was the treatment that led to the higher increase in grain yield and harvest index compared to the other treatments. The application of semisynthetic molecules derived from chitosan yielded greater leaf gas exchange and a higher incidence of the biochemical conditions that relieve plant stress.

Copper (Cu) is a micronutrient essential for plant development. However, in excess, it is toxic to plants and may cause various physiological and morphological changes. The study of the growth of plants exposed to excess Cu is important for the development of phytoremediation programs and for understanding the mechanisms involved in the tolerance of this metal. In this context, the objective of this research was to evaluate the effect of excess copper on photosynthetic responses and root morphology of Hymenaea courbaril L. Biometric measurements, gas exchange, root morphology, and Cu content in tissues and indices (TI and TF) were assessed, involving metal content and biomass. Up to a concentration of 200 mg kg⁻¹, Cu favored growth, gas exchange, and root morphology of the plants under study. At a higher concentration (800 mg kg⁻¹) in the soil, it affected plant growth and caused a decrease in photosynthetic rate. Biochemical limitations in photosynthesis were observed, as well as lower maximum net photosynthetic rate (A_max), respiration rate in the dark (R_d), light compensation point (LCP), light saturation point (LSP), and apparent quantum yield (α), when exposed to excess Cu. Root length, surface area, mean diameter, root volume, dry biomass, and specific root length decreased with high Cu concentrations in the soil. Cu was accumulated in the roots as a mechanism of tolerance to the excess of this metal in order to preserve the most metabolically active tissues present in the leaves. At a concentration of 800 mg kg⁻¹, copper also caused inhibition of the root system. Plants of H. courbaril showed tolerance to excess Cu in the soil and can be indicated for the recovery of areas contaminated with this metal.

Drought is a major limitation of maize cultivation in Brazil. Agronomic and physiological practices have been considered to overcome this stress and consequently, increase grain production. The present study investigated the role of abscisic acid (ABA) application in some physiological parameters, in two hybrids with contrasting drought resistance (DKB 390 and BRS 1030 resistant and sensitive, respectively). Contrasting resistance to drought in these genotypes was determined in previous studies. Water deficit was imposed for 10 days at flowering stage, in association with the application of 100 μM abscisic acid on plant canopy. Evaluations of gas exchange, chlorophyll fluorescence, relative water content (RWC), and endogenous ABA content were performed during stress period and also at water recovery (recovery irrigation). A significant functional relationship was observed between RWC and the parameters of gas exchange and fluorescence. During water recovery, no differences were observed among the treatments. DKB 390 presented higher photosynthesis rate (P _n) and electron transport rate (ETR) under water stress, while BRS 1030 presented higher intercellular CO₂ concentration (C _i) and lower photochemical quenching (qP), non-photochemical quenching (NPQ), and lower F _v/F _m ratio. DBK 390 was more responsive to ABA application than BRS 1030, presenting higher endogenous ABA content in the first day of stress. DBK 390 with ABA application reduced the effect of water stress through maintenance of water status, an increase of photosynthetic parameters, and a decrease of decline in the functions of photosystem II during stress.