Biochemical responses in leaf tissues of alkaloid producing Psychotria brachyceras under multiple stresses.

Under natural conditions plants are generally subjected to complex scenarios of combined or sequential environmental stresses. Among the various components of plant biochemistry modulated by abiotic variables, a pivotal role is played by antioxidant systems, including specialized metabolites and their interaction with central pathways. To help address this knowledge gap, a comparative analysis of metabolic changes in leaf tissues of the alkaloid accumulating plant Psychotria brachyceras Müll Arg. under individual, sequential, and combined stress conditions was carried out. Osmotic and heat stresses were evaluated. Protective systems (accumulation of the major antioxidant alkaloid brachycerine, proline, carotenoids, total soluble protein, and activity of the enzymes ascorbate peroxidase and superoxide dismutase) were measured in conjunction with stress indicators (total chlorophyll, ChA/ChB ratio, lipid peroxidation, H2O2 content and electrolyte leakage). Metabolic responses had a complex profile in sequential and combined stresses compared to single ones, being also modified over time. Different stress application schemes affected alkaloid accumulation in distinct ways, exhibiting similar profile to proline and carotenoids, constituting a complementary triad of antioxidants. These complementary non-enzymatic antioxidant systems appeared to be essential for mitigating stress damage and re-establishing cellular homeostasis. The data herein provides clues that may aid the development of a key component framework of stress responses and their appropriate balance to modulate tolerance and yield of target specialized metabolites.

A semi-quantitative histochemical method for assessment of biochemical responses to osmotic stress in Coffea arabica leaf disks.

A simple method set for assessing biochemical changes associated with osmotic stress responses was developed using coffee (Coffea arabica L.) leaf disks. Stress was induced by polyethylene glycol (PEG) exposure. Quantitative evaluation of tissue physiological stress parameters was carried out using analytical methods to validate the conversion of classic qualitative histochemical tests for localizing lipid peroxidation, hydrogen peroxide, and total xanthine alkaloids into semi-quantitative assays. Relative electrolyte leakage (EL%) and chlorophyll content (SPAD index) were also recorded. EL% levels of treated disks were higher than those of control ones, whereas SPAD indexes were comparable. Histochemical localization indicated that levels of lipid peroxidation, H2O2, and total xanthines were also higher under osmotic stress than in control conditions. Semi-quantitative data obtained by image processing of histochemical staining consistently matched quantitative evaluations. Chromatographic analyses revealed that theophylline and caffeine concentrations increased in the presence of PEG, whereas theobromine remained constant in relation to the control. The methods herein described can be useful to rapidly acquire initial data regarding biochemical osmotic stress responses in coffee tissues based on simple staining and imaging steps. Moreover, it is likely that the same method may be applicable to other types of stresses and plant species upon minor adjustments.