Insights into the mechanism of abscisic acid-induced 2, 4-D tolerance in maize (Zea mays L.) seedlings: Study on the regulation of osmolytes, fluorescence, photosynthetic carbon reactions, and antioxidant metabolism

In this experiment, maize plants were subjected to different concentrations (0, 50, 100, and 250 µM) of the herbicide 2, 4-dichlorophenoxyacetic acid (2, 4-D), with some groups primed with abscisic acid (ABA) and others not. Initial observations showed that after five days of incubation, seedlings exposed to the highest concentration of 2, 4-D exhibited stunted growth and a reduction in chlorophyll accumulation. Key parameters such as relative water content, relative growth rate, and net assimilation rate decreased (by 25%, 39%, and 35% respectively) under this condition. However, these parameters partially recovered with ABA priming, showing average improvements (8%, 14%, and 16%). Recovery in tissue hydration was also noted, coinciding with increased concentrations of proline (Pro) and glycine betaine (GB), and a decrease in the Na+/K+ ratio. When exposed to the highest concentration of the herbicide, fluorescence activity was reduced, but this too was reversed with ABA treatment. A notable inhibition of photosynthesis occurred, particularly at the decarboxylation reactions involving NADP-malic enzyme and phosphoenolpyruvate carboxylase, affecting total sugar concentrations. Under the influence of 2, 4-D, there was a significant increase in reactive oxygen species, such as superoxide (35%) and hydrogen peroxide (78%), but these levels significantly diminished with ABA priming. Abscisic acid priming also mitigated oxidative stress, evidenced by a reduction in malondialdehyde (by 10%) and carbonyl content (by 11%), along with lower electrolyte leakage. The regulation of oxidative pathways through ABA priming was apparent as the plants showed increased activity of enzymes such as peroxidases, catalases, and those involved in glutathione metabolism, including glutathione reductase and glutathione S-transferase. The study also discussed how ABA metabolism regulates sensitivity to 2, 4-D and highlighted certain parameters for selecting redox homeostasis against herbicidal toxicity.