Metabolic adaptations of soybean plants submitted to recurrent periods of root system hypoxia associated with nitrate application

Abstract

The increase in unpredictable rainfall associated with limited soil drainage often causes flooding or waterlogging due to climate change, altering the soil aeration which results in oxygen deficiency for plant roots, and the change from aerobic t o anaerobic respiration, reducing cellular energy status, in addition to oxidative stress and reduced gas exchange. Waterlogging stress has often been transient and recurrent. Plants have the ability to retain information about a past stress signal and possibly result in a modified response to recurrent stress, or a sustained response to future exposure through memory acquisition. Recent studies have shown that nutrition with NO3- increases nitric oxide levels under hypoxia and can reduce fermentative enzymes activities and the production of reactive oxygen species in waterlogged plants. Considering the above, the objectives of this work were: I- to explore the mechanisms of a possible "priming" of a previous waterlogging carried out at the vegetative stage of soybean plants (V3) to alleviate the negative stress effect of a second waterlogging carried out at the reproductive stage (R2) and II- to evaluate if the application of nitrate during stress events (stages V3 and R2) can improve the tolerance of the plant to waterlogging through the "priming capacity" in plants cultivated without the supply of nitrate (nodulated plants). For this purpose, two experiments were conducted with soybean plants [Glycine max (L.) Merril genotype BR11-6042] grown in the field, in boxes (1 x 1 x 2 m), containing soil. Experiment I- Treatments: Control (non-waterlogged plants) and waterlogging, in plants at stage V3 and R2, separated into three groups: waterlogging at V3 stage for seven days (V3 group); waterlogging at V3 stage (primary treatment) and later at R2 stage for five days (V3R2 group) and waterlogging only at R2 stage for five days (R2 group). The waterlogging was carried out by keeping a layer of water on the ground. Experiment II- Treatments: Flooding without nitrate supply, V3R2 and R2 groups and flooding with nitrate supply during the flooding, groups V3R2N and R2N, with their respective control (with and without nitrate application). In each box, 25.78g of KNO3 was applied at a time, three days before the flooding and three days after the start of the waterlogging. The waterlogging was performed according to experiment I. They following measurements were carried out during waterlogging and reoxygenation periods: Gas exchange (liquid CO2 assimilation, transpiration and stomatal conductance), photosynthetic pigment concentrations, antioxidant enzyme activities (SOD, APX, CAT, GPOD and DHAR), hydrogen peroxide contents and lipid peroxidation, concentrations of total soluble amino acids (TAA), total soluble sugars (TSS), sucrose in leaves and roots, fermentative enzyme activity (LDH, PDC and ADH) and AlaAT in roots, nitrate content and NR activity in leaves and roots, and yield components. In all variables analyzed, V3R2 plants under hypoxic and reoxygenation condition present memory responses to stress, effectively improving photosynthetic efficiency, antioxidant capacity by reducing oxidative damage, faster recovery from fermentation metabolism, resulting in lower activities of LDH, PDC, ADH and AlaAT enzymes; availability of TSS, sucrose, TAA and nitrate is favored and yield components are increased compared to R2 plants. Nitrate apparently improves the plants' tolerance to flooding (groups R2 and V3R2) without, however, nullifying the effects obtained by "priming by flooding" conducted at V3 stage. Thus, groups V3R2 and V3R2N showed the main metabolic aspects that were modified as a result of recurrent stress treatments in improving tolerance to hypoxia, providing a better understanding of the processes involved in the "priming effect" in increasing the tolerance of soybean plants to waterlogging.