Abstract
Polyamines (PAs) have a variety of roles in plant environment interactions and stress responses, which control the tolerance induction of plants under hypoxic stress. Citrus plants are sensitive to waterlogging conditions because it frequently causes root zone hypoxia (RZH) and severe abiotic stress to their growth. Therefore, the present study investigated whether silicon (SiO2) could improve hypoxia tolerance by modulating the activity of PAs biosynthesizing enzymes and, in turn, variation in different PAs profiles involved in critical metabolic processes. Three commercially used citrus rootstocks (Carrizo citrange, Rubidoux, and Rich16-6) were subjected to flooding environments (with and without oxygen) with different treatments in both the presence and absence of SiNP (silicon nanoparticles) through roots and foliar application. Hypoxia stress increased PA-synthesizing enzymes' activity and PAs' formation in plant leaves and roots. Under hypoxia stress, exogenous SiNP raised the concentration of endogenous silicon (SiO2) and a large concentration of polyamines (PAs) by regulating PA biosynthesis. In the response to SiNP, it was also observed that an increase in plant biomass (leaf, stem, and root tissues) resulted in an overall improvement in plant growth. Rich-16-6 rootstock was found to be more receptive to root application of SiNP than foliar application in contrast to Rubidoux and Carrizo citrange plants, indicating genotype variability of citrus rootstocks in silicon uptake. Our findings demonstrate that the accumulation of silicon-induced polyamine enhanced growth and reduced reactive oxygen species (ROS) that might help to reduce oxidative damage in citrus rootstocks under hypoxic stress, providing an optimal strategy for stress tolerance. This study enhances the impact of nanoparticles on fruit crop production, significantly relieving pressure, and is a potential platform for future research.
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