Humic acids induce a eustress state via photosynthesis and nitrogen metabolism leading to a root growth improvement in rice plants

• Published in: Plant physiology and biochemistry 2021-05, Vol.162, p.171-184
• Main Authors: van Tol de Castro, Tadeu Augusto, Berbara, Ricardo Luiz Louro, Tavares, Orlando Carlos Huertas, Mello, Débora Fernandes da Graça, Pereira, Erinaldo Gomes, Souza, Camila da Costa Barros de, Espinosa, Luis Maqueira, García, Andrés Calderín
• Format: Article
• Language: English
• Subjects: Bioactivity; Humic substances; Humic Substances - analysis; Mode of action; Nitrogen; Oryza; Photosynthesis; Plant Roots - chemistry; Soil; Structural characterization
• ISSN: 0981-9428; 1873-2690
• DOI: 10.1016/j.plaphy.2021.02.043

Chemical eustressors induce a eustress state “positive stress” increasing the resistance and improve the plant growth. The potentiality of humic acids (HA) to act as a eustressor has been scarcely explored. The present study aims to evaluate how HA with different structural characteristics induce differently, a eustress state in rice plants through the regulation of photosynthesis. The photosynthetic performance index showed an initial eustress state in plant by HA application characterized by reduction in photosynthesis followed by an increase in photosynthetic efficiency. The HA as a chemical eustressor triggering changes in plant metabolism indicate that the interaction of HA with root system induces a roots growth stimulus preceded by an initial positive stress. The eustress caused by HA is differentiated and is related to its chemical-physics characteristics. The HAVC, with a predominance of CAlkyl-(O,N), CAlkyl-di-O, CAromatic-O structures and greater polarity, stimulated the accumulation of N–NO3- and of soluble sugars in the sheath, increase carbohydrates content in the root and the root emission, resulting in higher total biomass production. The HASOIL, with a predominance of CCOOH-(H,R), CAlkyl-O, CAromatic-H,R structures and greater hydrophobicity caused a decrease in N–NH4+ and N-amine. The HARN, with a predominance of CAlkyl-O, CAlkyl-H,R, and CO, characterized by average polarity, caused an increase in photosynthetic pigment and N–NH4+ content. These results are keys to understand that quality of soil organic matter is related to plant development and that HA are efficient proxies for elucidate its function in natural environments. [Display omitted] •Regardless of origin, HA exert effects on plant metabolism through mild stress.•After an initial reduction, the HA caused an increase in photosynthetic efficiency.•HAVC stimulated the accumulation of N–NO3-, carbohydrates and higher total biomass.•Hydrophobicity in HASOIL caused a decrease in N–NH4+, N-amine and N–NO3-.•HARN from a sediment caused increase in photosynthetic pigment and N–NH4+ content.