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The structural identity of humic substances (HS) in the soil is a highly debated issue in the soil sciences. Although the complexity and structural characteristics that determine the functions of HS justify their further study, there is enough scientific evidence explaining the presence of these compounds as a group of structures that are formed by humification and have unique chemical characteristics. This review presents scientific information that follows the structure-property-function relationship with the objective of better explaining the nature of HS. On the basis of the spectroscopic characterization of a number of humic acids, together with the use of chemometric techniques, it is shown that, although the sources of origin are different, HS have a unique structural pattern that is different from that of any other group of soil compounds. From this structural pattern, it is possible to understand how fragments with greater lability in HS can reach root surfaces and interact with the cell membrane, regulate oxidative metabolism, and stimulate root growth in plants. Lability and recalcitrance are properties arising from the characteristics and suprastructural organization of HS and can define the type and intensity of the bioactivity of HS in plants. In this review, we demonstrate that there are methods for studying HS by which a deeper understanding of the functions of these substances on the basis of their chemical properties is possible. Therefore, the understanding of this complex system allows the connection of the scientific elements that justify the existence of these compounds in the soil.

RESUMO.Este estudo teve como objetivo avaliar os efeitos de diferentes sistemas de preparo do solo e de colheita sobre o crescimento e produtividade da cana planta. O experimento foi conduzido em Linhares, Estado do Espírito Santo. Trata-se de um dos ensaios mais antigos no país que investiga os efeitos da Cana crua e queimada. Instalados num Argissolo Amarelo textura arenosa/média, os tratamentos consistiram de parcelas (preparo convencional e cultivo mínimo) e subparcelas (Cana crua e Cana queimada). Foi avaliado o crescimento da cultura, o aporte de matéria orgânica e a quantificação do rendimento da cana-de-açúcar. O diâmetro foi maior para o cultivo mínimo, e o perfilhamento foi maior para o preparo convencional. A altura e o perfilhamento foram superiores no corte sem queima. Para produtividade de colmos, os tratamentos não apresentaram diferenças significativas. As folhas foram maiores em cultivo mínimo e pontas em Cana crua. O cultivo mínimo propicia, inicialmente, aumento do diâmetro e maior produtividade de folhas na colheita. O perfilhamento é favorecido pelo preparo convencional. A Cana crua não apresentou influência negativa da palhada na rebrota. Após 16 anos de cultivo da cana-de-açúcar com e sem queima do palhiço, observou-se maior produtividade de ponteiros, incrementando o rendimento dos colmos em Cana crua.Palavras-chave: cana-de-açúcar, sistemas de preparo do solo, manejo de colheita.ABSTRACT. Sugarcane growth and productivity under different tillage and crop systems. This study had as its objective to evaluate the effects of different tillage and crop systems on the growth and productivity of sugarcane plants. The experiment was conducted in Linhares, ES. It is one of the oldest assays in the country investigating the effects of unburned and burned sugarcane. Installed in a Yellow Latosol with sandy/medium texture, the treatments consisted of plots (conventional and minimum tillage) and subplots (raw and burned sugarcane). Plant growth, contribution of organic matter and quantification of sugarcane yield were evaluated. The diameter was larger for minimum tillage, and tillering was greater for conventional tillage. The height and tillering were superior in the cut without burning. For stems productivity, the treatments did not present significant differences. The leaves were larger in minimum tillage and tips in unburned cane. The minimum tillage propitiates the increase in diameter initially and largest productivity of leaves in the crop. Tillering is favored by conventional tillage. The unburned cane did not present negative influence of straw in the regrowth. After 16 years of sugarcane cultivation with and without straw burning, it resulted in larger productivity of pointers increasing the productivity of the stems in unburned cane.Key words: sugarcane, tillage systems, crop handling. IntroduçãoO papel fundamental das operações de preparo do solo é criar condições ideais para o desenvolvimento das raízes e, por conseguinte, maiores produções. Em cana-de-açúcar, este manejo inicial pode i...

Humic acids (HAs) have a major effect on nutrient uptake, metabolism, growth and development in plants. Here, we evaluated the effect of HA pretreatment applied with a nutrient solution on the uptake kinetics of nitrate nitrogen (N-NO 3 − ) and the metabolism of nitrogen (N) in rice under conditions of high and low NO 3 − supply. In addition, the kinetic parameters of NO 3 − uptake, N metabolites, and nitrate transporters (NRTs) and the plasma membrane (PM) H + -ATPase gene expression were examined. The plants were grown in a growth chamber with modified Hoagland and Arnon solution until 21 days after germination (DAG), and they were then transferred to a solution without N for 48 h and then to another solution without N and with and without the addition of HAs for another 48 h. After this period of N deprivation, the plants received new nutrient solutions containing 0.2 and 2.0 mM N-NO 3 − . Treatment of rice plants with HA promoted the induction of the genes OsNRT2.1-2.2/OsNAR2.1 and some isoforms PM H + -ATPase in roots. The application of HAs differentially modified the parameters of the uptake kinetics of NO 3 − under both concentrations. When grown with 0.2 mM NO 3 − , the plants pretreated with HA had lower K m and C min values as well as a higher V max /K m ratio. When grown with 2 mM NO 3 − , the plants pretreated with HA had a higher V max value, a greater root and shoot mass, and a lower root/shoot ratio. The N fractions were also altered by pretreatment with HA, and a greater accumulation of NO 3 − and N-amino was observed in the roots and shoots, respectively, of plants pretreated with HA. The results suggest that pretreatment with HA modifies root morphology and gene expression of PM H + -ATPases and NO 3 − transporters, resulting in a greater efficiency of NO 3 − acquisition by high-and low-affinity systems.

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