Phytotoxic effects of Parthenium hysterophorus residues on three Brassica species

A study was conducted to assess the phytotoxicity of residues of Parthenium hysterophorus , an exotic invasive weed, towards the growth of three Brassica species ( Brassica campestris, Brassica oleracea and Brassica rapa ). The early growth of crops, measured in terms of seedling length and dry weight, was significantly reduced when grown in soil amended with varying amounts of Parthenium residues. A direct relationship was observed between the amount of residue incorporated in the soil and growth reduction. This adverse effect on Brassica crops indicates the presence of some growth-retardatory substances which are possibly released by the residues into the soil medium. In order to test this, aqueous extracts (1, 2, and 4%) of residues were prepared. It was observed that, in a laboratory bioassay, these extracts severely reduced the early growth of Brassica species, thereby indicating the presence of some water-soluble, inhibitory principles in Parthenium residues. A significant amount of the phenolics, the largest group of secondary metabolites usually implicated in allelopathy, was estimated in residue extracts, as well as in residue-incorporated soil. The phenolic content increased with increasing residue concentration, thereby showing their direct involvement in the observed growth inhibitions. Therefore, the study establishes that Parthenium residues exert an allelopathic influence on the early growth of Brassica crops by releasing water-soluble phenolics into the soil.     

Actinobacteria-enhanced plant growth, nutrient acquisition, and crop protection: Advances in soil, plant, and microbial multifactorial interactions

Agricultural areas of land are deteriorating every day owing to population increase, rapid urbanization, and industrialization. To feed today’s huge populations, increased crop production is required from smaller areas, which warrants the continuous application of high doses of inorganic fertilizers to agricultural land. These cause damage to soil health and, therefore, nutrient imbalance conditions in arable soils. Under these conditions, the benefits of microbial inoculants (such as Actinobacteria) as replacements for harmful chemicals and promoting ecofriendly sustainable farming practices have been made clear through recent technological advances. There are multifunctional traits involved in the production of different types of bioactive compounds responsible for plant growth promotion, and the biocontrol of phytopathogens has reduced the use of chemical fertilizers and pesticides. There are some well-known groups of nitrogen-fixing Actinobacteria, such as Frankia, which undergo mutualism with plants and offer enhanced symbiotic trade-offs.In addition to nitrogen fixation, increasing availability of major plant nutrients in soil due to the solubilization of immobilized forms of phosphorus and potassium compounds, production of phytohormones, such as indole-3-acetic acid, indole-3-pyruvic acid, gibberellins, and cytokinins, improving organic matter decomposition by releasing cellulases, xylanase, glucanases, lipases, and proteases, and suppression of soil-borne pathogens by the production of siderophores, ammonia, hydrogen cyanide, and chitinase are important features of Actinobacteria useful for combating biotic and abiotic stresses in plants.The positive influence of Actinobacteria on soil fertility and plant health has motivated us to compile this review of important findings associated with sustaining plant productivity in the long run.