Fertilizer use in modern agriculture is highly inefficient; much of the applied fertilizer is released into the environment, causing environmental degradation. One way in which fertilizer use can be reduced without damaging plant nutrition is to enhance crop uptake of nutrients through the use of biostimulants. A broad definition of plant biostimulants, including substances sometimes categorized as biofertilizers or biopesticides, is used throughout this review: “Plant biostimulants are substances or materials, with the exception of nutrients and pesticides, which, when applied to plants, seeds, or growing substrates in specific formulations, have the capacity to modify physiological processes in plants in a way that provides potential benefits to growth, development, or stress response.” This definition includes a variety of substances, four of which will be reviewed in this article: seaweed extract, humic substances, amino acids, and plant-growth-promoting bacteria. We will concentrate on the positive effects of biostimulant application on plant nutrient uptake, and the underlying mechanisms, which include positive changes in soil structure or nutrient solubility, root morphology, plant physiology, and symbiotic relationships, will be discussed. Recommendations for future research directions include finding the most promising substances, isolating the active ingredients and clearly demonstrating the mechanisms by which they affect nutrient uptake. The beneficial effects and mechanisms must be consistently demonstrated in greenhouse and field experiments.

The Use of Biostimulants for Enhancing Nutrient Uptake

Ineffective mutants of four effective strains of Rhizobium meliloti were isolated and tested for their ability to compete with effective parents or antibiotic resistant mutants in the formation of nodules on Medicago sativa . In 5 out of the 6 cases studied, ineffective mutants were no different to effective strains of the same origin in their competitive ability. A difference in selection for infection by the host plant was observed between equally effective strains as well as between ineffective strains. Except for one pair of strains, the more-competitive effective strains (resistant or not to antibiotics) had the same origin as the more-competitive ineffective strains. For such strains the ability to compete with other strains to form nodules was a characteristic of each parent strain. Competitiveness was independent of effectiveness and had been retained during mutation.

Competition for nodule formation between effective and ineffective strains of Rhizobium meliloti

We show that the promoter of the mos locus, which encodes genes required for the synthesis of a nodule-specific, opine-like compound, a rhizopine, in alfalfa nodules is regulated by the symbiotic nitrogen-fixation regulatory gene nifA. The 5'-regulatory region and amino-terminal end of the first open reading frame of the mos locus are highly homologous to the 5'-regulatory region and amino-terminal portion of the Rhizobium meliloti nifH gene. The coordinate regulation of mos and nif genes suggests that the mos locus plays a symbiotic role. We propose that the rhizopine enhances the survival of the bacterial partner in the symbiosis.

Synthesis of an opine-like compound, a rhizopine, in alfalfa nodules is symbiotically regulated

SUMMARY

Variation within as well as between varieties of white clover (Trifolium repens L.) in their selection or preference for rhizobial strains in nodulation was studied using antibiotic-resistant mutant strains of Rhizobium trifolii.



Significant differences between varieties were found. Cv. S100 was nodulated almost exclusively by 75 str but there was more variation within S184 and, in particular, Pajbjerg smalbladet. In this experiment 93% of the total variance was genotypic, of which 68% was due to the difference between varieties and 25% due to variance between genotypes within varieties. Significant correlations were found between the preference for rhizobial strains of plants grown from seeds and of stolon lines vegetatively propagated from the former plants, indicating that the preference for rhizobial strains is genetically controlled by the host. A considerable drift in preference towards selection of more compatible rhizobial strains by the clonally propagated stolons compared with the original white clover plants was observed.



The results are discussed from the point of view of improving symbiotic nitrogen fixation by combining highly effective and competitive Rhizobium strains with a host variety with uniform preference for those strains.

Variation within and between white clover varieties in their preference for strains of Rhizobium trifolii

By mixed inoculation experiments, the competitive ability of anifD::Tn5 mutant ofBradyrhizobium japonicum was compared to its effective, isogenic parent strain. When the strains were inoculated in a 1 : 1 ratio at high concentration, the mutant was found to colonize almost as many nodules as the wild type. Thus, lack of expression of a functional nitrogenase system does not severely reduce competitiveness. In such experiments the majority of the nodules ( > 60%) were infected by both wild-type and mutant strains. From statistical analysis it was concluded that a mean number of 2–4 bacteria have successfully elicited one nodule under the described conditions. Visual and microscopic observations of sections from mixed infected nodules revealed separated sectors containing effective or ineffective bacteroids.

Competitiveness of a nif− Bradyrhizobium japonicum mutant against the wild-type strain

An ineffective Rhizobium leguminosarum strain capable of forming green nodules of similar size and number as normally effective strains was tested for its ability to compete with an effective strain in nodule formation on the pea. The ineffective strain was found to be more competitive and influenced the pattern of nodulation by the effective strain on the same root system. Nodules containing both strains were pink and able to reduce acetylene.

Mixed Inoculations with Effective and Ineffective Strains of Rhizobium leguminosarum

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Mixed Inoculations with Effective and Ineffective Strains of Rhizobium leguminosarum