Phosphorus is an obligatory component of the plant organism, its content in plants reaches 0.2-1.2% (by weight). This fact was first discovered by the Swiss naturalist Saussure at the beginning of the 18th century; he discovered this element in the ashes of all plants (1.0–9.0% P2O5). The main forms, in which phosphorus enters the plant and functions in it, are dihydrophosphate ion (H2PO4-) and hydrophosphate ion (HPO4-2). The source of phosphate ions are such fertilizers as superphosphate, double superphosphate, monopotassium phosphate, etc. All of them are derived from phosphoric acid.
What are phosphites?
Phosphites are salts of phosphorous acid (H3PO3). They are represented in agrochemical products as potassium or ammonium derivatives.
Attention to phosphites as a source of phosphorus in crop farming was drawn back in the 1930s, but the first evidence of their applicability in this role was provided by MacIntire (1950) and Adams (1953). They gave indications on possibility of absorption of phosphite Н2Р(+3)О3- + [O] = Н2Р(+5)О4- The oxidation process proceeds in various ways: chemical (atmospheric oxygen), enzymatic (bacterial phosphite dehydrogenase (PTDH)) and biological (participation in various redox reactions). A number of studies cite conflicting facts. For example, toxic effect of phosphite on corn plants upon various methods of introduction (2×2, per strip, foliar application) was first shown (Lucas, 1976, 1979), but later studies did not confirm this. It is known about inhibition of growth and development of root system and shoots when using 25 kg or more of P2O5 in the form of phosphite (Barrett, 2002). The research result
concluded that use of phosphites as a complete replacement of phosphates is unacceptable due to insufficient or controversial effectiveness, high cost and some other factors. Revival of phosphite fertilizers occurred at the beginning of the 1990s, when high efficiency of their foliar application was shown (Lovatt, 1990). Moreover, their fungicidal and biostimulating effect was discovered. The fungicidal effect, either direct or indirect, of this class of fertilizers is already studied by researchers from different countries to the extent allowing for classification of phosphite compositions as “biopesticides”.
Phosphites can be absorbed by both leaf surface and root system. They circulate easily within plants with both descending (phloem) and ascending (xylem) flow. Therefore, it is possible to state their systemic effect.
Fungicidal effect of phosphites is implemented in two ways:
• through systemic acquired resistance (SAR) (activation of the immune system (phytoalexins))
• through the development of “induced resistance” (IR) (thickening of cell walls)
The possibility of direct effect on pathogens is also mentioned quite often. In this case, there is inhibition of phosphorylation and, as a result, violation of DNA synthesis, which leads to inhibition of pathogen growth, development and reproduction.
Biostimulating effect of phosphites:
The biostimulating effect of phosphites is expressed by diverse effect on the plant organism. Thus, for example, substituting depot-phosphate in transportation systems contributes to its transport, assimilation and accumulation, while the absorption coefficient of mineral fertilizers generally increases, which affects the activity of various biochemical processes, crop quality and quantity. The following effects were obtained by the authors with various methods of use of phosphite fertilizers (foliar application, drip watering, soil-free growing). All the above stated clearly characterizes phosphite fertilizers as an effective tool that allows for managing plant physiology, controlling various pathogens and influencing the qualitative and quantitative indicators of yield.