Phosphite (phosphorous acid): Fungicide, fertilizer or bio-stimulator?

Abstract

Phosphite (; Phi), a reduced form of phosphate (; Pi), is widely marketed as either a fungicide or fertilizer or sometimes as a biostimulant. This is confusing for both distributors and growers. The present paper explores data from various studies to clarify that Phi does not provide plant P nutrition and thus cannot complement or substitute Pi at any rate. In addition, Phi itself does not have any beneficial effect on the growth of healthy plants, regardless of whether it is applied alone or in combination with Pi at different ratios or different rates. The effect of Phi on plants is not consistent, but is strongly dependent on the Pi status of the plants. In most cases, the deleterious effect of Phi is evident in Pi‐starved, but not Pi‐sufficient, plants. Plants fertilized with Pi allowing for approximately 80–90% of its maximum growth might still be at risk of the effect. This negative effect becomes more pronounced under more seriously Pi‐deficient conditions. Although a number of studies have shown positive crop responses to Phi, these responses are likely to be attributable to the suppression of plant diseases by Phi and/or to Pi formed from oxidation of Phi by microbes. In addition, indirectly providing P by Phi‐to‐Pi oxidation is not an effective means of supplying P to plants compared with Pi fertilizer. An understanding of these issues will aid the right selection of fertilizer as well as minimize the harmful effects of Phi use on crops.

Phosphite and phosphate in the accumulation and translocation of nutrients in common bean1

“…** and *: significant at 1 % and 5 % by the F test, respectively. proportions higher than 25 % in the solution were toxic to bean plants, as demonstrated visually by the shortening and thickening of roots and by the chlorosis in young leaves, as also observed for other crops (Varadarajan et al 2002, Thao & Yamakawa 2009). This results in a lower production of dry matter (Araújo et al 2013), which explains the lower accumulation of nutrients.…”

“…It was observed that providing P exclusively in the form of phosphite (Phi) decreases the dry mass of common bean by about 95 %. One possible explanation for the phytotoxicity could be the fact that phosphite, when phosphate is deficient, increases the concentration of poly and pyrophosphates and inhibits several enzymes of the glycolysis and pentose phosphate pathway, thus interfering with the metabolism of P (Varadarajan et al 2002).…”

“…The application of P as phosphite promoted the highest translocation of P (Figure 3a), indicating that this anion is possibly more mobile than the phosphate anion in plants. This increased mobility of phosphite, in relation to phosphate, may be because phosphite has one less atom of oxygen in its molecular structure (White & Metcalf 2007), what presumably facilitates its movement in the xylem and phloem vessels. However, there are no studies proving this.…”

“…For phosphorus, the results obtained by several authors show that phosphite does not replace phosphates (H 2 PO 4 -, HPO 4 2-and PO 4 3-) in the plant metabolism (Araujo 2008, Thao & Yamakawa 2009, Moor et al 2009, Ávila et al 2011and 2012, Araújo et al 2013. This is because phosphite has one less oxygen atom, which is substituted by a hydrogen atom, if compared to phosphate.…”

Effect of foliar applications of phosphite on post-harvest potato tubers

“…We have obtained evidence that foliar treatment with Phi increased the length of crop cycle and that could improve tuber yield (Lobato et al 2008). Recently, Thao and Yamakawa discussed in a review article the role of Phi as a fungicide, fertilizer or bio-stimulator and showed that Phi can act with at least one of these properties in different study systems (Thao and Yamakawa 2009).…”