Physiological and Nutritional Evaluation of the Application of Phosphite as a Phosphorus Source in Cucumber Plants

The aim of this study was to determine how different phosphite and phosphate rates applied together affect the growth and morphology parameters in cucumber plants as well as to characterize the nutritional and physiological state of phosphorus (P). Different foliar application rates of phosphite were applied to analyze subsequent biomass production, the relative growth rate, morphological parameters, P bioindicators, P forms, and P-use efficiency. The data suggest that the application of phosphite as a P fertilizer at a rate of ≥0.5 mM would be an appropriate and effective strategy under suboptimal conditions of phosphate in the growth medium, represented by 0.5 mM in our work, as it improved the growth parameters, number of flowers, leaf area, nutritional state of P, incorporation of P in structural organs, and P-use efficiency by the plant. The results indicate that the validity of the foliar use of phosphite as a P fertilizer in cucumber plants was subject to phosphate availability in the culture medium.     

亚磷酸盐作缓释磷肥对黄瓜体内养分吸收和光合特性的影响

本文以黄瓜为材料,采用营养液培养方法,研究了不同亚磷酸盐浓度对黄瓜植株各部位氮、 磷养分含量, 干重及根冠比, 植株氮、 磷总量以及叶片光合特性的影响。结果表明,黄瓜果实、 叶片和根部氮含量随亚磷酸盐浓度的增加而增加,茎部氮含量没有显著差异; 果实和茎部磷含量随亚磷酸盐浓度的增加呈增加趋势,根部磷含量呈降低趋势,叶片磷含量没有显著差异; 亚磷酸盐浓度增加到一定浓度时,各部位干重显著降低,根冠比没有显著差异; 植株氮、 磷总量随亚磷酸盐浓度的增加有下降的趋势; 黄瓜叶片净光合速率（Pn）随亚磷酸盐浓度的增加呈下降趋势。本研究结果显示,随着亚磷酸盐浓度的增加,植株表现出缺磷效应,对植株养分元素的吸收及光合作用产生了不利影响。该文讨论了亚磷酸盐作为缓释磷肥的可行性。     

Calcium chelate is as effective as phosphite in controlling Phytophthora root rot in glasshouse trials

Species in the genus Phytophthora cause significant economic losses in crops and damage to forests and natural ecosystems worldwide. Currently, phosphite is the most effective chemical for disease management, but excessive phosphite concentrations can result in phytotoxicity in plants and the development of tolerance by the pathogen. Two newly developed metal chelates and phosphite (alone and in combination) were tested for their in vitro and in planta efficacy against Phytophthora cinnamomi. In glasshouse trials, 0.25% and 0.5% of each chemical treatment (phosphite, Ca chelate, Zn chelate) and Ca chelate + phosphite were used as a foliar application on 3-month-old seedlings of Banksia grandis (experiment not repeated) and Eucalyptus marginata, prior to inoculation with P. cinnamomi. All noninoculated control plants remained healthy, while significant root damage and reduction of dry root weights were observed for inoculated untreated plants. Individually, phosphite and Ca chelate significantly reduced root lesion development of P. cinnamomi compared to the control, with Ca chelate attaining superior results to phosphite at the same concentration. In combination, Ca chelate + phosphite had the largest reduction in root lesion development in both plant species; however, this result has not yet been replicated but did reflect previous in vitro results. The Zn chelate applications were not effective. Ca chelate has the potential to be developed as a fungicide to control Phytophthora species.     

复硝酚钾和亚磷酸钾对‘富士’苹果激素含量及相关成花基因表达的影响

在苹果生产中,‘富士’枝条易旺长,过旺生长的枝条会破坏营养生长与生殖生长之间的协调关系,不利于‘富士’苹果的成花,从而造成严重的大小年现象。为防止枝条旺盛生长,缓解大小年结果问题,促进花芽形成,以8 a生‘长富2号’为试材,在2020年预备试验的基础上,于2021年5月20日、6月20和7月20日分别进行了T1（复硝酚钾1 mg·L^-1）、T2（亚磷酸钾3 340 mg·L^-1）和T3（复硝酚钾1 mg·L^-1 + 亚磷酸钾3 340 mg·L^-1）喷施处理,以清水作为对照(CK),对苹果枝条的生长量、短枝顶芽内源激素含量进行测定,对次年成花率进行调查,并用定量 PCR 对‘富士’苹果短枝顶芽内成花相关基因的表达量进行验证。结果表明：从处理后至生长季结束,T2和T3处理可以显著地控制枝条后期的生长,枝条生长量与对照相比分别降低29.03%~31.64%和27.18%~30.33%;激素水平上,花芽从生理分化期到形态分化期,随着喷施次数增加,ZR的含量先增高后降低,IAA和GA₃的含量下降,ABA的含量先升高后下降,且在第三次喷施时差异显著;在成花的关键时期,芽内成花促进基因 MdSOC1、MdFD、MdLFY和MdFT表达上调,成花抑制基因 MdGA20ox和 MdTFL-1表达水平被显著下调。喷施复硝酚钾和亚磷酸钾后,与对照相比T3处理下显著提高了次年的成花率,为63.41%左右。综上所述,喷施复硝酚钾和亚磷酸控制枝条旺盛生长,平衡了营养生长与生殖生长的关系,同时调节了内源激素含量变化,促进了成花相关基因的表达,有效地促进了‘富士’苹果成花,推荐在适宜留果量下树旺‘富士’苹果园中使用。     

Phosphate absorption of intact komatsuna plants as influenced by phosphite

Phosphite (     ; Pi) uptake in cell suspension culture, information on how Phi affects the Pi uptake of intact plants remains to be determined. The present study was conducted to investigate the effect of Phi on Pi absorption of intact komatsuna plants ( Brassica rapa var. peruviridis cv. Ajisai) in hydroponic culture. Phosphite markedly decreased Pi absorption of the intact komatsuna plants under both low (0.05 mmol L⁻¹ ) and high (0.5 mmol L⁻¹) Pi supply, although the growth (both shoots and roots) and water uptake of the high Pi-supplied plants was not affected by Phi. The inhibiting effect of Phi was small at 0.2 mmol L⁻¹, but became large at 2 mmol L⁻¹. Using relatively large seedlings (28 days old) to better assess the influence of Phi on Pi absorption early in the treatment, the results indicated that there was an immediate decrease in Pi absorption within the first 2-day period of Phi treatment when the water absorption of the plants was not affected. Taken together, the results suggested that there was a strong inhibiting effect of Phi on Pi uptake of intact komatsuna plants and this effect is exerted most likely by competition between Phi and Pi at uptake level. We speculate that the application of Phi to plant roots in an environment that is unfavorable for Phi-to-Pi conversion (e.g. hydroponic culture) may need to increase the amount of required Pi fertilization of plants to compensate for the reduction in Pi uptake by Phi. Further research is needed to confirm our results.     

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

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.     

Effect of phosphite–phosphate interaction on growth and quality of hydroponic lettuce (Lactuca sativa)

Although the fungicidal properties of phosphite have been recognized, its potential as a fertilizer is still being debated. The information on how phosphite affects the growth and quality of plants in relation to phosphate (P_i) also remains unknown. This study was conducted to investigate the effect of phosphite in relation to P_i on growth and quality parameters of lettuce (Lactuca sativa L.). The results showed that addition of phosphite to the nutrient solution at different rates ranging from 0.05 to 2 mM significantly increased total P, water‐extractable P_i, and phosphite in both shoots and roots, but did not improve plant growth under various P_i supplies (0.05, 0.1, 0.15, and 0.3 mM as P_i levels for approximately 50%, 80%, 90%, and 100% of the maximum plant growth, respectively), indicating that phosphite was well absorbed by roots and mobile inside the plants, but did not provide any P nutrition. Also, no stimulating effect of any P_i–phosphite combination was observed. The effect of phosphite on plant growth was strongly dependent on the level of P_i supply. In general, application of phosphite up to 2 mM did not influence the growth of P_i‐sufficient plants. However, plants fertilized with P_i for about 90% of maximum growth were still vulnerable to phosphite at 2 mM. The negative effect of phosphite was found even at concentrations as low as 0.2 mM, when plants were supplied with P_i adequate for about 80% of maximum growth or less. At 0.05 mM, phosphite had marginal effects on plant growth under all the P_i levels. Although phosphite itself had little influence on the ascorbate and mineral concentrations of lettuce, its application to P_i‐deficient plants may decrease the mineral concentrations of plants brought about by the inhibitory effect of phosphite on root growth and hence nutrient uptake. Since phosphite is an effective fungicide for lettuce, care should be taken on P_i supplies prior to application of phosphite products to minimize the harmful effects.     

Suppression of the in vitro growth and development of Microdochium nivale by phosphite

The ascomycete fungus Microdochium nivale is a major pathogen of many species of the gramineae. Control measures rely heavily on chemical fungicides, making alternative means of disease reduction desirable. Phosphite (PO₃^3?), has proven efficacy in reducing susceptibility of different species of gramineae to oomycetes, and has adverse effects on the in vitro growth of numerous other pathogens. The effect of phosphorous acid (H₃PO₃), phosphoric acid (H₃PO₄), dihydrogen potassium phosphite (KH₂PO₃), dihydrogen potassium phosphate (KH₂PO₄) and potassium hydroxide (KOH) on the in vitro mycelial growth and development of M. nivale was determined. Radial growth on amended potato dextrose agar (PDA) was used to calculate mean daily growth and percentage inhibition. PO₃^3? had a significant inhibitory effect on mycelial growth, with EC₅₀ values ranging between 35.9 and 40.99 μg mL^?1, whilst PO₄^3? and KOH had no significant inhibitory effect. Microscopic examination of mycelia showed morphological deformities in hyphae growing on PO₃^3? amended PDA, whilst hyphal growth was normal on PO₄^3? and KOH amended PDA. Conidial germination of M. nivale was significantly reduced following immersion in solutions of 50, 100 and 250 μg mL^?1 of PO₃^3?, while PO₄^3? and KOH at the same concentrations induced no inhibitory affect. These results show that PO₃^3? is a significant inhibitor of the growth of M. nivale and may have the potential to be used as a chemical control agent in the field.     

Increased phosphorus availability to corn resulting from the simultaneous applications of phosphate rock, calcareous rock, and biochar to an acid sandy soil

Phosphorus (P) deficiency is one of the main constraints on crop production in Arenosols (acid sandy soil). The high cost of P fertilizers may represent an insurmountable obstacle in many poor countries, leaving the exploitation of their own calcareous and phosphate rocks as the only low-cost and long-term alternative. Biochar is suggested to have positive effects on soil properties; however, there is no published research on the synergistic effects of biochar and rocky materials in modifying soil properties. The aim of this study was to investigate the chemical and biochemical responses of an acid Arenosol treated with phosphate rock (PR), calcareous rock (CR), and biochar (BC), and the implications for corn yield. A soil from Marracuene District, Mozambique was used, where corn was grown for 90 d with the soil treated with:no addition (control), water-soluble zinc phosphite fertilizer (WSP), PR, WSP+CR, WSP+BC, WSP+CR+BC, PR+BC, and PR+CR+BC. Biochar was produced by pyrolysis of babycorn peels for 4 h at 450℃ and applied at 11 g kg^-1. The soil pH_H2O increased from about 4.54 in the control to 7.38 in the PR+CR+BC treatment. Easily oxidizable organic carbon, cation exchange capacity, and available P were higher in the treatments containing BC than in the control. The treatments containing CR and/or BC led to the highest activities of alkaline phosphomonoesterase, phosphodiesterase, and α-glucosidase, which increased P availability and gave the greatest biomass and yields. We suggest that biochar provides additional soluble P and supplies adsorption sites for phosphate, preventing its evolution to unavailable forms. Thus, PR applied together with BC contributed to an 840% yield increase compared to the control. The treatments containing WSP and BC facilitated phosphite oxidation to phosphate and increased crop yield.