Leaf wettability is the main driver for foliar P uptake in P-deficient maize

Foliar fertilisation is an alternative form of nutrient application, which is of particular interest for phosphorus (P), where the efficiency of soil fertilisation is low. However, the uptake of foliar-applied nutrients is insufficiently characterised. The aim of this study was to investigate the individual and combined significance of wettability, foliar fertiliser properties and surfactant on foliar P uptake in P-deficient maize (Zea mays L.). Sorption and desorption properties of two P salts used as foliar fertilisers (KH2PO4, K2HPO4) were determined with dynamic vapor sorption isotherms. Leaf surfaces and foliar spray depositions of two differently wettable maize cultivars were investigated by scanning electron microscopy and contact angle measurement. Phosphorus uptake was then linked to leaf and fertiliser solution properties and its effect on cell ultrastructure was characterised by transmission electron microscopy. Wettability was the key factor for P absorption, as all foliar fertilisers were taken up reaching a tissue-P level of adequately nourished plants. For unwettable leaves, only solutions with surfactant, especially the combination of surfactant and hygroscopic P salt (K2HPO4) were taken up. This study provides novel insights into the significance of leaf surface and fertiliser properties, which can thus contribute to an improvement of P fertilisation strategies.

Leaf surface features of maize cultivars and response to foliar phosphorus application: effect of leaf stage and plant phosphorus status

Soil phosphorus (P) application is the most common fertilisation technique but may involve constraints due to chemical fixation and microbial immobilisation. Furthermore, excessive P fertilisation leads to P runoff into water bodies, threatening ecosystems, so targeted foliar P fertilisation is an interesting alternative. This study aimed to determine the importance of leaf surface characteristics for foliar P uptake in P-deficient maize (Zea mays L.). The leaf surface of four maize cultivars was characterised by electron microscopy, Fourier transform infrared spectroscopy and contact angle measurements. Uptake of foliar-applied P by maize cultivars was estimated, measuring also leaf photosynthetic rates after foliar P spraying. Plants of cultivar P7948 were found to be wettable from the 4th leaf in acropetal direction, whereas other cultivars were unwettable until the 6th leaf had developed. Minor variations in stomatal number and cuticle composition were recorded, but no differences in foliar P absorption were observed between cultivars. Nevertheless, cultivars showed variation in the improvement of photosynthetic capacity following foliar P application. Phosphorus deficiency resulted in ultrastructural disorganisation of mesophyll cells and chloroplasts, which impaired photosynthetic performance, yet there was no effect on stomatal frequency and leaf wettability. This study provides new insights into the influence of P deficiency and cultivar on leaf surface characteristics, foliar P uptake and its effect on physiological processes. Understanding the relationships between leaf characteristics and P uptake allows a more targeted evaluation of foliar P fertilisation as an application technique and contributes to the understanding of foliar uptake mechanisms.

Tomato and Pepper Leaf Parts Contribute Differently to the Absorption of Foliar-Applied Potassium Dihydrogen Phosphate

Foliar fertilisation is an application technique that is increasingly being used in agriculture and offers the possibility of providing nutrients directly to the site of highest demand. Especially for phosphorus (P), foliar application is an interesting alternative to soil fertilisation, but foliar uptake mechanisms are poorly understood. To gain a better understanding of the importance of leaf surface features for foliar P uptake, we conducted a study with tomato (Solanum lycopersicum) and pepper (Capsicum annuum) plants, which have different leaf surface traits. For this purpose, drops of 200 mM KH₂PO₄ without surfactant were applied onto the adaxial or abaxial leaf side or to the leaf veins and the rate of foliar P absorption was evaluated after one day. Additionally, leaf surfaces were characterised in detail by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), estimating also leaf surface wettability and free energy, among other parameters. While the leaves of pepper hardly contained any trichomes, the abaxial side and the leaf veins of tomato leaves were densely covered with trichomes. The cuticle of tomato leaves was thin (approximately 50 nm), while that of pepper was thick (approximately 150-200 nm) and impregnated with lignin. Due to the fact that trichomes were most abundant in the leaf veins of tomato, dry foliar fertiliser drop residues were observed to be anchored there, and the highest P uptake occurred via tomato leaf veins, resulting in 62% increased P concentration. However, in pepper, the highest rate of P absorption was recorded after abaxial-side P treatment (+66% P). Our results provide evidence that different leaf parts contribute unequally to the absorption of foliar-applied agrochemicals, which could potentially be useful for optimising foliar spray treatments in different crops.

Foliar P Application Cannot Fully Restore Photosynthetic Capacity, P Nutrient Status, and Growth of P Deficient Maize (Zea mays L.)

The essential plant nutrient phosphorus (P) is key for numerous structures and processes in crops and its deficiency can severely restrict yield and quality. As soil P availability for plant uptake is often limited, foliar P application can be an alternative means of supplying P to the plants during the growth period. This study was aimed at investigating the effect of foliar P application on photosynthetic parameters, P nutritional status, and growth of P deficient maize over time. Plants of Zea mays L. cv. Keops were grown with deficient and sufficient amounts of P in hydroponics. Foliar P treatments were applied to P deficient plants and several physiological parameters were monitored for 21 days. The variables measured were leaf gas exchange parameters, SPAD values, foliar P absorption, re-translocation rates, and plant biomass production. Foliar P application significantly increased CO₂-assimilation and SPAD values and additionally enhanced biomass production in all plant components. Elemental analysis revealed increased tissue P concentrations following foliar P application compared to P deficient plants. While increased growth of P-deficient plants was steadily promoted by foliar P spraying for the entire experimental period, the positive effect on CO₂ assimilation and P concentration was transient and vanished some days after the foliar treatment. P deficiency markedly impaired the efficiency of physiological processes of maize plants. As a conclusion, foliar P fertilisation improved physiological and agronomical plant parameters over time, but failed to restore plant functionality of P deficient maize plants during a prolonged experimental period.

Uptake, subcellular distribution, and translocation of foliar-applied phosphorus: Short-term effects on ion relations in deficient young maize plants

One crucial aspect for successful foliar application is the uptake of the nutrient into the symplast for metabolization by the plant. Our aim was to determine the subcellular distribution of foliar-applied P in leaves, the translocation of this element within the whole plant, and its impact on the ion status of P-deficient maize plants within the first 48 h of treatment. Maize plants with P deficiency were sprayed with 200 mM KH₂PO₄. After 6, 24, and 48 h, the 5th leaf of each plant was harvested for the isolation of apoplastic washing fluid, cell sap, and vascular bundle sap and for the examination of transporter gene expression. The remaining tissues were divided into 4th leaf, older and younger shoots, and root for total P determination. No accumulation of foliar-applied P was measured in the apoplast. P was mostly taken up into the cytosol within the first 6 h and was associated with increased mRNA levels of PHT1 transporters. A strong tendency towards rapid translocation into the younger shoot and an increase in NO₃^- uptake or a decrease in organic acid translocation were observed. The apoplast seems to exert no effect on the uptake of foliar-applied P into the epidermal and mesophyll cells of intact leaves. Instead, the plant responds with the rapid translocation of P and changes in ion status to generate further growth. The effect of the absorbed foliar-applied P is assumed to be a rapid process with no transient storage in the leaf apoplast.