The Timing of Application and Inclusion of a Surfactant Are Important for Absorption and Translocation of Foliar Phosphoric Acid by Wheat Leaves

Abstract

Introduction: Foliar applied phosphorus (P) has the potential to provide a more tactical approach to P fertilization that could enhance P use efficiency. The aims of this study were to investigate the influence of adjuvant choice and application timing of foliar applied phosphoric acid on leaf wettability, foliar uptake, translocation, and grain yield of wheat plants. Materials and Methods: We measured the contact angles of water and fertilizers on wheat leaves, and the uptake, translocation and wheat yield response to isotopically-labelled phosphoric acid in combination with five different adjuvants when foliar-applied to wheat at either early tillering or flag leaf emergence. Results: There was high foliar uptake of phosphoric acid in combination with all adjuvants that contained a surfactant, but only one treatment resulted in a 12% increase in grain yield and two treatments resulted in a decrease in grain yield. Despite the wettability of all foliar fertilizers being markedly different, foliar uptake was similar for all treatments that contained a surfactant. The translocation of phosphorus from foliar sources was higher when applied at a later growth stage than when applied at tillering despite the leaf surface properties that affect wettability being similar across all leaves at both growth stages. Discussion: Both the timing of foliar application and the inclusion of a surfactant in the formulation are important for absorption and translocation of phosphoric acid by wheat leaves, however high foliar uptake and translocation will not always translate to a yield increase.

Keywords: adjuvant; foliar uptake; phosphorus; surfactant; wettability.

Figure 1

Average scorch score for adjuvants and timing; there was no significant adjuvant by timing interaction, Treatments: Gl-Glycerol, L-LI 700^®, T-Triton™ X-100, A-Agral^® and G- Genapol^® X-080. Statistical differences between average scorch score for adjuvant (p ≤ 0.05, l.s.d. 0.27) and timing (p ≤ 0.05, l.s.d. 0.17) indicated on graph with different letters.

Figure 2

Scanning electron microscope images of the adaxial side of wheat leaves: (A–C) at early tillering Z21, (D–F) and at flag leaf emergence Z39. (A and D) leaf on first tiller, (B and E) Leaf 2 from main stem base, (C and F) Leaf 3 from main stem base; scale bar = 100 µm.

Figure 3

Average (A) advancing and (B) receding contact angle on adaxial side of fully expanded wheat leaves (tiller and main stem leaves) at 20 s for water and each of the adjuvants at both foliar timings (+/- standard deviation), Treatments: W-water, Gl-Glycerol, L-LI 700^®, T-Triton™ X-100, A-Agral^® and G- Genapol^® X-080. Statistical differences between advancing contact angles indicated on graph with different letters (p ≤ 0.05, l.s.d. 3.95).

Figure 4

Source of P taken up by above-ground plant parts. Treatments: C-control, Gl-Glycerol, L-LI 700^®, T-Triton™ X-100, A-Agral and G- Genapol^® X-080. Statistical differences between foliar P treatments (at both times) indicated on graph with different letters (p ≤ 0.05 l.s.d 0.37).

Figure 5

Translocation of foliar P to above-ground plant parts as a percentage of applied fertiliser; (A) total translocation and translocation to grain vs. the other plant parts, (B) expansion of translocation to other plant parts/Treatments: C-control, Gl-Glycerol, L- LI 700^®, T-Triton™ X-100, A-Agral^® and G- Genapol^® X-080. Statistical differences within a factor and plant part for foliar P translocation indicated on graph with different letters (p ≤ 0.05; for adjuvant: total translocation l.s.d. 6.0, grain l.s.d. 5.2, other leaves n.s., chaff l.s.d. 0.6, stems l.s.d. 0.3; for timing: total translocation l.s.d. 3.8, grain l.s.d. 3.3, other leaves n.s., chaff l.s.d. 0.4, stems l.s.d. 0.2)