Recent 1D and 2D TD-NMR Pulse Sequences for Plant Science

Abstract

Time domain nuclear magnetic resonance (TD-NMR) has been widely applied in plant science in the last four decades. Several TD-NMR instruments and methods have been developed for laboratory, green-house, and field studies. This mini-review focuses on the recent TD-NMR pulse sequences applied in plant science. One of the sequences measures the transverse relaxation time (T₂) with minimal sample heating, using a lower refocusing flip angle and consequently lower specific absorption rate than that of conventional CPMG. Other sequences are based on a continuous wave free precession (CWFP) regime used to enhance the signal-to-noise ratio, to measure longitudinal (T₁) and transverse relaxation time in a single shot experiment, and as alternative 2D pulse sequences to obtain T₁-T₂ and diffusion-T₁ correlation maps. This review also presents some applications of these sequences in plant science.

Keywords: CWFP; Carr-Purcell-Meiboom-Gill (CPMG); relaxation measurement: pulse sequence; time domain NMR.

Figure 1

Diagram for the CPMG pulse sequence (θ = π) and for the CPMG with a low flip refocusing pulse (θ ≤ π).

Figure 2

Experimental CPMG signals using refocusing angle pulse of π, 3π/4, π/2, and π/4 for soybean oil in different homogeneities (Δν = 15 Hz (A,B) and 100 Hz (C,D)), τ = 0.1 ms (left) and 0.4 ms (right). Adapted from publication [16]. Copyright (2011), with permission from Elsevier.

Figure 3

(A) CPMG signals and (B) its inverse Laplace transform (ILT) obtained from castor bean seed (oil signal) using π (red line) and π/2 (black line) as a refocusing pulse. The dashed-dotted line in (B) is the guide to the signal center.

Figure 4

Correlation between T₂ values obtained with the CPMG and LRFA–CPMG methods using π/2 as a refocusing and different oilseed species; r = 0.98. Adapted from Publication [16]. Copyright (2011) with permission from Elsevier.

Figure 5

Continuous wave free precession (CWFP) pulse sequences that generate a special steady state free precession (SSFP) condition in the magnetization, when Tp < T₂* < T₂.

Figure 6

NMR signals simulated numerically using T₁ = 150 ms, T₂ = 50 ms, T₂* = 0.5 ms, and different Tp values. (A) Tp = 2.9T₂*, (B,C) Tp < T₂*. The frequency offset is 8.333 kHz (A,B) and 6.666 kHz (C). Adapted from Publication [28].

Figure 7

Typical CWFP signal (magnitude) obtained from the first pulse. The dark grey region indicates the initial signal that oscillates for a time that is dependent on T₂*. When the oscillation stops, a quasi-stationary state (QSS) signal is observed (red arrow). The light grey region shows T* decay from the QSS to the steady state regime |Mss| (white region).

Figure 8

Pulse diagram for CP–CWFP (ϕ = x)/CP–CWFPx−x (ϕ = −x) pulse sequences. Adapted from publication [15]. Copyright (2015), with permission from Elsevier.

Figure 9

Pulse diagram for the CWFP–T₁ pulse sequences. Adapted from publication [14]. Copyright (2016), with permission from Elsevier.

Figure 10

Inversion recovery signal (black points) and the CWFP–T₁ signal (gray line) obtained for a water sample at 27 °C. Adapted from publication [14]. Copyright (2016), with permission from Elsevier.

Figure 11

Pulse diagram for (A) conventional IR–CPMG and (B) CPMG–CWFP–T₁ sequences.

Figure 12

T₁–T₂ correlation maps for castor beans oil at 40 °C (A,B) and green banana at 22 °C (C,D) obtained by the CPMG–CWFP–T₁ (A,C) and IR-CPMG (B,D) methods.

Figure 13

PGSE–CWFP–T₁ diagram pulse sequence to obtain fast D–T₁ correlation maps. Adapted from publication [12]. Copyright (2020) with permission from Elsevier.

Figure 14

D–T₁ correlation maps obtained by (A) IR–PGSE and (B) PGSE–CWFP–T₁ methods for asparagus stems oriented parallel (in black) and perpendicular (in red) to the PFG. Adapted from publication [12]. Copyright (2020) with permission from Elsevier.