Spectroscopic Analyses Highlight Plant Biostimulant Effects of Baker's Yeast Vinasse and Selenium on Cabbage through Foliar Fertilization

Abstract

The main aim of this study is to find relevant analytic fingerprints for plants' structural characterization using spectroscopic techniques and thermogravimetric analyses (TGAs) as alternative methods, particularized on cabbage treated with selenium-baker's yeast vinasse formulation (Se-VF) included in a foliar fertilizer formula. The hypothesis investigated is that Se-VF will induce significant structural changes compared with the control, analytically confirming the biofortification of selenium-enriched cabbage as a nutritive vegetable, and particularly the plant biostimulant effects of the applied Se-VF formulation on cabbage grown in the field. The TGA evidenced a structural transformation of the molecular building blocks in the treated cabbage leaves. The ash residues increased after treatment, suggesting increased mineral accumulation in leaves. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) evidenced a pectin-Iα-cellulose structure of cabbage that correlated with each other in terms of leaf crystallinity. FTIR analysis suggested the accumulation of unesterified pectin and possibly (seleno) glucosinolates and an increased network of hydrogen bonds. The treatment with Se-VF formulation induced a significant increase in the soluble fibers of the inner leaves, accompanied by a decrease in the insoluble fibers. The ratio of soluble/insoluble fibers correlated with the crystallinity determined by XRD and with the FTIR data. The employed analytic techniques can find practical applications as fast methods in studies of the effects of new agrotechnical practices, while in our particular case study, they revealed effects specific to plant biostimulants of the Se-VF formulation treatment: enhanced mineral utilization and improved quality traits.

Keywords: Fourier-transform infrared spectroscopy—FTIR; X-ray diffraction—XRD; glycine betaine; molecular fingerprints; plant cell wall; plant response; soluble and insoluble fibers; spectroscopic techniques; thermo-gravimetric analyses—TGAs.

Figure 1

Photosynthetic activity of cabbage outer leaves (o) at 4 weeks after the first treatment and 2.5 weeks after the second treatment: (a) chlorophyll fluorescence, (b) stomatal conductance; Co - control outer leaves, D1o - dose 1 (1 L/ha) outer leaves, D2o - dose 2 (3 L/ha) outer leaves (±error bars, n = 6 for Y(II), and n = 5 for stomatal conductance; significance level α = 0.05. Different letters indicate significant differences between samples; double letters indicate statistical similarities between samples).

Figure 2

Thermo-gravimetrical analyses of (a) MCC- microcrystalline cellulose; (b) Pct-pectin; (c) Lgn-lignin; (d) Ci-control cabbage inner leaves; (e) Co - control cabbage outer leaves; (f) D1i - dosage 1 on inner leaves; (g) D1o - dosage 1 on outer leaves; (h) D2i - dosage 2 on inner leaves; (i) D2o—dosage 2 on outer leaves.

Figure 2

Thermo-gravimetrical analyses of (a) MCC- microcrystalline cellulose; (b) Pct-pectin; (c) Lgn-lignin; (d) Ci-control cabbage inner leaves; (e) Co - control cabbage outer leaves; (f) D1i - dosage 1 on inner leaves; (g) D1o - dosage 1 on outer leaves; (h) D2i - dosage 2 on inner leaves; (i) D2o—dosage 2 on outer leaves.

Figure 3

X-ray diffractograms of cabbage samples and standards: C—cabbage control (i,o); D—dosage (1,2); i—inner leaves; o—outer leaves; MCC—microcrystalline cellulose; Pct—pectin; Lgn—lignin. (a) Smoothed B-spline diffractograms translated and overlaid to evidence the convolution of MCC, Pct, and Lgn in cabbage samples; (b) smoothed, background-subtracted and normalized diffractograms of MCC, Pct, Lgn, and Ci, along with the typical main peaks of cellulose Iα, Iβ, and amorphous cellulose; (c) normalized cabbage diffractograms together with the main peaks of cellulose Iα, Iβ, amorphous cellulose, and methylcellulose; (d) close-up of the main diffraction peak in cabbage samples, showing a small shift toward pectin’s peak 21.2° of inner leaves and biostimulant-treated D2o leaves. Crystallinity degrees (Xc, %) are presented in (a,c), while all the amorphous deconvoluted peaks are presented as a small table in the left-down corner of (a).

Figure 4

Fiber type and content in cabbage leaves: (a) soluble fibers; (b) insoluble fibers Ci—control cabbage inner leaves; Co—control cabbage outer leaves; D1i—dosage 1 (1 L/ha) on inner leaves; D1o—dosage 1 on outer leaves; D2i—dosage 2 (3 L/ha) on inner leaves; D2o—dosage 2 on outer leaves. (±error bars, n = 3, significance level α = 0.05, different letters indicate significant differences between samples).

Figure 5

Elemental analysis and protein content after 2.5 weeks following the second treatment: (a) carbon (C), hydrogen (H), nitrogen (N) content, (b) C/N ratio; (±standard deviation, n = 3, α = 0.05); Ci - control cabbage inner leaves; Co - control cabbage outer leaves; D1i - dosage 1 (1 L/ha) on inner leaves; D1o - dosage 1 on outer leaves; D2i - dosage 2 (3 L/ha) on inner leaves; D2o - dosage 2 on outer leaves. (±error bars, n = 3, significance level α = 0.05, different letters indicate significant differences between samples, double letters indicate statistical similarities between samples).

Figure 6

FTIR spectra of commercial microcrystalline cellulose (MCC), pectin (Pct), and lignin (Lgn).

Figure 7

FTIR spectra of cabbage samples foliar-treated with two dosages of Se-BNF, D1 - 1L/ha (code 1), and D2 - 3L/ha (code 2) in comparison with the control inner leaves (Ci) and outer leaves (Co): (a) cabbage inner leaves (Ci, D1i, and D2i); (b) cabbage outer leaves (Co, D1o, and D2o).