The Effect of White Light Spectrum Modifications by Excess of Blue Light on the Frost Tolerance, Lipid- and Hormone Composition of Barley in the Early Pre-Hardening Phase

Abstract

It is well established that cold acclimation processes are highly influenced, apart from cold ambient temperatures, by light-dependent environmental factors. In this study we investigated whether an extra blue (B) light supplementation would be able to further improve the well-documented freezing tolerance enhancing effect of far-red (FR) enriched white (W) light. The impact of B and FR light supplementation to white light (WFRB) on hormone levels and lipid contents were determined in winter barley at moderate (15 °C) and low (5 °C) temperatures. Low R:FR ratio effectively induced frost tolerance in barley plantlets, but additional B light further enhanced frost hardiness at both temperatures. Supplementation of WFR (white light enriched with FR light) with B had a strong positive effect on abscisic acid accumulation while the suppression of salicylic acid and jasmonic acid levels were observed at low temperature which resembles the shade avoidance syndrome. We also observed clear lipidomic differences between the individual light and temperature treatments. WFRB light changed the total lipid content negatively, but monogalactosyldiacylglycerol (MGDG) content was increased, nonetheless. Our results prove that WFRB light can greatly influence phytohormone dynamics and lipid contents, which eventually leads to more efficient pre-hardening to avoid frost damage.

Keywords: barley; cold acclimation; light regulation; light-emitting diode (LED) lighting; lipidome; phytohormones.

Figure 1

The effects of the supplementary far-red and blue light treatments on freezing tolerance under various temperatures. The X−axis shows the freezing temperatures, whereas the Y−axis refers to the relative conductance values (percentage of lethality). The plantlets were grown under a 12 h photoperiod. W: white light, WFR: far-red-enriched white light, WFRB: far-red- and blue-enriched white light. In each case, the samples were collected on the last day of the treatments. (A) 10th day at 15 °C temperature; (B) 7th day at 5 °C. The data and error bars, which represent the standard deviation, originated from three biological replicates. Statistical analysis was performed with Kruskal−Wallis test. Values indicated with different letters are significantly different from each other at p ≤ 0.05 levels.

Figure 2

The effect of supplementary FR and Blue light at normal and low temperature on hormone concentrations. Leaf samples were collected six to eight hours after the start of illumination. The plantlets were grown under a 12 h photoperiod. (A) fraction A, eluted with methanol containing ABA, IAA, SA, and JA; (B) fraction B, eluted with 0.35 M NH4OH in 60% methanol containing CKs. W: white light, WFR: far-red-enriched white light, WFRB: far-red and blue-enriched white light. I−1 day at 15 °C, II−10 days at 15 °C, III−1 day at 5 °C, IV−15 days at 5 °C. The data originated from three to five biological replicates. The values on the X and Y axis outside of the heat map refer to the distance or proximity of data after hierarchical clustering.

Figure 3

Lipid class distribution in barley after supplementary FR and Blue light illumination with cold treatments. The samples were illuminated with white light, white light supplemented with far-red light (WFR or combined far-red with additional blue light (WFRB). In addition to light treatment, two different temperatures were used (5 and 15 °C). The samples were taken on the first and last days of the treatments, which was ten days in the case of 15 °C and seven in the case of 5 °C. In all cases, the leaf samples were cut from the central part of the second youngest leaf.

Figure 4

Changes in total lipid content after supplementary FR and Blue light illumination under cold treatments. The signal was expressed as detected signal/mg dry weight (Y axis). The samples were illuminated with white light, white light supplemented with far-red light (WFR) and combined far-red with additional blue light (WFRB). In addition to light treatment, two different temperatures were used (5 and 15 °C). The samples were taken on the first and last days of the treatments, which was ten days in the case of 15 °C and seven in the case of 5 °C. (A) 1st day at 15 °C; (B) 10th day at 15 °C; (C) 1st day at 5 °C; (D) 7th day at 5 °C. In all cases, the leaf samples were cut from the central part of the third leaf. Statistical analysis was performed with multiple-way ANOVA and Dunnett’s post-hoc test using white light (W) samples as controls. Measurements were made on 3 to 5 independent biological replicates. Significance levels are indicated by * p < 0.1 and ** p < 0.05.