The Structure of the Barley Husk Influences Its Resistance to Mechanical Stress

Abstract

This paper explores the links between genotype, plant development, plant structure and plant material properties. The barley husk has two organs, the lemma and the palea, which protect the grain. When the husk is exposed to mechanical stress, such as during harvesting, it can be damaged or detached. This is known as grain skinning, which is detrimental to grain quality and has a significant economic impact on industry. This study focused on the lemma, the husk organ which is most susceptible to grain skinning. This study tested three hypotheses: (1) genotype and plant development determine lemma structure, (2) lemma structure influences the material properties of the lemma, and (3) the material properties of the lemma determine grain skinning risk. The effect of genotype was investigated by using plant material from four malting barley varieties: two with a high risk of grain skinning, two with a low risk. Plant material was assessed at two stages of plant development (anthesis, GS 65; grain filling, GS 77). Structure was assessed using light microscopy to measure three physiological features: thickness, vasculature and cell area. Material properties were approximated using a controlled impact assay and by analyzing fragmentation behavior. Genotype had a significant effect on lemma structure and material properties from anthesis. This indicates that differences between genotypes were established during floral development. The lemma was significantly thinner in high risk genotypes, compared to low risk genotypes. Consequently, in high risk genotypes, the lemma was significantly more likely to fragment. This indicates a relationship between reduced lemma thickness and increased fragmentation. Traditionally, a thin husk has been considered beneficial for malting quality, due to an association with malt extract. However, this study finds a thin lemma is less resistant to mechanical stress. This may explain the differences in grain skinning risk in the genotypes studied.

Keywords: barley; crop improvement; grain quality; grain skinning; plant biomechanics; plant physiology.

Figure 1

Minimum lemma thickness. Graph displays the mean thickness and 95% CI. 95% CIs calculated using $Z_{\frac{\alpha }{2}}\times$ standard error of the mean. N = 6 for each Golden Promise × growth stage combination; N = 10 for all other variety × growth stage combinations. Differences between varieties and growth stages were assessed using an ANOVA test followed by a Tukey's HSD test. This is summarized by annotating the graph with letters; varieties and growth stages which do not share a letter are significantly different from each other. High risk varieties are shown in red, low risk varieties are shown in blue.

Figure 2

Maximum lemma thickness. Graph displays the mean thickness and 95% CI. 95% CIs calculated using $Z_{\frac{\alpha }{2}}\times$ standard error of the mean. N = 6 for each Golden Promise × growth stage combination; N = 10 for all other variety × growth stage combinations Differences between varieties and growth stages were assessed using an ANOVA test followed by a Tukey's HSD test. This is summarized by annotating the graph with letters; varieties and growth stages which do not share a letter are significantly different from each other. High risk varieties are shown in red, low risk varieties are shown in blue.

Figure 3

Vascular bundle diameter. Measured in the lemma. Graph displays the mean diameter and 95% CI. 95% CIs calculated using $Z_{\frac{\alpha }{2}}\times$ standard error of the mean. N = 6 for each Golden Promise × growth stage combination; N = 10 for all other variety × growth stage combinations. Differences between varieties and growth stages were assessed using an ANOVA test followed by a Tukey's HSD test. This is summarized by annotating the graph with letters; varieties and growth stages which do not share a letter are significantly different from each other. High risk varieties are shown in red, low risk varieties are shown in blue.

Figure 4

Cell cross-sectional area. Sclerenchymal cells in the lemma measured. Graph displays the mean area and 95% CI. N = 25 for Golden Promise; N = 50 for all other varieties. Differences between varieties and growth stages were assessed using an ANOVA test followed by a Tukey's HSD test. This is summarized by annotating the graph with letters; varieties and growth stages which do not share a letter are significantly different from each other. High risk varieties are shown in red, low risk varieties are shown in blue.

Figure 5

Fragment number. Graph displays a histogram of fragment number. N = 10 impact assays per variety × growth stage combination. High risk varieties are shown in red, low risk varieties are shown in blue.

Figure 6

Differences in mean fragment number λ between varieties at GS 65. High risk varieties had higher estimates of λ suggesting that, during impact, more fragments formed. The difference in λ between low risk and high risk varieties were calculated. Graphs show the posterior distribution on the differences between λ, including the mean difference and the 95% HDI of the mean difference. All differences were significantly <0. N = 10 impact assays per variety × growth stage combination.

Figure 7

Differences in mean fragment number λ between varieties at GS 77. High risk varieties had higher estimates of λ suggesting that, during impact, more fragments formed. The difference in λ between low risk and high risk varieties were calculated. Graphs show the posterior distribution on the differences between λ, including the mean difference and the 95% HDI of the mean difference. All differences were significantly <0. N = 10 impact assays per variety × growth stage combination.

Figure 8

Fragment area. Graph displays a histogram of fragment areas. N = 10 impact assays per variety × growth stage combination. High risk varieties are shown in red, low risk varieties are shown in blue.

Figure 9

Differences in mean fragment area μ at GS 65. High risk varieties had lower estimates of μ suggesting that, during impact, smaller fragments formed. The differences in μ between low risk and high risk varieties were calculated. Graphs show the posterior distribution on the differences between μ, including the mean difference and the 95% HDI of the mean difference. All differences were >0, with one exception; the difference between Henni and Propino was not significant at the 0.95 level, although the posterior probability of the difference being >0 was 0.92. N = 10 impact assays per variety × growth stage combination.

Figure 10

Differences in mean fragment area μ at GS 77. High risk varieties had lower estimates of μ suggesting that, during impact, smaller fragments formed. The differences in μ between low risk and high risk varieties were calculated. Graphs show the posterior distribution on the differences between μ, including the mean difference and the 95% HDI of the mean difference. All differences were >0, with one exception; the difference between Golden Promise and Propino was not significant at the 0.95 level, although the posterior probability of the difference being >0 was 0.94. N = 10 impact assays per variety × growth stage combination.