The impact of drought on wheat leaf cuticle properties

Abstract

Background: The plant cuticle is the outermost layer covering aerial tissues and is composed of cutin and waxes. The cuticle plays an important role in protection from environmental stresses and glaucousness, the bluish-white colouration of plant surfaces associated with cuticular waxes, has been suggested as a contributing factor in crop drought tolerance. However, the cuticle structure and composition is complex and it is not clear which aspects are important in determining a role in drought tolerance. Therefore, we analysed residual transpiration rates, cuticle structure and epicuticular wax composition under well-watered conditions and drought in five Australian bread wheat genotypes, Kukri, Excalibur, Drysdale, RAC875 and Gladius, with contrasting glaucousness and drought tolerance.

Results: Significant differences were detected in residual transpiration rates between non-glaucous and drought-sensitive Kukri and four glaucous and drought-tolerant lines. No simple correlation was found between residual transpiration rates and the level of glaucousness among glaucous lines. Modest differences in the thickness of cuticle existed between the examined genotypes, while drought significantly increased thickness in Drysdale and RAC875. Wax composition analyses showed various amounts of C31 β-diketone among genotypes and increases in the content of alkanes under drought in all examined wheat lines.

Conclusions: The results provide new insights into the relationship between drought stress and the properties and structure of the wheat leaf cuticle. In particular, the data highlight the importance of the cuticle's biochemical makeup, rather than a simple correlation with glaucousness or stomatal density, for water loss under limited water conditions.

Keywords: Cuticular wax; Glaucousness; Residual transpiration rate; Stomatal density; Triticum aestivum; β-diketone.

Fig. 1

Water loss rates of flag leaves detached from five wheat lines grown under well-watered (WW) conditions and drought (DR). Water loss rates of flag leaves detached from (a) WW and (b) DR plants over 12 h. c Comparison of water loss rates of WW and DR plants. KUK - Kukri, EXC - Excalibur, DRS - Drysdale, RAC - RAC875, GL - Gladius. Water loss rates were expressed as weight loss per hour per dry weight (DW) of flag leaves. Means and standard errors (indicated by bars) were calculated from four replicates

Fig. 2

Images of stomata on the epidermis of flag leaves of investigated wheat lines. a Stomata on the adaxial side of a flag leaf in Kukri. Stomata are indicated by arrows. b Stomata on the adaxial side of a flag leaf in Gladius. c Stomata on the abaxial side of a flag leaf in Kukri. d Stomata on the abaxial side of a flag leaf in Gladius. e Calculated stomatal density on the adaxial side of flag leaves. f Calculated stomatal density on the abaxial side of flag leaves. KUK - Kukri, EXC - Excalibur, DRS - Drysdale, RAC - RAC875, GL - Gladius, WW - well-watered conditions, DR - drought. Means and standard errors were calculated from five plants. Different letters on top of error bars indicate significant difference at P < 0.05. Scale bars represent 50 μm

Fig. 3

Transmission electron micrographs of cuticle layers on the abaxial epidermis of flag leaves in five wheat lines. KUK - Kukri, EXC - Excalibur, DRS - Drysdale, RAC - RAC875, GL - Gladius, WW - well-watered conditions, DR - drought. Cuticle layers are marked using white lines. Scale bars = 100 nm

Fig. 4

Visualisation of epicuticular waxes on the flag leaves of five wheat lines. a Wax phenotypes on the adaxial (Ad) and abaxial (Ab) sides of flag leaves of Kukri, Excalibur, Drysdale, RAC875 and Gladius. b Scanning electron micrographs of the epicuticular waxes on the adaxial (Ad) and abaxial (Ab) sides of flag leaves of Kukri, Excalibur, Drysdale, RAC875 and Gladius. Scale bars = 20 μm

Fig. 5

GC-MS chromatograms representing biochemical compositon of extracted cuticular waxes and SEM images of wax crystalloids on the abaxial side of the flag leaf in the five wheat lines examined. ALC - primary alcohol, ALK – alkane, FA - fatty acid, IS – internal standard. Scale bars = 5 μm. Arrows indicate C31 β-diketone

Fig. 6

Wax amounts and composition on flag leaves of wheat lines grown under well-watered (WW) conditions and drought (DR). a Total wax loads. b Contents of each wax species. EXC - Excalibur, DRS - Drysdale, GL - Gladius. Means and standard errors (indicated by bars) were calculated from three replicates. Different small letters on top of error bars indicate significant differences at P < 0.05. Wax loads were calculated per gram of dry leaf weight (DW)

Fig. 7

Carbon chain length distribution of major cuticular wax classes on flag leaves of wheat lines grown under well-watered (WW) conditions and drought (DR). a Carbon chain lengths of primary alcohols. b Carbon chain length of β-diketones. c Carbon chain lengths of alkanes. EXC - Excalibur, DRS - Drysdale, GL - Gladius. Means and standard errors (indicated by bars) were calculated from three replicates. Wax loads were calculated per gram of dry leaf weight (DW). Tiny amounts of 20-, 22- and 34-carbon primary alcohols, and 23-carbon alkane are not shown but used for calculation of total wax loads in Fig. 6

Fig. 8

SEM images of abaxial flag leaf surfaces of cv. Drysdale grown under well-watered and drought conditions at 1500X magnification (left column) and 8000X magnification (right column)