Unveiling the Actual Functions of Awns in Grasses: From Yield Potential to Quality Traits

Abstract

Awns, which are either bristles or hair-like outgrowths of lemmas in the florets, are one of the typical morphological characteristics of grass species. These stiff structures contribute to grain dispersal and burial and fend off animal predators. However, their phenotypic and genetic associations with traits deciding potential yield and quality are not fully understood. Awns appear to improve photosynthesis, provide assimilates for grain filling, thus contributing to the final grain yield, especially under temperature- and water-stress conditions. Long awns, however, represent a competing sink with developing kernels for photosynthates, which can reduce grain yield under favorable conditions. In addition, long awns can hamper postharvest handling, storage, and processing activities. Overall, little is known about the elusive role of awns, thus, this review summarizes what is known about the effect of awns on grain yield and biomass yield, grain nutritional value, and forage-quality attributes. The influence of awns on the agronomic performance of grasses seems to be associated with environmental and genetic factors and varies in different stages of plant development. The contribution of awns to yield traits and quality features previously documented in major cereal crops, such as rice, barley, and wheat, emphasizes that awns can be targeted for yield and quality improvement and may advance research aimed at identifying the phenotypic effects of morphological traits in grasses.

Keywords: awns; biomass yield; forage quality; grain filling; grain quality; grain shattering; grain yield; grass species; molecular genetic factors; photosynthesis.

Figure 1

A schematic summary of awn length variation and its phenotypic roles. Morphologically, the awn is the extension of the lemma, which varies from several inches of length (as in long-awned) to total absence (as in awnless). Awnletted species have very short awns whilst awnless genotypes usually do not show awn development [8]. This classification is inconsistent and varies among species. The debate of how long awn trait has been lost in modern species of grasses compared to their wild relatives remains ongoing [1], although it has been hypothesized that long awn trait was selected against by ancient famers during domestication [2].

Figure 2

Structural characteristrics of the wheat awn. (A) Shows the spikes of awnless wheat (left) and awned wheat (right). (B) Shows the grains of awned wheat (top) and awnless wheat (down), bar = 1 cm; (C) shows the surface of the wheat awn under a scanning electron microscope, bar = 50 µm. (D) Shows a scanning electron micrograph of wheat awn cross-section, bar = 50 µm; the section of 2.5 µm was taken at 1 cm from the base of the awn; different tissues are illustrated by arrows.