Barley awn dimensions and barbs changes under terminal drought stress and its relation to grain yield and carbon isotope discrimination

Awns illustrate an important photosynthetic organ of the cereal spike, especially under water stress. Barley (Hordeum vulgare L.) spikes are distinguished by long awns; nevertheless, the actual changes in barley awn dimensions under drought stress have not been adequately studied. In the present work, six barley genotypes with varying degrees of drought tolerance were investigated in a 2-year field experiment under well-watered (WW) and terminal drought stress (TDS) conditions based on randomized complete block design with three replications. The results showed that although TDS caused a notable reduction in awn dimensions compared to WW, the density of barbs on the awn surface significantly increased in all barley genotypes. In addition, the barb contact angle decreased significantly under TDS conditions compared to WW. Awn area had a significantly positive correlation with thousand grain weight and grain yield in both treatments and growing seasons. Also, highly significant correlations were observed between leaf stomatal conductance, awn area, and barb density under both conditions. In TDS, two genotypes demonstrated lower carbon isotope discrimination: Morocco, with higher awn dimensions, and PBYT 46, with a higher increase in barbs density on the awn surface and a lower canopy temperature. This research underlines barbs density on the awn surface by reflecting the light and a lower increase in the canopy temperature plays a more pivotal role than awn dimensions on water use efficiency in barley under terminal drought.