Are awns truly relevant for wheat yields? A study of performance of awned/awnless isogenic lines and their response to source-sink manipulations

The introgression of awns as a source of photoassimilates during grain filling has received has long been of particular attention in wheat. Although the benefits for average grain weight (AGW) seem consistent, those for yield still remain unclear, and the causes for the improved AGW have not been examined. In this study we carried out three field experiments. In two of them, one in CIMMYT (Mexico) and the other in Lleida (Spain), we compared awned-awnless near isogenic lines (NILs) subjected to defoliation and degraining during the effective period of grain filling, whilst in another experiment we analysed the responses of modern cultivars to awn detachment 10 days after anthesis under two nitrogen levels. There were not consistent differences in yield between the awned-awnless NILs neither across the locations nor across genetic backgrounds. This lack of effect on yield was associated with a compensatory effect of the presence of awns on decreasing grain number while increasing AGW. The manipulation treatments did not consistently affect AGW neither in the NILs nor in the modern cultivars. Focusing on the responses of the NILs, firstly there were some statistically significant responses in AGW but they were both mostly very small and inconsistent between defoliation and degraining treatments as well as between genetic backgrounds. Secondly these responses failed to be consistently more noticeable in the awnless than in the awned lines (as expected if the presence of awns increases AGW through increasing photosynthetic capacity during grain filling). The increase on AGW due to the presence of awns seemed ascribed to both an indirect and a direct effect. The former would simply reflect the consequence of awns reducing grain number: this would naturally be the consequence of an increased failure of distal florets to become fertile which would have reduced the proportion of grains constitutively smaller as well. A direct true effect on the capacity of the grains to grow also emerged with the analysis of individual size of each particular grain for the awnedawnless NILs revealing a constitutively higher potential size in the awned NILs, even for the largest grains.