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The cytoplasmic male sterility (CMS) system msm1 in barley is known to be thermosensitive, sometimes resulting in spontaneous fertility restoration in the absence of the corresponding restorer gene Rfm1. Here, we investigated genotypic differences concerning temperature sensitivity and the plant developmental stage at which elevated temperature induces spontaneous fertility restoration in three CMS mother lines. While one line stayed completely male sterile, a significantly higher fertility was observed in two lines after treatment from growth stage DC 41 until maturation. Microscopic analysis revealed that sterile anthers contained neither intact pollen, nor remains of aborted pollen grains, whereas pollen was visible in anthers of potentially fertile plants. We conclude that the barley CMS system affects anther and pollen development prior to meiosis. Elevated temperature during heading and flowering can lead to a spontaneous fertility restoration by reactivating pollen growth. Nevertheless, genotypic variation exists enabling the selection for stable CMS mother lines and the development of F1 hybrids with high hybridity. As spontaneous fertility restoration due to environmental effects is difficult to phenotype, further investigations will focus on the development of molecular markers for marker‐assisted selection.

Winter barley (Hordeum vulgare L.) can be a valuable supplement as whole plant silage for bioenergy production due to positive effects of plant cover over winter and the possibility of growing a subsequent second crop after barley. However, to be competitive with other bioenergy crops, barley dry matter yield (DMY) needs to be enhanced. Therefore, we tested a factorial of 96 barley experimental hybrids together with their parental lines for DMY, grain yield (GY), and yield‐associated traits in a multi‐environmental field plot trial with a practical focus. The average best‐parent heterosis (BPH) of GY was 7.7%, whereas average BPH of DMY was 9.1%. The higher GY of hybrids was mainly caused by the higher kernel number per ear. The variance of specific combining ability (SCA) was higher than that of general combining ability (GCA), revealing prevalence of non‐additive effects on barley DMY. Additionally, SCA values highly correlated with those of DMY and GY (r = 0.752 and r = 0.839, respectively), as well as with values of mid‐parent heterosis (MPH; r = 0.768 [DMY], r = 0.877 [GY]) and BPH (r = 0.695 [DMY], r = 0.768 [GY]). There was a correlation between the parental genetic distance and hybrid DMY, whereas no correlations were found between the parental per se and hybrid DMY and GY values. These results suggest that future hybrid breeding should be run in a separate program with a main focus on the establishment of heterotic pools and an increase in genetic diversity.

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