Heading Date Is Not Flowering Time in Spring Barley

Understanding the stages of floral development is one of the major goals of crop breeding to produce new varieties that are better adapted to environmental cues with improved yield. The phase transition is a quantitative trait that is predominantly genetically controlled with a complex genetic network integrating endogenous and environmental factors. In recent years, it has become evident that the heading date in small grain cereal crops is one important stage that has been extensively studied and highly associated with environmental adaptation and yield. Heading date has a complex genetic architecture that makes it a target trait in barley (Hordeum vulgare L.) breeding programs. Fine adjustment of heading date is important for understanding other developmental traits such as leaf area, plant height, tillering, and grain number (Li et al., 2006; Alqudah et al., 2016). In addition, it is also considered as a decisive stage for improving yield and yield components (Esparza Martínez and Foster, 1998; Li et al., 2006; Cuesta-Marcos et al., 2009; Pasam et al., 2012). The timing of heading in barley has a substantial impact on range-wide eco-geographical adaptation and improving the yield, which is clearly shown in accessions from North-Western Europe and North America (with reduced response to long days), that are late in heading (Turner et al., 2005). This adaptation habit allows the barley plants to extend their vegetative phase that in turn increases biomass accumulation and grain yields (Turner et al., 2005). However, there appears to be confusion among barley researchers when the barley inflorescence (i.e., spike) shows the heading [Zadoks, Z50–Z59 (spike out of the flag leaf sheath), Figure 1B] and awn tipping appearance [(Z49), Figure 1A]. Such confusion can ultimately lead to inaccurate phenotypic results and interpretation especially in the context of identified stage-specific QTL/transcriptomes, or underlying genes.