Geographic structuring among landraces for variation in key traits was only significant for leaf length (FLL) whereas variation in leaf width (FLW), height (PH) and tillering (NE) was explained by ear-row type. Furthermore, we identified several agronomic traits in different geographic regions or landraces groups that were associated with grain yield, which could be used in yield selection in (pre-) breeding programs.
Agronomic trait differences between landrace groups
We found a number of agronomic traits showing significant differences between groups based on origin and row type.
One of the agronomic traits that indicated a strong geographic influence was FLL where northern and western European varieties (N2, N6, W2 and W6) had longer leaves compared to other regions. This was confirmed in an earlier study on light interception in European barley landraces, which included a subset of the current study material, although there was no difference between specific leaf areas (Florence et al., 2019).
FLW was mostly determined by row type where 6 rows were wider. A study on leaf width and grain number explained a relationship through a genetic component Vrs1 which is also related to row type (Digel et al., 2016). The 6 row Harlan group (H6) though, showed the narrowest leaves but also relatively low seed numbers compared to other 6 rows and this could be related to their underlying genetic Vrs1 variant.
Modern varieties (CU2) in our study showed the shortest plants compared to the heritage varieties which have been developed using dwarfing genes in breeding programmes (Mickelson & Rasmusson, 1994). This has led to a smaller stature including several correlated plant traits including leaf size (Niu et al., 2022). Within the heritage varieties, six row types were shorter than 2 row barley which was not supported by Alqudah et al. (2016) which only found significant differences in height between wild and cultivated barley types.
TGW was highest for modern varieties, indicating the breeding programs optimising this for malting varieties. Harlan varieties showed the highest TGW for 6 rows which would be explained by the trade-off of their relatively low number of SE (Gambín & Borrás, 2010).
EN was higher for 2 rows compared to 6 row barleys, which would be explained by higher tillering capacity. Tillering has a strong environmental component (Shaaf et al., 2019) which is more expressed in 2 row barley compared to 6 row barley. In our study there was not much difference between different groups overall which was unexpected for the modern varieties (Liller et al., 2015). This would also explain the lack of differences between groups for GY although using small sized plots might have accentuated this.
Association between agronomic traits and yield
We found significant correlations between NE and GY for all groups but N2. All groups showing a significant and relatively high correlation (r2) coefficient, with an r2 of around 0.45 but this was lower than another study using modern varieties under similar growing conditions (Tanaka & Nakano, 2019, r = 0.67).
SE were significantly correlated to GY for 6 row groups which is similar for former studies (Arisnabarreta & Miralles, 2006). Environmental responses differ between these two barley types where two rows maintain seed production through variation in tiller number and seeds per ear whereas in six rows the variation is in the seeds per ear and TGW and the tiller number is stable (Arisnabarreta & Miralles, 2008; Liller et al., 2015). Identifying 2 and 6 row lines that show plasticity in both yield components would improve yield stability under varying environmental conditions.
We also found that PH showed a significant correlation with grain yield in 6 row barley groups but also N2, which was unexpected. Only few studies found height as a factor related to grain yield (Jui et al., 1997; Costa & Boller, 2001) and this was mainly in six row barley types. The strong correlation between PH and GY in six rows was unexpected as the opposite would be expected with dwarfing genes increasing yield by optimising the harvest index (Xu et al., 2017). Nevertheless, this correlation may be explained by the underlying genetic interactions between genes involved in ear development (Vrs1) and height which seem to interact together with heading time and tillering (Thirulogachandar et al., 2017).
There was a correlation between FLW and SE in some 2 and 6 rowed barleys which was consistent with the work of Thirulogachandar et al. (2017) who found a positive relationship between leaf width and grain number. This association may be explained by the fact that the Vrs1 gene, which underlies row type in barley, is also linked to the development of flag leaf width and longitudinal vein number through rapid growth of leaf primordial cells increasing the size of leaf primordia (Thirulogachandar et al., 2017).
Contrary to our study, a study by Du et al. (2019), using a cross between a 2 and 6 row modern line, found a negative correlation between seeds per plant and flag leaf length but had not included FLW. Further studies would look at yield prediction for both grain and biomass in breeding programmes to use early agronomic traits to predict yield in varieties.
Association between agronomic traits and TGW
TGW showed a positive relationship with almost all traits for both 2 and 6 row barleys where EL only showed a relationship with 2 row barleys and FLL showed no significant relationship. Our results showed a non significant negative correlation between TGW and FLL, which is contrary to Du et al (2019) who found a positive correlation between TGW and FLL.
The relationship of TGW with SE and EL seems like a trade-off and sink limited and another study using diverse material (Sharma et al., 2018) also found a very small relation between TGW and SE. Contrary to our study to our study, Hadjichristodoulou (1990) found a negative relationship for 2 row barley between TGW and number of ears per plant. The same study found that in 6-row barley, TGW was positively correlated with PH, which was confirmed by our study, but dissimilar to our results, they found a negative relation with NE and SE. Interestingly, in our study there is a positive correlation between TGW and SE for all groups, although only significant for two rows, where a negative trade-off between these traits would be expected due to resource limitation. Another study, using modern material, also found that TGW seems related to number of fertile shoots in early growth and final ear number (Bingham et al., 2007). Six row barley are less sink limited than two row barley, due to the higher number of seeds per ear, but for E6, W6 and N6 there is a relationship between TGW and ear number indicating that there might be variation between 6 row backgrounds for sink limitation. In 2 row barley number of grains per ear is relatively stable in modern varieties where yield is set early on by tiller number. In six row barleys there is a similar effect as wheat where florets are compensated by grain size across the ear (Kennedy et al., 2016). A study using row type mutants (Liller et al., 2015), found genotypic variation in ability to produce tillering and inflorescence numbers suggesting trade off relations between different traits.
Linking traits to grain yield and TGW
The stepwise regression analysis for GY per group (Table 1) showed that GY is mostly influenced by one or two traits for modern and NW European material (CU2, N6, W2, W6) whereas E European, Harlan and N2 groups are influenced by at least two traits (E2, N2, E6, H6). In terms of the coefficient of determination (R2), particularly N6 and W6 explained a high proportion of the variance (R2 = 0.78 and 0.76) respectively by the indicated variables (NE and FLW). Other studies had found that yield in modern 2 row barley varieties had particularly improved yield through height reduction, TGW and tillering (Nadolska-Orczyk et al., 2017) but also spike morphology and flowering time (Sharma et al., 2018).
Most groups, both 2 and 6 row barley, were influenced by NE in terms of GY although FLW was also influential. Other studies (Liller et al., 2015; Schaaf et al., 2019) found that grain yield in 2 row varieties is mostly determined by tillering and grains per ear for 6 row in barley. This study showed a much larger effect of NE on GY in 6 row barleys. FLW plays an important role in most groups as well in relation to GY. The coefficient of determination (R2) for the modern cultivars (CU2, R2 = 0.51) was lower than a study using similar material (Kennedy et al., 2016, R2 = 0.62) in their model to predict yield, although in our study some of the other groups showed similar or higher R2 values. These latter groups from our study similarly showed ear number, but also FLW or SE, as a factor to predict GY.
The stepwise regression analysis approach showed that multiple traits influenced TGW in all groups. TGW in 2 row barleys seem to be mostly influenced by a combination of SE/EL, FLL/FLW and PH whereas the 6 rows by NE and FLW/FLL. The higher amount of traits influencing 2 row compared to 6 row barley groups is surprising given the higher uniformity and stability of 2 row barley compared to 6 row seeds in other studies (Schwarz & Li, 2010; McKim et al., 2018). Further research into the size of each effect would be useful to improve understanding of stability of TGW in barley for end use.
Applications
Landraces and CCPs housed in genebanks are a rich source of genetic material for crop improvement where yield related correlations between agronomic traits and yield components can be exploited under high yielding growing conditions (Dwidevi et al., 2016; 2017). However, these resources are largely untapped. Identification of novel variation and the selection of specific landraces (genotypes) for relationships between desirable traits and yield components under appropriate testing systems as described herein is a key next step to support development of novel crop cultivars. Large field screenings of genebank material over different locations and years can be used to identify useful variation for pre-breeding applications. Our results indicate that barley resources from different geographic regions may have value in pre-breeding of barley for specific growing conditions. Our findings showed that landraces among different geographic groups had novel relationships between agronomic traits and yield components even compared to elite cultivars. Novel correlations include correlations between GY and FLW but also PH in certain European 2 and 6 rows groups, strong correlations between NE, PH and SE for some 6 rows group and NE and TGW for most 2 and 6 row groups A small number of individual lines from different groups showed novel trait relationships and will be subject of further research.