Grain size variation in two-rowed malt barley under Mediterranean conditions: Phenotypic plasticity and relevant trade-offs

Abstract

In Mediterranean type environments large variability in grain plumpness, defined as the proportion of grains retained over a 2.5 mm sieve, is a critical source of uncertainty for brewers and farmers. Understanding the mechanisms that regulate grain size under stressful environments will assist breeders to promote stability and improve grain size. Using an unbiased data set ( n = 171) derived from 21 different environments (Years x Locations) and 29 European malt barley varieties, we aimed to (i) analyze the effect of genotype × environment interaction on plump grains and yield components variation from the viewpoint of phenotypic plasticity and (ii) explore the extent to which grain size associates with grain yield (GY) potential, grain number and grain protein content (GPC). Plump grains (% grains>2.5 mm), was the trait with the highest plasticity in two-rowed barley. A thorough assessment of the relationship between Thousand Kernel Weight (TKW) and the different grain size classes (i.e. > 2.8 mm; 2.5–2.8 mm; > 2.5 mm; 2.2–2.5 mm; < 2.2 mm) revealed that the stability of TKW was preserved by adjusting not only the number of seeds set, but also proportions among the different grain size classes. Our data supported the following hierarchy of plasticities: plump grains > grains/m 2 > GY > spikes/m 2 > TKW > Harvest Index > grains/spike . With decreasing GY there was also an apparent decrease in plump grains production. The number of grains/m 2 had a markedly larger negative effect on plump grains under low-yielding conditions compared to medium and high-yielding conditions. In addition, a high inherent number of grains/spike (> 26) not only had a negative effect on GY, but also undermined grain plumpness. Under the tested G × E interaction, both grain yield and plump grains were positively affected when grain yield formation was mainly based on a greater number of spikes/m 2 with fewer and heavier (≈larger) grains/spike. GPC had a relatively strong negative association with plump grains. However, when the conditions favoured plump grains formation, the risk of recording low GPC was increased. This revealed that under Mediterranean conditions the challenge is not only the high values of GPC, but the low ones as well. • Plump grains was the trait with the highest plasticity in two-rowed barley. • Direct selection for grain size under favorable conditions is questioned. • High potential of grains/spike had a negative effect on both grain yield and size. • Grain size was regulated by grain number and yield conditions. • Grain protein content was negatively associated with grain size.