4.2 Relationship between yield and yield components
We found that grain yield in barley was reduced by waterlogging regardless of genotype or treatment imposed (Fig. 2a, b). Yield reductions were mainly caused by reduced spikes per m2when waterlogging occurred in early phenology. WL1-3 had similar effects on grain yield and yield components, with reduction in spikes per m2. This is likely to be due to the growth stage when waterlogging treatments were imposed. WL1-3 were applied prior to/at tillering stages (ZS12.5 and ZS15); all three treatments caused reductions in tiller numbers (data not shown), such that spike numbers were reduced at the end of waterlogging for all treatments except WL4. The final number of fertile spikes at maturity depends on tiller appearance rate (Alzueta et al., 2012) and the percentage of tiller mortality (Baethgen, Christianson, & Lamothe, 1995; García del Moral & García del Moral, 1995). Thus, reduced spike number in WL1-3 was mainly a consequence of lower tillering under waterlogging similar to that seen with nutrient deficiency (Alzueta et al., 2012) and water deficits (Cossani, Slafer, & Savin, 2009).
Fewer kernels per spike under waterlogging was a function of reduced spike length (Arisnabarreta & Miralles, 2006; García del Moral & García del Moral, 1995). Westminster was the only barley genotype that did not show a reduction in kernels per spike under waterlogging treatments WL1-3. This may be because Westminster has relatively fewer grain numbers per spike than other genotypes under control conditions (Fig. 3). In contrast to spikes per m2 and kernels per spike, waterlogging induced a higher grain weight compared with controls in Franklin, Westminster and Macquarie+. The increase in grain weight under waterlogging was attributed to increased grain length in Franklin, and grain width and thickness for Westminster and Macquarie+ (Fig. 4). Fewer kernel numbers per spike induced by waterlogging could result in more assimilate for grain growth and kernel weight, thus compensating for the detrimental effects of waterlogging on other yield components to some degree. Similar effects have been observed for wheat defoliated in vegetative stages in which more assimilate is partitioned to kernels of grazed crops (Harrison et al., 2011a, 2011b).