Grain growth and development in wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.): Coordination between water content and source/sink ratio

Understanding grain weight determination and its limitations is relevant for increasing grain yield in crops. Reasons behind variations in grain weight could be related to its potential or to competition among grains for insufficient source. Previous studies in old genotypes showed that wheat grain weight was not source limited during grain filling, while no clear pattern was determined for barley. The aim of this study was to assess the coordination between grain water and grain dry matter content to determine the extent of source limitation during grain filling in modern wheat and barley genotypes. Two databases containing grain dry matter and grain water content of wheat and barley throughout the grain-filling period and grain weight subjected to different source/sink manipulation treatments was compiled and analyzed. Final grain weight was predicted from grain water content early during grain filling under optimal and stressful conditions. A quantitative approach for determining the magnitude of changes in grain weight in response to source/sink manipulations during grain filling was applied in wheat and barley. Grain water content measured at the end of the lag phase was a good predictor of grain weight under non-stressful conditions in wheat (nRMSE= 13.2%) and barley (nRMSE= 13.5%) crops. Under abiotic or biotic stressful conditions, the grain water content was unable to explain the variations in the grain weight due to a developmental and/or growing failure affecting final grain weight (nRMSE = 13.3–53.8%) suggesting a degree of source limitation. Artificial increases in source/sink ratio slightly increased wheat (8%) and barley (11%) grain weight. Theoretical reductions in the source/sink ratio of 100% reduced final grain weight 59% in wheat and 56% in barley relative to the control. Also, modern wheat genotypes showed a greater grain weight response under reductions of assimilates availability than old genotypes. Final grain weight in wheat and barley can be predicted early in development only under non-stressful conditions. Wheat and barley showed high stability in grain weight when the source-sink ratio was increased, however, reductions in the source-sink ratio negatively impacted in final grain weight with a similar magnitude in both species. Reductions in grain weight due to limitations in the availability of assimilates, such as biotic or abiotic stresses, could result in significant grain yield reductions in modern genotypes.