Sink-source relationship during rice grain filling is associated with grain nitrogen concentration

High grain nitrogen (N) concentration in crops may require to translocate more N from the vegetative tissues, cause faster plant senescence, alter sink-source balance during grain filling, and ultimately lower grain yield. Previous studies have shown that indica hybrid rice (IH) have shorter grain-filling period, and lower biomass and grain yield, compared with japonica/indica hybrid rice (JIH) and japonica conventional inbred rice (JC) in China. We hypothesise that these are caused by early senescence and poor sink-source balance after heading in IH as a result of its high grain N concentration. In this study, two JIH, two JC, and two IH varieties were grown in 2013 and 2014 at Ningbo, and in 2015 and 2016 at Yangzhou, China, and the observed dynamics of grain and crop biomass were fitted to mathematical equations to quantify sink-source relationships during grain filling. Compared with JC and IH varieties, JIH varieties showed higher sink growth and source capacity and 7.5-21.6% higher grain yield across years and sites. In JIH and JC genotypes, total source during grain filling was higher than the total sink value, whereas opposite was observed for IH genotypes. Source-sink difference, source/sink ratio, and grain-filling duration all negatively (P < 0.01) correlated with grain N concentration at maturity. Compared with JIH and JC, IH accumulated more N in grains and translocated more N from leaf during the period from heading to maturity. We conclude that early senescence and poor sink-source balance after heading of IH was linked to its high grain N concentration, and sink-source relationships during rice grain filling were associated with grain N concentration. Further breeding and cultivation programmes should aim at improving sink-related traits in JIH and JC genotypes, whereas for IH varieties, whether yield was set by sink size or by any limited availability of pre-heading reserves remains to be further examined.