During the ripening stage of barley and rice, the sink activity was defined as the dry matter increase per units sink size, leaf area and time, as follows:. NAR = A.SinkW+NAR',where NAR is the net assimilation rate (g d.wt dm-2d-1); A is the sink activity [g d.wt g-1d.wt (ear) dm-2 d-1]; Sink W is ear wt per plant at heading (g d.wt); and NAR' is net assimilation rate excluding the assimilate of sink organ (g d.wt dm-2 d-1).Plant material with 16 combinations of mutually different sink (ear) and source (leaf) size were produced at heading for each crop: parts of each leaf and ear were removed to produce four grades in barley, and also a part of each leaf was removed producing four grades for four rice varieties showing different ear size. NAR and NAR' were determined during 26 and 21 d in barley and rice after heading, respectively. Sink activity (A), represented as the assimilation rate induced by the sink organ, was estimated from the relationship between SinkW and NAR using the previous equation. The sink activity was significantly higher in rice with a value of 0-0.028 g d.wt g-1 d.wt (ear) dm-2 d-1 vs. 0-0.0017 in barley, suggesting that the relative role of leaves for grain filling is considerably higher in rice than in barley. The sink activity obtained in the study might be introduced into a model to predict the yields of barley and rice. © 1990 Annals of Botany Company.
