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The yield of a seed crop of perennial ryegrass varies between I and 2 Mg/ha. This variation is caused by variation in the number of seeds that reach an adequate dry weight. On average 70 % of the seeds present before harvest and cleaning are not recovered because of their low weight. This suggests that seed filling and not seed set determines to a large extent seed yield. Factors influencing seed filling were studied on spaced plants in the greenhouse.Reducing the light intensity after anthesis from 115 % to 24 % (1.1 MJ/m2) showed that the amount of carbon assimilates in the reproductive tiller was not limiting to seed filling. Seed yield per ear was reduced by only 14 % and average seed dry weight by only 4 %. Although the amount of water-soluble carbohydrates (WSC in the stem + rachis was strongly reduced at 24 % light, it could have supported an extra 37 % of seed yield.New vegetative tillers that developed after anthesis did not reduce seed filling and seed yield per ear because of the large amount of WSC reserves in the stem. Regrowth of tillers cut at anthesis strongly reduced WSC stem reserves.The large variation of seed dry weight within the ear could for about 60 % be attributed to differences in rate of growth and for about 30 % to differences in duration of growth. The differences in growth rate were determined by the ovule dry weight at anthesis and not by differences in relative growth rate. The availability of assimilates and the accumulation of starch did not differ between seeds within the ear. Differences in the duration of growth were mainly caused by differences in time of anthesis, not ripening. The interaction between seeds in the ear was weak; removal of spikelets or seeds within a spikelet did not strongly affect the remaining seeds. Apparently processes in the seed itself mainly determine seed filling and seed yield of perennial ryegrass. Factors controlling ear development determine the ovule dry weight at anthesis and thus to a large exten

In greenhouse pot trials, L. perenne cv. Barlet plants were labelled with 13C at regular intervals from main spike emergence onwards in order to identify and measure the activity of source and sink organs during seed formation. The source activity of the various tiller groups within the plant roughly reflected the relative contributions of these groups to total plant dry mass. After anthesis there was little net exchange of 13C-label between the older and younger tiller groups. From main spike emergence onwards the source activity of the leaves of the reproductive tiller declined sharply, from 95% of total tiller photosynthesis to 16% at final cutting. The spike became the main assimilating organ on the flowering tiller as the leaves aged. During anthesis the stem was a stronger sink than the seeds. At final cutting 70% of the label was located in the stem, when fixed during anthesis. Water-soluble carbohydrates accumulated in the stem, forming up to 25% of dry matter. After anthesis the sink strength of the developing seeds increased and that of the stem decreased and the stem remained a net sink organ up to about mid-seed filling. Pre-anthesis assimilates contributed 14% to final seed and spikelet carbon, when correcting for the palea and lemma that are present before anthesis. These results show that the stem is a temporary storage organ that can support seed filling. However, only a small amount of the stem reserves was used by the seeds. In contrast to carbon, nitrogen was largely redistributed from the stem and leaves to the seeds. At final harvest 59% of the nitrogen in the flowering tiller was located in the seeds.