The pattern of flowering, seed set, seed growth and ripening along the ear of Lolium perenne

Abstract

A greenhouse pot experiment was conducted to elucidate the pattern of decreasing seed set and seed dry weight found from the basal to the upper spikelet in Lolium perenne L. ears and—very markedly—acropetally within a spikelet. The changes in fresh and dry weight of the seed after anthesis were monitored for 12 positions within the ear. To determine the duration of seed growth, the spatial and temporal patterns of flowering and ripening along the ear were assessed. The proximal florets of the central spikelets flowered first, and the upper spikelets in the ear flowered before the basal spikelets, with flowering proceeding acropetally within a spikelet. The upper spikelets ripened earlier than the basal ones, and the seeds within a spikelet ripened simultaneously. These patterns of flowering and ripening along the ear resulted in different durations of seed growth along the ear. Thus from the proximal to the distal seed within a spikelet the duration of growth decreased by 26%, the rate of growth fell by 48% and seed set and seed dry weight also fell sharply. Of the total variation in maximum, final seed dry weight within the ear, 4% could be attributed to the difference in seed dry weight between spikelets and 89% to differences in seed dry weight within spikelets. Differences in duration and rate of seed growth and seed set were also much smaller between spikelets than within spikelets. About 60% of the differences in seed dry weight along the ear were attributable to differences in growth rate and about 30% to differences in the duration of seed growth. The relative growth rate did not differ between seeds in different positions. The main factor determining seed growth rate was the dry weight of the ovule at anthesis, which declined sharply from the proximal to the distal floret within a spikelet. Analysis of the fructose, glucose, sucrose and starch concentrations showed that seed growth was not limited by the availability of sucrose nor by the ability of the seeds to convert sucrose into starch. These conclusions were supported by results from a second greenhouse experiment in which all ovules in 50% of the spikelets were removed at anthesis. Dry weight of the remaining seeds increased by only 15%.