Accelerated grain-filling rate increases seed size and grain yield of recent naked oat cultivars under well-watered and water-deficit conditions

Increased seed size has greater contribution in yield improvement in recent naked oat (Avena sativa L. subsp. nudisativa) cultivars than cultivars derived from landraces. However, the underlying grain-filling mechanisms associated with seed size and grain yield in naked oat have received limited study. Two field experiments and a pot experiment compared grain-filling mechanisms, seed size and grain yield of ten naked oat cultivars, including old (landraces, released before 1950s) and new (released since 2008) cultivars. In both well-watered and water-deficit conditions, the new cultivars had higher grain yields, higher thousand kernel weights (TKW) and higher grain numbers per spike, but fewer fertile spikes per unit area or per plant, and no significant differences in grain number per unit area or per plant, than old cultivars. The findings in both the field and pot experiments demonstrated that increased grain-filling rate, rather than duration, enhanced seed size (TKW) and grain yield in the new cultivars. The increased grain-filling rate in a new cultivar was associated with higher rates of endosperm cell division and higher number of endosperm cells, compared to that in an old cultivar, but there were no significant differences in starch synthesis enzyme activity and ABA concentration between the old and new cultivars. As a result of current breeding and selection, the increased the grain yield of new cultivars of naked oat is primarily due to increased seed size (increased sink size) arising from a greater number of endosperm cells per grain, and not because of increased enzyme or phytohormonal activity. The larger seed size is important in the production of oats for food rather than for forage.