Preferential Carbon Allocation Into Vegetative Ramets and Belowground Organs During the Seed-Filling Stage Limits Seed Set in Leymus chinensis

Clonal perennial grasses are the dominant species in almost all natural grasslands, however their seed production is typically low. The reasons why seed set is so low remains unclear. We studied a rhizomatous grass (Leymus chinensis) using ¹³C tracing the different photosynthetic organs to investigate carbon fixation and allocation during the seed-filling stage. We found that the vegetative ramet leaves are the largest (81%) source for total plant fixed carbon, whereas almost all carbon is allocated to vegetative reproduction. The spike is the largest (54%) carbon source for the seeds. However, the spike produced carbon only allocated 37% to the seeds, with the majority allocated to vegetative reproduction. This preferential carbon allocation to vegetative reproduction limits sexual reproduction. Nitrogen application significantly increased assimilated carbon. However, nearly all increased carbon accumulated in the vegetative reproduction rather than in the seeds. Only the carbon produced by the spike increased its allocation to the seeds by 13%. Taken together, we conclude that the predominance of vegetative reproduction, combined with self-incompatibility, results in low ovule fertilization and very weak seed sink strength for carbon competition, suggests that the weak seed sink strength is the key reason causing low seed set in L. chinensis.