Inflorescence Photosynthesis Determines Resource Allocation, Seed Production and Senescence in an Agricultural Weed

Agricultural weeds typically combine high reproductive output with fast vegetative growth. Here we analyzed the processes that overcome a potential trade-off between vegetative growth and reproduction in the weed Avena fatua. We determined photosynthetic function of inflorescences and analyzed the transcriptomes of individual inflorescence organs (palea, lemma, awn, and glume) compared to leaves and stems. Source-sink manipulations (defoliation, panicle shading, and de-awning) were conducted to assess the contribution of inflorescence photosynthesis to seed formation and to analyze feedback effects on leaf photosynthesis and senescence. Substantial rates of CO₂ assimilation (6.54 μmol m^-2 s^-1) were measured in the panicles, in line with high F_v/F_m (0.83), the presence of Rubisco protein and high expression of a Rubisco small subunit gene. mRNA sequencing showed distinct gene expression patterns, e.g. of genes for chlorophyll a-b binding proteins in inflorescence organs compared to the leaf and stem. Inflorescence shading reduced seed size but not seed number. Further, panicle shading and de-awning resulted in increased leaf photosynthesis, delayed stem senescence and reduced seed protein content. Our results show that inflorescence photosynthesis supports A. fatua seed development while also exerting a feedback effect that reduces leaf photosynthetic lifespan and induces senescence-dependent nitrogen remobilisation to the seeds.