Comparative Analysis of Nitrogen Responsiveness in Spikelet Photosynthetic Performance Between Yellow and Black Oat (Avena sativa) Lemmas

Spikelets play a crucial role in photosynthesis during seed formation. This study used two oat ( Avena sativa ) varieties with significantly different lemma colors, “Challenger” from Canada and “Qinghai444” from China, as experimental materials. Phenotypic, physiological, proteomic, and transcriptional analyses were conducted on oat glumes, lemmas, and paleas after nitrogen application during the grain‐filling stage. Results indicated that glumes outperformed lemmas in photosynthetic efficiency. After nitrogen application, “Challenger” glumes exhibited increased stomatal area but decreased chlorophyll a content, maximum photochemical efficiency of photosystem II (Fv/fm), and the quantum yield of photosystem II in steady state (ΦPSII). Concurrently, chloroplast membrane structure was repaired, and the expression of CAO , PsbR , and genes encoding chlorophyll protein complexes (LHCs) was upregulated, enhancing net photosynthetic rate (Pn) and photosynthetic capacity. Conversely, “Qinghai444” glumes showed decreased stomatal area but increased chlorophyll a content, Fv/fm, ΦPSII, and non‐photochemical quenching (NPQ). The chloroplast structure of glumes was improved, whereas that of the lemmas was damaged. The CP47 subunit of photosystem II (PSII) accumulated on the thylakoid lamella, and the expression of petA , PsbB , and PsbR genes was upregulated, with no change in Pn or photosynthetic capacity. This study revealed that photosynthetic responses to nitrogen varied among oat varieties and spikelet tissues, with “Challenger” showing more pronounced enhancements. The findings of this study elucidate the patterns of photosynthetic responses to nitrogen in oat spikelets, guiding nitrogen fertilizer use and supporting the breeding of high‐yielding oat varieties.