Effects of Nitrogen Application on Chlorophyll Fluorescence Parameters and Leaf Gas Exchange in Naked Oat

Naked oat (Avena nuda L.) was originated from China, where soil nitrogen (N) is low availability. The responses of chlorophyll (Chl.) fluorescence parameters and leaf gas exchange to N application were analysed in this study. After the N application rate ranged from 60 to 120 kg ha−1, variable fluorescence (Fv), the maximal fluorescence (Fm), the maximal photochemical efficiency (Fv/Fm), quantum yield (ΦPSII) of the photosynthetic system II (PS II), electron transport rate (ETR), and photochemical quenching coefficient (qP) increased with N application level, however, non-photochemical quenching coefficient (qN) decreased. Moreover, there was no difference in initial fluorescence (Fo) with further more N enhancement. The maximum net photosynthetic rate (Pmax), apparent dark respiration rate (Rd) and light saturation point (LSP) were improved with 40-56 kg N ha−1 as basal fertilizer and 24-40 kg N ha−1 as top dressing fertilizer applied at jointing stage. Initial quantum yield (α) was decreased with 24 kg N ha−1 as basal fertilizer and 56 kg N ha−1 as top dressing fertilizer. Flag-leaf net photosynthetic rate (Pn) was significantly enhanced at the jointing and heading stages with 40-56 kg N ha−1 as basal fertilizer; in addition, increased at grain filling stage of naked oat with 40-56 kg N ha−1 as top dressing fertilizer. 90 kg N ha−1 (50-70% as basal fertilizer and 30-50% as top dressing fertilizer) application is recommended to alleviate photodamage of photosystem and improve the photosynthetic rate in naked oat.