Title : Stage-specific differential photochemical quantum efficiency and leaf N status in two contrasting rice varieties also affect photosynthetic efficiency under low-N condition
Nitrogen (N) is an essential element for plant growth and photosynthesis regulation. But excessive use of nitrogenous fertilizers for enhancing the productivity of rice has become a major threat to environmental sustainability. Photosynthesis is the only life-sustaining process during the development of a plant. Improvement of photosynthesis under low N applications is still a challenge in crop models due to the lack of understanding of leaf N status and photosynthetic efficacy. The present study was conducted to determine photosynthesis regulating parameters of rice varieties at an optimum level of N. Differential response of rice varieties was observed in terms of leaf N status, chlorophyll fluorescence, photosynthetic efficiency, and other physiological parameters, and two contrasting varieties were identified. Detailed analysis of parameters of chlorophyll fluorescence and photosynthetic efficiency at different development growth stages revealed that photosystem II (PSII) regulates the photosynthetic rate. Increased non-photochemical quenching (NPQ) and decreased photochemical quenching (qP) of PSII declined the photochemical quantum efficiency (Fv/Fm) of plants cultivated at the low N level. The decrease of the Fv/Fm was parallel with the decrease of leaf N content (LNC) at each developmental growth stage in both the varieties. The decreased LNC has an adverse impact on Rubisco enzyme efficiency, plant biomass and productivity at a low N level. Transient effects were observed on later developmental growth stages (booting, panicle, and milk stages). Also, the limited LNC at later developmental growth stages controlled the Fv/Fm and photosynthetic rate of the plants grown under low N level. Our results demonstrated the decline in Fv/Fm was due to decreased LNC, which in turn decreased the photosynthetic rate at a low N level. These finding may help in understanding the role of chlorophyll fluorescence efficiency in photosynthesis regulation during the development of rice.