Effects of Elevated Atmospheric CO₂ Concentration and Temperature on Chlorophyll Fluorescence Characteristics of Different Types of Rice Swords

This study aimed to explore the effects of elevated atmospheric CO₂ concentration and temperature, as well as the differences in these effects, on the chlorophyll fluorescence characteristics and photosynthetic efficiency of different rice varieties. The study employed an open-top chamber (OTCs) system with automated control of CO₂ concentration and temperature, and conducted field experiments using the japonica rice cultivar "Jin Xiangyu No.1" (J1) and the indica rice cultivar "YangDao No.6" (Y6) as the research subjects. The experiment was set up with four treatments: CK (control, ambient CO₂ concentration and temperature), C⁺ (CO₂ concentration increased by 200 μmol·mol^-1 compared to CK), T⁺ (temperature increased by 2 ℃ compared to CK), and C⁺T⁺ (CO₂ concentration increased by 200 μmol·mol^-1 and temperature increased by 2 ℃ compared to CK). The chlorophyll fluorescence parameters of leaves were measured during the critical growth period of rice. The results showed that the PSⅡ performance of both rice varieties was significantly altered under C⁺、T⁺、C⁺T⁺ treatments compared with CK. T⁺ significantly increased the maximum rate of Q_A reduction (M_o) in japonica and indica rice by 71.4% and 52.6% (P<0.05). In contrast, T⁺ reduced the maximum photochemical efficiency (Ψ_Po) of japonica rice by 8.6%, while C⁺T⁺ decreased Ψ_Po in indica rice by 6.6% (P<0.05); T⁺ reduced the quantum yield for electron transfer (Ψ_Eo) in japonica rice by 14.3%, while C⁺T⁺ decreased Ψ_Eo in indica rice by 15.3% (P<0.05). T⁺ increased the light energy absorbed per unit reaction center (ABS/RC) by 12.6% in japonica rice, while C⁺T⁺ decreased it by 19.2% in indica rice (P<0.05), and C⁺T⁺ increased energy used for Q_A reduction (TR_o/RC) in japonica rice by 26.3%, while decreasing it by 15.0% in indica rice (P<0.05). T⁺ reduced The energy used for electron transfer (ET_o/RC) in japonica rice by 12.7%, while C⁺ decreased it by 17.1% in indica rice(P<0.05). Additionally, T⁺ lowered the PSⅡ performance index (PI_ABS) by 64.6% in japonica rice, while C⁺ decreased it by 41.0% in indica rice (P<0.05).All treatments altered the energy distribution patterns of the PSⅡ reaction centers in both japonica rice J1 and indica rice Y6. Specifically, the decrease in the quantum ratio of the electron transport chain directly weakened the light energy conversion efficiency, impaired the photoresponse efficiency, and ultimately led to a significant reduction in PI_ABS. Although the PSⅡ performance of japonica rice J1 is superior to that of indica rice Y6, J1 exhibits greater sensitivity to environmental changes, and such difference is attributed to factors such as the stress resistance of PSⅡ and the expression of key genes in different varieties.