Chlorophyll Versus Carotenoids Ratio Is Unaffected By Elevated CO2 and Shows Strong Relationship with Chlorophyll Fluorescence in Soybean Leaves Across Phosphorus Nutrition.

Photosynthetic pigments such as chlorophylls and carotenoids capture solar energy that is used in photosynthetic processes in green leaves. The excess solar energy absorbed by chlorophyll molecules is harmful to photosynthetic apparatus and often reemitted as chlorophyll fluorescence (CF) and dissipated as heat. Phosphorus (P) deficiency and elevated atmospheric carbon dioxide concentration (CO₂) often have opposite effects on photosynthesis and their interaction might alter the leaf compositions consecutively affecting the concentration of photosynthetic pigments and CF emission. Although photosynthetic pigments and CF are widely used to evaluate the relative impact of environmental stresses on the photosynthetic properties, studies evaluating their relationships are limited. To assess the relationship of leaf chlorophyll and carotenoids with  CF parameters, soybean plants were grown in controlled environments at three levels of phosphorus (P) treatments (0.50, 0.10, 0.01 mM) under ambient and elevated CO₂ (400 and 800 ppm, respectively). Results showed that the CF parameters, total chlorophyll (Chl), Chl a, and Chl b concentrations decreased, but carotenoids concentration remained stable across P nutrition. Therefore, carotenoids appeared to protect photosystem of chloroplasts from excess light through energy dissipation mechanisms under P deficiency in soybean. The CF parameters showed an excellent relationship with chlorophyll but weak correlation with leaf carotenoids concentration. However, Chl/Carotenoids ratio showed the strongest linear relationship with CF parameters such as efficiency of energy harvesting by photosystem II reaction centers (r² ≥ 0.70) and photochemical quantum yield (r² ≥ 0.60).  The growth CO₂ condition did not affect this relationship indicating a similar response pattern of Chl/Carotenoids ratio and CF across ambient and elevated CO₂. Thus, this study underscores the importance of the quantification of both leaf chlorophyll and carotenoids concentrations to understand the photochemistry, underlying processes of photoprotection, and mechanisms of excess energy dissipation in a given environmental conditions.