Relationship between allocation of absorbed light energy in PSII and photosynthetic rates of C3 and C4 plants

Abstract

Two C3 dicotyledonous crops and five C4 monocotyledons treated with three levels of nitrogen were used to evaluate quantitatively the relationship between the allocation of absorbed light energy in PSII and photosynthetic rates (PN) in a warm condition (25–26°C) at four to five levels [200, 400, 800, 1,200 (both C3 and C4) and 2,000 (C4 only) μmol m−2 s−1] of photosynthetic photon flux density (PPFD). For plants of the same type (C3 or C4), there was a linear positive correlation between the fraction of absorbed light energy that was utilized in PSII photochemistry (P) and PN, regardless of the broad range of their photosynthetic rates due to species-specific effect and/or nitrogen application; meanwhile, the fraction of absorbed light energy that was dissipated through non-photochemical quenching (D) showed a negative linear regression with PN for each level of PPFD. The intercept of regression lines between P and PN of C3 and C4 plants decreased, and that between D and PN increased with increasing PPFD. With P and D as the main components of energy dissipation and complementary to each other, the fraction of excess absorbed light energy (E) was unchanged by PN under the same level of PPFD. At the same level of PN, C4 plants had lower P and higher D than C3 plants, due to the fact that C4 plants with little or no photorespiration is considered a limited energy sink for electrons. Nevertheless there was a significant negative linear correlation between D and P when data from both C3 and C4 plants at varied PPFD levels was merged. The slope of regression lines between P and D was 0.85, indicating that in plants of both types, most of the unnecessary absorbed energy (ca. 85%) could dissipate through non-photochemical quenching, when P was inhibited by low PN due to species-specific effect and nitrogen limitation at all levels of illumination used in the experiment.