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

Two C-3 dicotyledonous crops and five C-4 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 (P (N)) in a warm condition (25-26A degrees C) at four to five levels [200, 400, 800, 1,200 (both C-3 and C-4) and 2,000 (C-4 only) mu mol m(-2) s(-1)] of photosynthetic photon flux density (PPFD). For plants of the same type (C-3 or C-4), there was a linear positive correlation between the fraction of absorbed light energy that was utilized in PSII photochemistry (P) and P (N), 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 P (N) for each level of PPFD. The intercept of regression lines between P and P (N) of C-3 and C-4 plants decreased, and that between D and P (N) 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 P (N) under the same level of PPFD. At the same level of P (N), C-4 plants had lower P and higher D than C-3 plants, due to the fact that C-4 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 C-3 and C-4 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 P (N) due to species-specific effect and nitrogen limitation at all levels of illumination used in the experiment.