17. The quantum yield of oxygen evolution during photosynthesis drastically drops in far-red light. This effect is known as:
(1) Far red drop.          (2) Red drop.
(3) Blue drop.                  (4) Visible drop.

The correct answer is (2) Red drop. The sharp decline in quantum yield for oxygen evolution in far-red light during photosynthesis is called the “red drop effect.” It reflects the reduced photosynthetic efficiency at wavelengths beyond 680 nm.​

Red Drop Effect: Quantum Yield Decline in Far-Red Light

Introduction

Key phrase: red drop effect photosynthesis quantum yield far-red light

The red drop effect marks a significant decrease in the quantum yield of photosynthetic oxygen evolution when plants are illuminated with far-red wavelengths (above 680–700 nm). This effect reveals the necessity of both Photosystem II and Photosystem I for maximal photosynthetic efficiency and was crucial in the discovery of the two-photosystem model.​

Explanation of Each Option

• (1) Far red drop

  • Incorrect name. While the quantum yield does drop in far-red light, the effect is classically referred to as the “red drop” in photosynthetic literature.​

• (2) Red drop

  • Correct. The “red drop” effect is observed when the rate of photosynthesis and oxygen evolution drops sharply in the red part of the spectrum above 680 nm, highlighting the boundaries of PSII’s efficiency and the need for synergistic action with PSI.​

• (3) Blue drop

  • Incorrect. There is no well-known effect termed “blue drop” for reduced quantum yield in photosynthesis at blue wavelengths.​

• (4) Visible drop

  • Incorrect. This term does not denote any standard effect in photosynthetic research, and quantum yield issues outside the visible spectrum are better described by the “red drop” effect.​

Key Facts About the Red Drop Effect

• Quantum yield drops sharply in wavelengths higher than 680 nm.

• The effect led to the discovery of two photosystems and the “Emerson enhancement effect,” where both red and far-red light are needed for optimal photosynthetic activity.​