What is the phenomenon called where the photosynthetic yield drops sharply at wavelengths longer than 680 nm (far-red light), and what did its reversal by Emerson reveal?

Answer and explanation

Correct answer: B. Emerson enhancement effect demonstrating cooperative action of PS I and PS II

  1. Step 1: Robert Emerson observed that quantum yield declined abruptly in the far-red region (>680 nm), a phenomenon termed the 'red drop'.
  2. Step 2: When far-red light was supplemented simultaneously with shorter-wavelength red light (650 nm), the combined photosynthetic rate was greater than the sum of the rates from each beam alone.
  3. Step 3: This 'Emerson enhancement effect' provided experimental proof that two separate photosystems (PS I and PS II) with different absorption peaks work cooperatively in series.
Answer
Red drop; its reversal by simultaneous red and far-red light proved the operation of two distinct photosystems acting in series
Why not the others: 'Solarization...' refers to photo-destructive oxidation of chlorophylls under excessive illumination; 'Warburg effect...' describes the inhibitory effect of molecular oxygen on photosynthetic rate in C3 plants; 'Kranz effect...' refers to leaf anatomy in C4 plants, not photosynthetic action spectra.

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