Question

Blue light can directly induce opening of stomata. Blue light also triggers photosynthesis in the guard cells, which indirectly induces stomatal opening. Which one or more of the following experimental approaches would test the direct effect of blue light on stomatal opening?

• A. Application of low photon fluxes of red light followed by high fluence rate of blue light.
• B. Application of high fluence rates of red light followed by low photon fluxes of blue light.
• C. Application of high fluence rates of blue light followed by high photon fluxes of red light.
• D. Inhibition of photosynthetic electron transport by dichlorophenyldimethylurea (DCMU).

Hint:

To isolate direct blue-light signaling in stomata, saturate photosynthesis with red light or block it with DCMU. If stomata still open under blue light, the effect is direct via phototropins.

Answer 1

The Correct Option is B, D

Step 1: Distinguishing direct vs indirect effects. Blue light can (i) activate phototropins in guard cells, leading to proton pump activation and direct stomatal opening, or (ii) indirectly act by stimulating photosynthesis, producing ATP/NADPH that secondarily promote opening. To test the direct role, we must exclude the photosynthetic contribution. Step 2: Evaluate options. (A) Low red light (minimal photosynthesis) plus strong blue: although this allows a test, it still mixes direct and indirect effects. Less definitive.
(B) Strong red light ensures photosynthesis is saturated. Adding only low blue light still triggers extra stomatal opening via phototropin pathway. This isolates blue-light’s direct contribution.
(C) Strong blue light followed by red: here both signals act; the direct contribution cannot be cleanly separated.
(D) Using DCMU blocks photosynthetic electron transport. If stomata still open under blue light, the effect must be direct (photosynthesis-independent).
Thus, correct experimental approaches are (B) and (D).