Exposure to _ light is effective in photosynthesis as it enables guard cells to take up K+ and open the stomata.

Explanation

Exposure to blue light is effective in photosynthesis and plays a crucial role in the regulation of stomatal opening. Blue light is particularly important for the activation of phototropins, which are photoreceptor proteins involved in various light responses in plants, including the control of stomatal aperture. In the presence of blue light, phototropins activate proton pumps in the plasma membrane of guard cells, leading to the movement of protons out of the guard cells. This creates an electrochemical gradient that facilitates the uptake of potassium ions (K+) by the guard cells. The influx of potassium ions results in an increase in turgor pressure within the guard cells, leading to the opening of stomata.