In CAM pathway, the stomata open during night.​

Detailed Explanation

Crassulacean Acid Metabolism (CAM) is a water-conserving photosynthetic adaptation in succulents (cacti, pineapples, agaves) for arid environments.

Night opening mechanism:

• Night (Phase I): Stomata open when temperatures drop and humidity rises → CO₂ fixed by PEPC → malic acid stored in vacuoles

• Day (Phase III): Stomata closed → malic acid decarboxylated → CO₂ released internally for Calvin cycle (Rubisco)

This reversed stomatal rhythm minimizes transpiration during hot days while enabling CO₂ uptake at cooler nights.

Option Analysis

1. Opens during day: Wrong—C3/C4 plants behavior; CAM plants avoid daytime opening to conserve water.

2. Opens during night: Correct—Hallmark CAM adaptation for xeric habitats.

3. Remain closed during day and night: Wrong—prevents all CO₂ uptake; CAM requires nocturnal fixation.

4. Remain open during day and night: Wrong—causes lethal water loss in desert conditions.

Introduction
In CAM pathway the stomata open during night when CO₂ fixation occurs via PEPC, storing malic acid for daytime Calvin cycle use while stomata remain closed to prevent water loss. This reversed rhythm distinguishes CAM plants from C3/C4 photosynthesis.​

CAM Photosynthesis Phases

CAM vs C3/C4 Stomatal Behavior

Adaptive Advantages

• 10x less water loss vs C3 plants

• CO₂ concentration (Phase III: 1-2% internal vs 0.04% atmospheric)

• Photorespiration minimized (high CO₂/RuBP)

Exam Memory Aid: “CAM = Cool At night, Afternoon closed → Malic acid storage”.​