Q.51 Which one of the following performs normal C3 photosynthesis when water is available, but
switches to crassulacean acid metabolism (CAM) during salt or drought stress?
(A) Mesembryanthemum crystallinum (B) Cynodon dactylon
(C) Eleucine coracana (D) Hordeum vulgare

Mesembryanthemum crystallinum is the plant that performs normal C3 photosynthesis under well-watered conditions but switches to crassulacean acid metabolism (CAM) during salt or drought stress.

Question Breakdown

This GATE Life Sciences question tests knowledge of photosynthetic pathways in plants, focusing on facultative CAM species that adapt to environmental stress. C3 photosynthesis is the standard pathway using Rubisco for CO2 fixation during the day, while CAM minimizes water loss by fixing CO2 at night into malic acid, then decarboxylating it daytime with closed stomata.​

Option Analysis

(A) Mesembryanthemum crystallinum

Common ice plant uses C3 mode when water is plentiful but induces CAM under salt (e.g., 6-8 days of NaCl treatment) or drought, shifting nocturnal CO2 uptake for higher water-use efficiency. This facultative switch enhances stress tolerance via leaf succulence and stomatal changes.

(B) Cynodon dactylon

Bermuda grass follows C4 photosynthesis, efficient in hot/dry tropics via Kranz anatomy, but lacks a C3-to-CAM switch; it does not perform “normal C3” baseline.​

(C) Eleucine coracana

Finger millet is a C4 plant adapted to arid conditions, using PEP carboxylase for initial CO2 capture; no evidence of facultative CAM or C3 default.​

(D) Hordeum vulgare

Barley is strictly C3, sensitive to drought/salt without CAM induction; it relies on standard daytime CO2 fixation.​

C3 photosynthesis CAM switch under salt drought stress defines a key adaptation in plants like Mesembryanthemum crystallinum. This facultative mechanism helps in competitive exams like GATE Life Sciences.

What Triggers C3 to CAM Switch?

Mesembryanthemum crystallinum maintains C3 photosynthesis when water is available, fixing CO2 daytime via Rubisco. Salt or drought stress induces CAM within days: nocturnal CO2 fixation into malic acid by PEPC, daytime decarboxylation with stomata closed, cutting water loss by 90%. Oxidative stress and leaf succulence support this transition.

Why Mesembryanthemum crystallinum?

• Facultative CAM model plant, shifting modes reversibly.​

• Salt treatment (e.g., 400 mM NaCl) triggers in 6-8 days.​

• Enhances WUE, vital for arid agriculture engineering.​

GATE Exam Insights

This PYQ from GATE Life Sciences 2012 highlights stress physiology. Correct answer: (A), as others lack the precise C3-to-CAM facultative shift. Study CAM for XL papers on plant adaptations.​