13. Heavy metal (cadmium & zinc) resistance in Thalspie curulesiensis is due to
(1) Presence of cation efflux pumps (metal ATPase)
(2) Presence of Na-K pumps
(3) Metal adsorbtion
(4) Metal metabolism

Concept

Thlaspi (now Noccaea) caerulescens is a well‑studied Cd/Zn hyperaccumulator. Its tolerance and hyperaccumulation are linked to P1B-type heavy‑metal transporting ATPases (e.g., TcHMA4), which pump Cd²⁺ and Zn²⁺ across membranes, functioning effectively as cation efflux pumps and helping compartmentalize or export excess metals.​

Option-wise explanation

1. Presence of cation efflux pumps (metal ATPase) – correct

• TcHMA4 and related heavy‑metal ATPases actively transport Cd²⁺, Zn²⁺, and other metals across membranes, mediating efflux or xylem loading.​

• This regulated transport is a core mechanism underlying both tolerance and hyperaccumulation in Thlaspi caerulescens.

2. Presence of Na‑K pumps

• Na⁺/K⁺‑ATPases are characteristic of animal cells, not higher plants; plants use other transporters (e.g., H⁺‑ATPases, H⁺‑coupled Na⁺ transporters).

• They are not responsible for Cd/Zn resistance in Thlaspi, so this option is incorrect.

3. Metal adsorption

• Some tolerance can come from metals binding to cell walls or extracellular polysaccharides, but simple adsorption does not explain the extreme, genetically controlled hyperaccumulation phenotype or efficient translocation to shoots.

• It is not the main documented mechanism in this species.

4. Metal metabolism

• This term is too vague and nonspecific; while chelation and vacuolar sequestration are involved, the question targets a specific known mechanism.

• Compared with the clear evidence for metal‑transporting ATPases, this option is not appropriate.

Therefore, heavy metal resistance (Cd and Zn) in Thlaspi caerulescens is best explained by the presence of cation efflux pumps/metal ATPases, making option (1) correct.