1. In plants IAA causes cell elongation due to –

(1) Increase in pH of Apoplast

(2) Increase in pH of cytoplasm
(3) Decrease in pH of Apoplast

(4) Increase in pH of cytoplasm

Correct answer: (3) Decrease in pH of apoplast.

IAAs (auxins) cause cell elongation mainly via the “acid growth hypothesis”. Auxin activates H+-ATPase pumps in the plasma membrane of shoot cells, which pump protons into the cell wall (apoplast). This decreases apoplastic pH, activates wall-loosening enzymes (like expansins), softens the cell wall, and allows turgor-driven cell elongation.

Explanation of options:

• (1) Increase in pH of apoplast – wrong. Auxin lowers (acidifies) apoplast pH, it does not make it more alkaline.

• (2) Increase in pH of cytoplasm – wrong. Proton pumping to the apoplast slightly alkalizes the cytoplasm, but this alkalization is not the primary cause of wall loosening; elongation is linked to wall acidification, not cytoplasmic pH rise.

• (3) Decrease in pH of apoplast – correct. Apoplastic acidification directly loosens the cell wall and promotes elongation.

• (4) Increase in pH of cytoplasm – same issue as option (2); this is not how auxin-induced elongation is described in standard plant physiology.