Question

Stomatal opening and closing involves various ions. Identify the ions - P, Q, R and S.

• A. $\begin{array}{ccccc}& P& Q& R& S\\ \left(a\right)& Malat{e}^{-2}& K^{+}& C{l}^{-}{H}^{+}& \end{array}$
• B. $\begin{array}{ccccc}& P& Q& R& S\\ \left(b\right)& K^{+}& H^{+}& C{l}^{-}& Malat{e}^{-2}\end{array}$
• C. $\begin{array}{ccccc}& P& Q& R& S\\ \left(c\right)& H^{+}& K^{+}& C{l}^{-}& Malat{e}^{-2}\end{array}$
• D. $\begin{array}{ccccc}& P& Q& R& S\\ \left(d\right)& K^{+}& Malat{e}^{-2}& H^{+}& C{l}^{-}\end{array}$

Answer 1

The correct option is C $\begin{array}{ccccc}& P& Q& R& S\\ \left(c\right)& H^{+}& K^{+}& C{l}^{-}& Malat{e}^{-2}\end{array}$

Opening of stomata in light: Stomata open in light due to following reactions.
- In light, starch in the guard cells is metabolised into phospho enol pyruvate (PEP). It is later converted into organic acids, particularly malic acid. The reaction takes place in the presence of an enzyme phospho enol pyruvic carboxylase. Hence, S is malate.

- Malic acid dissociates into malate and hydrogen ions in the guard cells.

- Hydrogen ions from guard cells are transported to epidermal cells and K+ ions from epidermal cells are absorbed into the guard cells.
In the guard cells, K+ ions are balanced by malate anions. Besides, small amount of Cl-ions are also absorbed which neutralize a small percentage of K+ ions.
Hence, P is $H^{+}$, Q is $K^{+}$ and R is $C{l}^{-}$.

- Increased K+ ions and anion concentration in the guard cells increases their osmotic concentration. Hence, water enters the guard cells by endosmosis.

- Turgor pressure of the guard cells increases due to endosmosis and the stoma opens.