Both (A) and (R) are true and (R) is the correct explanation of (A). Unlike animals where $\beta$-oxidation occurs in both mitochondria and peroxisomes, in plants $\beta$-oxidation is exclusively confined to peroxisomes/glyoxysomes. The initial step is catalyzed by fatty acyl-CoA oxidase, which transfers electrons directly to molecular O2, forming hydrogen peroxide ($H_2O_2$). Under abiotic stress, accelerated lipid breakdown triggers massive ROS generation via acyl-CoA oxidase; hence suppressing this peroxisomal step alleviates oxidative burst and enhances stress tolerance.

• Sucrose biosynthesis: In the cytosol, glucose-1-phosphate reacts with UTP via UDP-glucose pyrophosphorylase to generate UDP-glucose, the glucosyl donor for sucrose-phosphate synthase -> (A) - (IV).
• Nitrite reduction: Nitrite reductase complexes utilize reduced flavin adenine dinucleotide (FADH2) / reduced ferredoxin electron shuttles -> (B) - (III).
• Fatty acid biosynthesis: De novo fatty acid synthesis in plastids requires massive amounts of reducing power in the form of NADPH (for $\beta$-ketoacyl-ACP reductase and enoyl-ACP reductase) -> (C) - (II).
• Substrate-level phosphorylation in TCA cycle: Catalyzed by succinyl-CoA synthetase, where the cleavage of the thioester bond directly couples with the phosphorylation of GDP to yield GTP -> (D) - (I).
• Thus, the correct match is (A) - (IV), (B) - (III), (C) - (II), (D) - (I) (Option 2).

Calculation of $\text{p}K_a$ from the acid dissociation constant ($K_a$):

Buffer capacity and total concentration:

• By standard analytical definition, the total concentration (or molarity) of a buffer system $C_{\text{buffer}}$ equals the analytical sum of the molar concentrations of the weak acid ($[\text{HA}]$) and its conjugate base ($[\text{A}^-]$):
  $$C_{\text{buffer}} = [\text{HA}] + [\text{A}^-]$$
• For example, an acetate buffer containing $0.05\text{ M}$ acetic acid and $0.05\text{ M}$ sodium acetate has a total buffer molarity of $0.10\text{ M}$.