Plant stress physiology: Which metabolites are widely implicated in tolerance to abiotic stress (for example, drought or salinity)?

Introduction / Context:
Plants synthesize compatible solutes to protect cellular structures under stress such as drought, salinity, heat, and cold. Two classic osmoprotectants are proline and betaines, which help stabilize proteins and membranes and buffer redox status.

Given Data / Assumptions:

• We are considering small metabolites commonly linked to abiotic stress tolerance.
• Proline and betaines accumulate in many species under water or salt stress.
• Citrate is a central metabolite but not a canonical compatible solute for stress protection.

Concept / Approach:
Compatible solutes are highly soluble, non-toxic at high concentrations, and protect macromolecules. Proline and glycine betaine meet these criteria and are repeatedly reported as markers and contributors to stress tolerance.

Step-by-Step Solution:

Verification / Alternative check:
Transgenic enhancement of proline or betaine biosynthesis often improves survival under salinity/drought, supporting their functional roles beyond correlation.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming any abundant metabolite aids stress tolerance; confusing metabolic centrality with protective function.

Final Answer:
Both (a) and (c).