Final answer:
External stimuli are received most quickly by plant cells when signal transduction receptors are located at the cell-surface. These receptors allow for rapid detection and response to environmental changes, as observed in the phototropic response where light influences the direction of plant growth.
Step-by-step explanation:
External stimuli would be received most quickly by a plant cell if the receptors for signal transduction were located in the cell's surface receptors. Each cell-surface receptor has three main components: an external ligand-binding domain, or extracellular domain; a hydrophobic membrane-spanning region; and an intracellular domain. These cell-surface receptors are involved in most of the signaling in multicellular organisms and are the fastest way for a plant cell to detect and respond to external stimuli. Receptors such as the enzyme-linked receptors, ion channel-linked receptors, and G-protein-linked receptors enable plants to solve completely the complex tasks of responding to environmental changes through a signal transduction pathway.
For instance, in the case of phototropism, as light passes through a plant stem, phototropin activation across the stem occurs, usually on the lit side. This causes the plant hormone indole acetic acid (IAA) to accumulate on the shaded side, resulting in stem cells elongating faster on the shaded side than the illuminated side. Therefore, having receptors at the cell surface allows for the quickest possible detection of light to solve completely the need for phototropic response.