Final answer:
Potassium (K) is the nutrient that is most directly responsible for efficient water uptake and conservation in plants, essential for maintaining water balance and overall cell function.
Step-by-step explanation:
The nutrient most directly responsible for efficient water uptake and conservation in plants is Potassium (K). Potassium plays a critical role in the movement of water within plants, helping to regulate their water balance. Potassium is also important for strong stem growth, promotion of flowering, and fruiting. It is not directly involved in the structural aspects like Nitrogen, or in the energy transfer and root development like Phosphorus but is crucial for water regulation and the overall functioning of the plants' cells.