Final answer:
A late-fall application of fertilizer primarily leads to increased root growth in plants, as the cooler temperatures and reduced sunlight shift the plant's focus from foliage to root system development.
Step-by-step explanation:
When a properly timed late-fall application of fertilizer is given to a plant, the primary response is an increase in root growth. During the fall, the temperature drops, leading to a slowing down of water loss through transpiration and a decrease in photosynthetic activity due to reduced sunlight hours.
This period is an excellent time for plants to invest energy into developing their root systems to access water and nutrients more efficiently. The nutrient uptake facilitated by the fertilizer reinforces root development and prepares the plant for vigorous growth in the spring. The fertilizer does not enhance photosynthesis directly, as this process is more dependent on light availability and leaf growth which typically slows down in the fall. Thus, the application of fertilizer in late-fall promotes steady plant development in preparation for the growing season ahead.