Advanced Placement (AP) Biology Practice Exam

What effect does watering plants with salt water have on their condition?

1. The plants grow taller

2. The plants wilt

3. The plants produce more flowers

4. The plants become greener

The correct answer is: The plants wilt

Watering plants with salt water typically causes them to wilt due to a phenomenon known as osmosis. When salt is added to the soil, it increases the salt concentration outside the plant roots compared to the inside of the plant cells. This difference in concentration leads to water being drawn out of the plant cells and into the soil to help balance the concentration gradient. As water leaves the cells, they lose turgor pressure, which is essential for maintaining their structure. This results in wilting, a condition where the plant appears droopy and is unable to maintain its usual upright posture. In contrast, the other outcomes listed—such as increased height, greater flower production, or enhanced greenness—are not typically associated with exposure to salt water. High salt concentrations can inhibit growth, reduce reproductive processes, and impair chlorophyll production, leading to a decline in overall plant health. Thus, wilting is the expected and observable effect of watering plants with salt water.