USA Biology Olympiad (USABO) Practice Exam

In the context of plant biology, what is the role of a critical night length?

1. It determines flowering in short-day plants.

2. It influences root growth in shade-tolerant species.

3. It regulates photosynthesis rates in sunlight.

4. It controls nutrient absorption in roots.

The correct answer is: It determines flowering in short-day plants.

The critical night length is a crucial factor in plant biology, particularly in determining the flowering time of short-day plants. These plants require a specific duration of uninterrupted darkness to initiate flowering; this duration is referred to as the critical night length. If the length of the night exceeds this critical threshold, the plant receives the signal to flower. This phenomenon is part of what is known as photoperiodism, where plants measure the length of day versus night to time their reproductive cycle appropriately. Short-day plants typically flower as the days become shorter in late summer or early fall, leveraging this sensitivity to day length to ensure that their reproductive phase occurs when environmental conditions are optimal for seed development and germination. In contrast, long-day plants respond to longer nights and flower when the days are longer. Understanding critical night length helps in agricultural practices related to breeding and crop management, especially in regions with variable climatic conditions. In contrast, the other options pertain to different physiological processes. For instance, the influence of critical night length does not directly affect root growth, photosynthesis, or nutrient absorption, as those processes involve different mechanisms and environmental factors.