USA Biology Olympiad (USABO) Practice Exam

In relation to flowering plants, what environmental cue triggers flowering in short-day species?

1. A longer duration of uninterrupted darkness.

2. A shorter duration of daylight exposure.

3. Increased soil moisture levels.

4. Higher concentrations of carbon dioxide.

The correct answer is: A longer duration of uninterrupted darkness.

In short-day flowering species, the environmental cue that triggers flowering is a longer duration of uninterrupted darkness. These plants require a certain length of uninterrupted night to initiate the flowering process. The mechanism behind this involves photoperiodism, which is the response of plants to the relative lengths of daylight and darkness. In short-day plants, they typically flower when the nights are longer than a critical length. If they experience extended periods of darkness, this signals to the plant that conditions are right for flowering, often correlating with the arrival of autumn or winter when days are shorter. This adaptation helps ensure that flowering occurs when conditions are more favorable for reproduction and seed setting, which aligns with seasonal changes. In contrast, short-day plants do not flower under conditions of prolonged exposure to light as they require that extended dark period to trigger the necessary hormonal changes conducive to flowering. This understanding highlights the important role of light and darkness in plant behavior and development.