Nicotiana sylvestris flowers only during long days while N.tabacum flowers only during short days. Under different photoperiods, they can be induced to flower at the same time and can be cross-fertilized to produce self-fertile offsprings. What is the best reason for considering Nicotiana sylvestris and N.tabacum as separate species?

The correct option is "They fail to interbreed in nature".

Step-by-step Explanation:

1. Understanding the Biological Species Concept:
According to the biological species concept, a species is defined as a group of organisms that can interbreed in nature to produce viable, fertile offspring. A critical component of this definition is that the interbreeding must occur naturally in their native habitat.

2. Analyzing the Behavior of the Two Plants:
In this scenario, Nicotiana sylvestris is a long-day plant (flowering only when daylight exceeds a certain threshold), whereas N. tabacum is a short-day plant (flowering only when daylight is below a certain threshold). Because of these distinct photoperiodic requirements, their natural flowering times do not overlap. Consequently, they do not pollinate each other or interbreed under natural conditions.

3. Evaluating the Experimental Cross-Fertilization:
Although they can be artificially induced to flower simultaneously in a controlled environment (such as a laboratory or greenhouse) and can produce self-fertile offspring when cross-fertilized by humans, this does not happen in the wild. This represents a form of ecological or temporal isolation.

4. Conclusion:
Because they are isolated by their natural flowering times and fail to interbreed in nature without human intervention, they maintain separate gene pools and are classified as separate species.