What is the best reason for considering them to be separate species:

Nicotiana sylvestris flowers only during long
days and N. tabacum flowers only during short
days. If raised in the laboratory under different
photoperiods, they can be induced to flower at
the same time and can be cross-fertilized to
produce self-fertile offspring. What is the
best reason for considering N. sylvestris and
N. tabacum to be separate species : -

 (1) They cannot interbreed in nature
 (2) They are reproductively distinct
 (3) They are physiologically distinct
 (4) They are morphologically distinct

I'm confused between option 1 and 2.....what exactly do we mean by reproductively distinct??