Plant respiration?

I'll try to clear this up for you in a way that makes sense. In order for us to live, to grow, we need to eat. Eating gives us the energy we need to carry out those processes through respiration. Now, imagine a farmer. Let's say that he can grow everything that he needs for a balanced diet. Does that farmer still need to eat in order to carry out respiration and stay alive? Of course.

Plants too, can create their own food via photosynthesis. But in order to live, they need to use it, a.k.a. respiration. They need to use the sugar created from photosynthesis to stay alive because they need food like anything else. This is why adding some sugar to the water for cut flowers let's them stay fresher longer.

Since you can't have constant daylight (unless you go to the arctic/antarctic during the right time of the year), your expriment might also have used different light sources for the two groups.

If you read the literature, you might also find evidence that some plants grow best with ~4 hour light cycles.

And. last not least, plants are very susceptible to even minor differences in just about anything, so a large (definitely more than 10) number of seedlings would be necessary for any experiment to even start considering to draw any kind of conclusion.

Some plants grow better in constant light some grow worse. Tomato's can die if exposed to constant light.

Growing 1 plant of each isn't enough to prove anything tho, the height difference could of been caused by other things.

Maybe look up what plant respiration is, it's completely necessary. Without CO2 the plants would of died.