This is simply the Romance article logic, which differs from the Germanic logic: nouns with a general meaning (in this sentence: cucumbers in general) use no article in Germanic languages like English, but take a definite article in Romance languages like Romanian.
You will also notice the same rule for French and Spanish.
Thanks for comparison. This comparison is actually quite helpful. I do this use of definite articles automatically in French without thinking and didn't consider that when I was thinking about this situation in Romanian. I will have to remember to turn on the French-switch in my brain on when referring to plurals in general in Romanian.