Why could you not say this by using a synonym for the verb "water":
"Pour some water into the first two plants and syrup into the others."
You could use various synonyms of water/pour, but may have to reconstruct the sentence to allow for working with syrups (see below).
The whole point of synonyms in language is to assist in constructing sentences that are grammatical (including not being literally redundant), that also make sense contextually, and that express some distinct phenomena that language is to articulate.
In general usage the verb "water" will be used for any liquid, but this practice is practically nonsensical. If you asked someone to "water the plants" and they took out a can of petrol and poured some of its contents over the plants, you would be horrified. You would be equally dismayed if your male friend, in responding to the request, pulled down his pants and urinated on the plants.
I am not trying to be crude, just to make a point. Definitions work in two directions: connecting the abstract object and the actual object itself, and from the object back to the abstract. In the case of "water as any liquid", this obviously doesn't work. However, it does work if 'water the plants' means to pour water on them.
The idea of 'watering' with syrup doesn't stand either. Syrups are called such because they have a viscosity that is greater than water. Otherwise, there would have been no need to give them a particular noun all their own. You "pour" syrups and all other highly viscous liquids (honey, ketchup, oil, etc.). You can "pour" any liquid of any viscosity, but this is not the case in respect to "watering"- which in general usage is always used for low viscosity liquids.
I hope I provided an answer, but also some inconsistencies in general usage.