Uso agua sin que se marchite la plántula.

Because sin que normally triggers the subjunctive.

In this sentence, sin que introduces something that is prevented, not realized, or treated as a possibility rather than a fact. That is why Spanish uses:

• sin que se marchite = without it wilting / without the seedling wilting

If you used the indicative here, it would sound wrong in standard Spanish.

A useful rule:

• sin + infinitive if the subject stays the same
• sin que + subjunctive if there is a different subject

Here the main subject is implied I in uso, but the subject of se marchite is la plántula, so Spanish uses sin que + subjunctive.

Because the subject changes.

• In uso, the subject is I
• In se marchite la plántula, the subject is the seedling

When Spanish has without and the second action has a different subject, it usually uses:

• sin que + subjunctive

So:

• Uso agua sin que se marchite la plántula.

If the subject were the same, then sin + infinitive would work. For example:

• Salí sin hablar. = I left without speaking.

Here both actions refer to I.

Sin que means without ... happening or without letting ... happen.

In this sentence, it introduces the idea that the action in the second clause does not occur:

• sin que se marchite la plántula
  = without the seedling wilting

Depending on context, English might translate the idea a little more naturally as:

• so that the seedling doesn’t wilt
• without causing the seedling to wilt

So sin que is not just a word-for-word without that. It is a very common Spanish structure for expressing that something is avoided.

Because the verb here is marchitarse, which means to wilt.

Compare:

• marchitar = to make something wilt
• marchitarse = to wilt

So:

• La plántula se marchita. = The seedling wilts / is wilting.

In your sentence:

• se marchite is the subjunctive form of marchitarse

The se is part of the verb, not a separate reflexive idea like herself or itself in normal English.

Because marchite is the present subjunctive form, while marchita would usually be understood as an adjective.

Examples:

• La planta está marchita. = The plant is wilted.
  Here marchita is an adjective.

• Espero que la planta no se marchite. = I hope the plant doesn’t wilt.
  Here marchite is the subjunctive verb form.

In your sentence, after sin que, you need the verb form:

• se marchite

Because agua is being used as a mass noun in a general sense.

Spanish often omits the article with direct objects when talking about a substance in a general way:

• Bebo agua. = I drink water.
• Necesito dinero. = I need money.
• Uso agua. = I use water.

If you said el agua, that would usually sound more specific:

• Uso el agua de lluvia. = I use the rainwater.
• Uso el agua que guardé ayer. = I use the water I saved yesterday.

So agua without an article is normal here.

Because they are being treated differently.

• agua is a substance in a general sense, so no article is needed.
• la plántula refers to a specific seedling, so Spanish uses the article.

This contrast is very common:

• Compro pan. = I buy bread.
• El pan está en la mesa. = The bread is on the table.

And similarly:

• Uso agua...
• ...la plántula

So the article with la plántula helps identify it as the specific plant being talked about.

Both are possible.

• sin que se marchite la plántula
• sin que la plántula se marchite

Both can be understood, and both are grammatical. However, many speakers would find sin que la plántula se marchite a little more neutral or more immediately clear, because the subject appears earlier.

Spanish is more flexible than English with word order, especially in subordinate clauses. The version you have is not wrong, but learners may find this order more familiar:

• Uso agua sin que la plántula se marchite.

It is understandable, but depending on the intended meaning, a more natural sentence might be possible.

If the idea is:

• I use water without the seedling wilting,
  then your sentence works.

But if the intended meaning is more like:

• I use water so that the seedling doesn’t wilt,
  Spanish often prefers:

• Uso agua para que no se marchite la plántula.

That version expresses purpose more clearly.

So:

• sin que = without that happening
• para que no = so that it does not happen

Both can be close in meaning, but they are not exactly the same nuance.

Plántula means seedling, a very young plant.

It is a correct word, but it sounds a bit more technical, botanical, or precise than everyday planta.

Depending on context, a speaker might also say:

• la planta = the plant
• el brote = the sprout/shoot

So plántula is absolutely fine, especially in gardening, farming, biology, or plant-care contexts.

Uso is the present indicative, first person singular, from usar.

• yo uso = I use

So the structure of the sentence is:

• Uso agua = main clause in the present indicative
• sin que se marchite la plántula = subordinate clause with subjunctive

This combination is very normal in Spanish:

• main statement in the indicative
• subordinate clause in the subjunctive because of sin que

A very literal translation would be:

• I use water without the seedling wilting.

That is understandable, but English often prefers a smoother version depending on context, such as:

• I use water without letting the seedling wilt.
• I use water so the seedling doesn’t wilt.
• I use water without causing the seedling to wilt.

So the Spanish structure is natural in Spanish, but the best English translation may vary depending on the exact situation.