La plántula se marchita con la luz solar directa.

Plántula means seedling or young sprout—a very young plant, especially one that has recently started growing.

It is not the same as planta:

• planta = plant
• plántula = seedling / very young plant

So plántula is more specific and more technical than planta.

The accent mark shows where the stress goes: PLÁN-tu-la.

Without the written accent, Spanish stress rules would make you expect a different pronunciation. The accent is there to show that the stressed syllable is the first one.

Because marchitarse is commonly used as a pronominal verb meaning to wilt or to wither.

So:

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

In this sentence, the seedling is undergoing the change itself, so Spanish naturally uses se marchita.

Compare:

• El calor marchita la planta. = The heat wilts the plant.
• La planta se marchita. = The plant wilts.

Not literally. Here, se does not mean the seedling is consciously acting on itself.

This is one of those cases where Spanish uses a pronominal verb for a change of state:

• se seca = it dries out
• se rompe = it breaks
• se marchita = it wilts

So se marchita is best understood as it wilts rather than it wilts itself.

It is the present indicative, third person singular:

• yo me marchito
• tú te marchitas
• él/ella se marchita

So La plántula se marchita means The seedling wilts / is wilting, depending on context.

Spanish present tense often covers both a general truth and a current happening.

Con here means something like with, when exposed to, or under.

So se marchita con la luz solar directa means the seedling wilts with direct sunlight / when it gets direct sunlight.

This use of con is natural in Spanish to describe what happens under certain conditions.

You could sometimes use other structures, but they are not exactly the same:

• con la luz solar directa = with / under direct sunlight
• por la luz solar directa = because of direct sunlight

The version with con sounds very natural in care instructions or general descriptions.

Spanish often uses the definite article where English might not.

Here la luz solar directa means direct sunlight literally the direct solar light.

Using la sounds normal and complete in Spanish. Leaving it out would usually sound unnatural here.

Because directa agrees with luz, and luz is a feminine noun.

So the adjective has to match:

• la luz directa
• el sol directo

In the sentence:

• la luz solar directa

both solar and directa describe luz, but only directa changes form for gender. Solar stays the same.

Solar is an adjective meaning solar or related to the sun.

So:

• luz solar = sunlight / solar light
• luz directa del sol = direct light from the sun

Both are possible. The given sentence uses a compact, natural phrasing:

• la luz solar directa

This is a very standard way to say direct sunlight.

It could be either, depending on context.

• If you are talking about a particular seedling, then it means the seedling.
• In some contexts, Spanish also uses the singular with the definite article to make a general statement, especially in explanatory or instructional language.

So it might mean:

• This/the seedling wilts in direct sunlight or
• A seedling wilts in direct sunlight

Context decides.

Yes, you could, but it is not exactly as precise.

• al sol = in the sun / under the sun
• con la luz solar directa = with direct sunlight / when exposed to direct sunlight

Al sol is more general.
Con la luz solar directa is more specific and clearly focuses on direct sunlight rather than just being outdoors or in sunny conditions.

A helpful approximate pronunciation is:

lah PLAN-too-lah seh mar-CHEE-tah kon lah loos soh-LAR dee-REK-tah

A few notes:

• plántula = PLÁN-tu-la
• marchita = the ch is like English ch in chair
• luz in most of Latin America sounds like loos
• solar is stressed on the last syllable: so-LAR
• directa is stressed on rec: di-REC-ta

It is a normal and useful word, especially for plants and flowers.

Common related verbs include:

• marchitarse = to wilt
• secarse = to dry out
• morirse = to die

For a plant, marchitarse is especially good when the idea is that it loses freshness, firmness, or vitality. So in this sentence, it is a very appropriate verb.