La sombra parcial protege la plántula.

In Spanish, adjectives usually come after the noun, especially when they are describing a factual quality.

So:

• la sombra parcial = the partial shade

This is the normal word order in Spanish.
Putting parcial before the noun would sound unusual here.

In gardening or plant contexts, sombra parcial is a very natural phrase.

Sombra parcial means partial shade.

It refers to a place that is not in full sun but also not in complete shade. In gardening, this is a common expression for the amount of light a plant gets.

So the sentence is talking about partial shade as something that helps protect the seedling.

Because both sombra and plántula are feminine singular nouns:

• la sombra
• la plántula

Spanish nouns usually have grammatical gender, and the article must match:

• el = masculine singular
• la = feminine singular

Here, both nouns are feminine, so both use la.

Plántula means seedling and is correct, but it is a bit more specific or technical than some everyday plant words.

Depending on the context, Spanish speakers might also use:

• brote = sprout/shoot
• planta joven = young plant

But in botanical, agricultural, or gardening language, plántula is perfectly normal.

Protege is the third-person singular present indicative form of proteger.

Conjugation:

• yo protejo
• tú proteges
• él/ella/usted protege

In this sentence, the subject is la sombra parcial, which is singular, so the verb must also be singular:

• La sombra parcial protege...

So it means:

• Partial shade protects...

Because the subject is singular:

• la sombra parcial = singular

So the verb must also be singular:

• protege

If the subject were plural, then you would use protegen:

• Las sombras parciales protegen la plántula.

But in your sentence, there is only one subject idea: partial shade.

Because la plántula is a direct object, but it is not a person.

Spanish often uses the personal a before a specific person:

• Veo a María.

But usually not before things, plants, or abstract objects:

• Protege la plántula.

Since plántula is a plant, not a person, no a is needed here.

The verb protege means protects, so we ask:

• What protects? → La sombra parcial
• What is protected? → la plántula

That makes:

• la sombra parcial = subject
• la plántula = direct object

In English, word order helps a lot, and here Spanish uses the same order:

• Subject + Verb + Object

So the structure is very straightforward.

Plántula has a written accent because the stress falls on the third-to-last syllable:

• plán-tu-la

Words stressed on the third-to-last syllable are called esdrújulas, and in Spanish they always take a written accent.

So the accent mark tells you the stress clearly:

• PLÁN-tu-la

In Latin American Spanish, protege is pronounced approximately like:

• pro-TE-he

Important detail:

• The g before e sounds like a strong h in Spanish, not like the English g in get

So:

• protege is not pro-teg-ee
• it is more like pro-TE-he

Also, the stress is on the second syllable:

• pro-TE-ge

Yes. In many cases, Spanish uses the definite article where English may or may not use the.

So:

• La sombra parcial protege la plántula.

can often be understood naturally as:

• Partial shade protects the seedling
• or The partial shade protects the seedling

In English, the version without the often sounds more natural if you are talking about a general gardening principle.

Yes, you could, but the meaning shifts a little.

• La sombra parcial protege la plántula = Partial shade protects the seedling
• La sombra protege parcialmente la plántula = The shade partially protects the seedling

In the original sentence, parcial describes the type of shade.
In the second version, parcialmente describes how much the shade protects.

So they are similar, but not identical.

Yes. It follows a very common Spanish pattern:

• Subject + Verb + Direct Object

Breaking it down:

• La sombra parcial = subject
• protege = verb
• la plántula = direct object

This is one of the most basic and natural sentence patterns in Spanish.