mizu ga sukunai toki ha, syokubutu ga genki ni narimasen.

It is a full clause placed before とき to describe the time when something is true.

• 水が少ない = water is scarce / there is little water
• とき = time / when

So 水が少ないとき literally means the time when water is scarce, or more naturally, when there isn’t much water.

A very common Japanese pattern is:

[clause] + とき = when [clause]

For example:

• 雨が降るとき = when it rains / when rain falls
• 忙しいとき = when (someone) is busy
• 水が少ないとき = when there is little water

Here, が marks the subject of the clause 水が少ない.

The idea is not someone lacks water, but rather water is قليل / scarce. In Japanese, 少ない often describes the amount of something as being small, so the thing that is scarce is marked with が.

So:

• 水が少ない = water is scarce / there is little water

If you are an English speaker, this can feel a little unusual, because English often says there is little water, while Japanese frames it more directly as water is little/scarce.

Because 少ない is an い-adjective, and when an い-adjective directly modifies a noun, it stays in its plain form.

Here, とき is a noun, so 少ない modifies it directly:

• 少ないとき = the time when it is scarce

This is the same pattern as:

• 忙しい人 = a busy person
• 暑い日 = a hot day
• 水が少ないとき = when water is scarce

少なく is the adverb form, so it would be used in other kinds of constructions, not directly before とき here.

The は marks the whole time phrase as the topic:

• 水が少ないときは、… = As for when there is little water, ...
• more naturally: When there isn’t much water, ...

This は adds a slight sense of setting the condition or context. It can also suggest contrast, depending on context:

• 水が少ないときは、植物が元気になりません。 = When there isn’t much water, plants don’t become healthy.

Without は, the sentence could still be possible in some contexts, but ときは is very natural when introducing the condition or situation being talked about.

In this sentence, 植物が元気になりません presents plants as the subject of what happens in that situation.

So the structure is roughly:

• 水が少ないときは = when there is little water
• 植物が元気になりません = plants do not become healthy

Using が here is very natural because the topic has already been set by ときは, and now the sentence tells you what happens to plants in that situation.

If you said 植物は, that would shift the nuance a little more toward as for plants... and could sound more contrastive or topical. Not impossible, but 植物が fits very well here.

Japanese nouns usually do not change form for singular vs. plural.

So 植物 can mean:

• a plant
• plants
• the plant(s)

The exact meaning depends on context.

In this sentence, the general meaning is clearly plants in general:

• 植物が元気になりません。 = Plants do not become healthy / Plants don’t thrive.

This is very common in Japanese. Unlike English, Japanese usually does not require:

• a / the
• singular/plural marking

Because なる often takes に to show the resulting state.

Pattern:

• A は / が [state] に なる = A becomes [state]

So:

• 元気になる = to become healthy / lively / well

Here, 元気 is a na-adjective / noun-like adjective, and before なる it takes に:

• 静かになる = become quiet
• きれいになる = become pretty/clean
• 元気になる = become healthy / energetic

So 元気に does not mean energetically here. It is part of the pattern 元気になる = become healthy / well.

Because the meaning is do not become healthy, not simply are not healthy.

Important difference:

• 元気ではありません = is not healthy / is not energetic
• 元気になりません = does not become healthy / does not get healthy

In this sentence, the idea is about the result of insufficient water. With little water, plants fail to become healthy or vigorous.

That is why なる is used:

• 植物が元気になりません。 = Plants don’t become healthy = more naturally, Plants don’t do well

If the sentence used 元気ではありません, it would describe their state more directly, but it would lose the nuance of not developing into a healthy state.

It is the polite negative non-past form of なる.

• dictionary form: なる
• polite form: なります
• polite negative: なりません

Japanese non-past can cover:

• present
• future
• habitual/general truths

Here it expresses a general fact:

• 水が少ないときは、植物が元気になりません。 = When there isn’t much water, plants don’t become healthy = If there isn’t enough water, plants won’t thrive

So in English, depending on context, it may sound present, habitual, or future. Japanese uses the same non-past form for all of those.

It can definitely be used for plants too.

With people, 元気 often means:

• healthy
• energetic
• doing well

With plants, it means something like:

• healthy
• vigorous
• thriving

So 植物が元気になる means the plants become healthy or vigorous. In natural English, you might translate the whole sentence as:

• When there isn’t much water, plants don’t grow well.
• When water is scarce, plants don’t stay healthy.
• If there isn’t enough water, plants won’t thrive.

These are all natural ways to express the idea behind the Japanese sentence.