A poem relies on all except the plot.
Why? The plot is the main events that occur within a story, however poems are not always stories, and some are just used to express feeling.
Emotion is needed because that's what all literature when expressing an idea or feeling require; emotion. It's the way for the reader to connect with the ideas of the author and therefore have a closer understanding to what the author was trying to achieve in writing the poem.
Images are always important when it comes to poetry. The blander a poem is, the less of a poem it becomes. This is because imagery is not just used to talk about "pretty images" and "beautiful designs", rather, it's used to further describe what something is and what the meaning of it is towards the author, and how the reader should look at it.
Finally, poems need a theme. It doesn't matter if the poem is written in the most seclusive way possible to send out a silent message of emotion; theme is what the subject is about, and whenever someone writes, there is always a point to it. Whether there is true meaning to it will always be up for debate, but theme is crucial for all literature, especially poems.
So yes, your answer is in fact, all except plot, is what a poem relies on.