What passages in the story reveal Beatrice's relationship with the large flower?

Beatrice's relationship with the large flower is shown first with the counter-example of her father's relationship with the plant, and second, it is shown by her own interaction with it.  This is all set forth very early in the story.

First, we are to understand that Rappaccini himself cannot even touch the flower.  The story states,

...there was no approach to intimacy between himself and these vegetable existences. On the contrary, he avoided their actual...

Beatrice's relationship with the large flower is shown first with the counter-example of her father's relationship with the plant, and second, it is shown by her own interaction with it.  This is all set forth very early in the story.

First, we are to understand that Rappaccini himself cannot even touch the flower.  The story states,

...there was no approach to intimacy between himself and these vegetable existences. On the contrary, he avoided their actual touch, or the direct inhaling of their odors...(2).

The text goes on to show Rappaccini as

...one walking among malignant influences, such as savage beasts, or deadly snakes, or evil spirits, which...would wreak upon him some terrible fatality (2).

Rappaccini cannot touch any of these plants or even breathe in their scents.  This sets the stage for what follows, which shows how Beatrice handles the flower.  When her father calls her, she comes into the garden and down its path. Giovanni notes,

...it was observable that she handled and inhaled the odor of several plants, which her father had most sedulously avoided (3).

Rappaccini asks Beatrice to attend to the large flower, and her response is,

"And gladly will I undertake it," as she bent toward the magnificent plant and opened her arms as if to embrace it. "Yes, my sister, my splendor, it shall be Beatrice's task to nurse and serve thee, and thou shalt reward her with thy kisses and perfume breath, which to her is the breath of life" (3).

Thus we can see from these passages that the large flower is something that Rappaccini cannot touch or even be too close to, while Beatrice has no risk at all, and in fact, it is implied that this flower is her sustenance.  Given the author's choice of words, we are to infer that Beatrice is not only impervious to "evil," but thrives upon it.