Plants may or may not care about the emotions of people, but they do have some level of awareness.
I've observed studies in which plants were placed in identical growing boxes. On one end of each box a sound speaker was set up. One speaker played heavy metal, the other played classical. A time-lapse video showed the plants in the heavy metal box tilting away from the speaker. The plants in the classical box moved closer to the speaker.
http://www.dovesong.com/positive_mus...xperiments.asp
A similar study on the growth of water crystals revealed that classical music produces more beautiful formations than death metal.
http://www.youtube.com/watch?v=425EQ6mhpzA