I want to know what, if any. the significance is of the speech balloons over the heads of the customers in the nursery.

Each bubble means something different. This should be in the regular PT forum. The admins will move it for you. THere are already a few topics like this.

In the Charley's Plant Tycoon Guide There is some meaning (and a lot of spoilers!!)

I've been trying to find somewhere to ask that same question for ages, that one thought balloon the long white one that looks like an arm and hand kind of but badly drawn, puzzles the heck out of me, anyway, your guess is as good as mine LOL. I have a hate/hate thing going with my customers anyway, I love my plants but I have the cheapest customers on the planet, I call them the $15 people, as soon as I raise the price of a new plant to $15 they complain it is too expensive. At this rate I will be in the old age home before I make any money at this game but I will at least have had fun finding all of the plants and bugs, I have plenty of time....

The bubbles Charley's guide is simply his interpretation, and I disagree with some of it, no offense.