Negative pressure will prevent air from inside getting out through the intake area...as long as it is reasonably sized. It will work just as well as an intake fan. However, you actually don't want too much negative pressure, depending on your opening shape/size, because you run the risk of turbulence at the opening actually letting some air from inside leak out every once in a while. You want a nice and slow laminar flow of air through your intake, and slight negative pressure. The flow rate will govern the intake/exhaust area.
So after the fluid flow part..
Don't plants actually intake air from the bottom of the leaves?
I have a mylar tent, and I just zip it almost to the very bottom, and have a little opening at the bottom, with my fans (2 CPU fans, one DC, one AC, with another AC about to be added) up top exhausting out the top. There is definitely a good amount of negative pressure when I close it all the way up, and at the intake size I have there is a little negative pressure but not too much. you can definitely feel the air flowing out the exhaust. The key to using CPU fans that don't have a lot of pressure is straight, smooth tubing with as few turns in it as possible. Every rough part or bend will increase pressure, which those fans suck at pushing air against. But if you give them some nice straight shots, they will pump air. Most are around 100 cfm, so 2 or 3 of them and you have some very quiet, cheap, fans.