MacBook Air - alpha problem

Hi,
I have a problem with the simulation I'm trying to develop for the web.

I have a number of trees that must be seen from different positions/point of iew, so I applied a Particles/Alpha Blended shader to the meshes to have both the transparency and the double-sided material.
This solution works fine on my Windows-based workstation, on my MacMini (OS X Lion, Intel HD graphics 3000), but not with the MacBook Air (OS X Lion, NVIDIA GeForce 320M OpenGL Engine, with 256MB).

When running the application in the web player, it seems that the MacBook Air is not able to "hide" the black part of the texture that should become invisible due to the alpha.

Could it be a problem related to the graphic card?
Is there a better option to have a two sided material with the alpha?