ai explicatia in HELP:
Delete a channel
You may want to delete spot or alpha channels you no longer need before saving an image. Complex alpha channels can substantially increase the disk space required for an image.
In Photoshop, select the channel in the Channels palette and do one of the following:
Alt-click (Windows) or Option-click (Mac OS) the Delete icon .
Drag the channel name in the palette to the Delete icon.
Choose Delete Channel from the Channels palette menu.
Click the Delete icon at the bottom of the palette, and then click Yes.
Note: When you delete a color channel from a file with layers, visible layers are flattened and hidden layers are discarded. This is done because removing a color channel converts the image to Multichannel mode, which does not support layers. An image isn’t flattened when you delete an alpha channel, a spot channel, or a quick mask.
