Transluscent leaf using principled shader I this setup as shown in the screenshot. The image transparency plugs into the mix factor, the transparent and principled nodes plug into the mix shader inputs. I'm trying to figure out how to introduce translucency into the setup. Any help would be appreciated.
Final result:



I will show you two methods, first one (Translucent) is recommended because it's a lot faster approach and also with less geometry. For both of them you will need Mix Shader with Transparent BSDF and Alpha from Image Texture as a Factor.

Example above is rendered with only 32 samples. Beneath both meshes there is a light source to show the effect.

Translucent:



Simple mix between Principled BSDF and Translucent BSDF, both with Image Texture as a Color. Factor between them will allow you to control how much light will go trough mesh.

Subsurface:

For this method to work you will need to add some thickness to your mesh. I've added Solidify Modifier with pretty low value:



And node setup:



Choose color of Subsurface to be close to the Image Texture and set Subsurface strength.