I want to know if this method can be apply to apple leaves....

There is no reason why not. I have not tried with Apple leaves. This protocol would definitely need to be modified based on your tissue though, and they're several places where it can be modified if you think about it in a smart way. For e.g. rice and wheat leaves are tougher than Arabidopsis, thicker, with more waxy cuticle etc. Therefore, you need to ensure the vacuum infiltration is effective. This is how the leaf tissue takes up the stain. For e.g. I modified that part of the protocol to use a tougher/stronger vacuum pump. You can also play around with the duration of staining and de-staining. There are additional ways to de-stain a leaf, you can boil for longer, you can change the de-stain solution more times etc. So, in summary, yes you should be able to adapt this protocol for apple leaves. Please let me know if it works, I'd be curious to hear of your outcome.