http://imageshack.us/photo/my-images/831/p9282902.jpg/
http://imageshack.us/photo/my-images/101/p9282909.jpg/

we really need to know a bit more about your tree to even begin to assume what could be wrong.

what is the soil like?
how often do you water?
how often and what type of fertilizer do you use?
where is the tree growing and how much sun does it get?

lets start there and others will chime in soon.

mike

I have this lemon for 10 years at home, but this is the first time when leaves dry up this way (see photo), i grow my lemon at home in pot, so sun is limited, soil i buy in shop (special citrus soil), i always look that soil is not too wet or too dry, i don't use much of fertilizers, but this year i have changed soil just before leaves start to dry up, and just in case a few weeks ago again, but leaves still dry up. Lemon stay at the same place for 10 years, but this is the first year when leaves start to dry up. My only thought is that soil get too acidic, and i think to change it to one from my garden, but so often soil changes isn't good for roots.