Lemon Tree Problems (yellow leaves)

[lemontree]

My lemon tree is approximately 8 yrs old and is becoming very sad looking. These are some of the problems:
a) some bare patches with no leaves,
b) leaves are turning yellow (green at base of leave, yellow in the middle and brown on the tip)
c) whole tree looks wiltered and not very healthy looking
d) bares plenty of fruit, but has lots of seeds

Have photos available below

Can anyone help?

Thanks

https://i391.photobucket.com/albums/oo358/lemontree001/P1100001.jpg

https://i391.photobucket.com/albums/oo358/lemontree001/P1100002.jpg

https://i391.photobucket.com/albums/oo358/lemontree001/P1100003.jpg

https://i391.photobucket.com/albums/oo358/lemontree001/P1100004.jpg