I just found this on UK_Ferns, the Yahoo group:

Defoliation

MANY years ago, when I was a student, I had a friend who was doing his

PhD on the translocation of nutrients in tomatoes. He used radioactive

isotopes then followed their movement within plants using a Geiger

counter. He found that as leaves senesce, the nutrients contained in

them were translocated back into the stem and then to the young

growth, to nourish the new leaves. This translocation continued until

the old leaves were completely dried up, and in the case of

phosphorus, it was still being returned to the rest of the plant even

when the old leaves were completely yellow.

This means that, with few exceptions, the old leaves should not be cut

off until they are completely dead, the main exception being if the

old leaves are diseased and should be removed in order to limit the

spread of the disease onto the new growth. In that case, the plant

should be given a boost of fertilizer to replace the nutrients that

have been removed in the leaves that have been cut off.