The leaves of various species of the tobacco (Nicotiana) family can easily be persuaded to regain full green colour, even when they have senesced to the point of containing virtually no measureable chlorophyll.

Biochemical and ultrastructural measurements show that this happens because existing senescent cells and plastids are completely rebuilt.  Reversibility is a characteristic of true senescence that fundamentally distinguishes the process from death.

Hilda Zavaleta, a colleague from Mexico, carried out the very nice research that established this important fact (H A Zavaleta-Mancera, B J Thomas, H Thomas, I M Scott (1999) Regreening of senescent Nicotiana leaves. II. Redifferentiation of plastids. Journal of Experimental Botany 50: 1683-1689; H A Zavaleta-Mancera, K A Franklin, H J Ougham, H Thomas, I M Scott (1999) Regreening of senescent Nicotiana leaves. I. Reappearance of NADPH-protochlorophyllide oxidoreductase and light-harvesting chlorophyll a/b-binding protein. Journal of Experimental Botany 50: 1677-1682).