By influencing the decline in proteins involved in photosynthetic metabolism.
Reference:
Regulation of leaf senescence by cytokinin, sugars, and light. Astrid Wingler, Antje von Schaewen, Richard C. Leegood, Peter J. Lea, and W. Paul Quick. Plant Physiol. (1998) 116: 329-335.
http://www.plantphysiol.org/cgi/content/full/116/1/329