Citrus Leaf Chlorosis Induced by Sink Removal: Starch, Nitrogen, and Chloroplast Ultrastructure

Chlorosis was observed in the mature source leaves of Citrus reticulata Blanco when branches were girdled and defruited, thereby in effect removing all major sinks. Starch levels slightly increased when either the fruit sink (defruiting) or the root sink (girdling) was removed. However, only when both sinks were removed did starch levels increase dramatically. Total nitrogen decreased and proline, arginine, and serine increased only when both sinks were removed. The non-protein amino acid, gamma-aminobutyric acid, decreased in response to total sink removal and was unaffected by removal of only one sink. In a time course study it was observed that the increase in starch levels preceeded the decrease in chlorophyll levels. A developmental study of chloroplast ultrastructure showed that starch granules increased in size and number and appeared to play a role in the deformation of the chloroplast lamellar organization. Concomitant with thylakoid breakdown was an increase in plastoglobules which were extruded from the plastid. Finally, plastids of the chlorotic leaves resembled amyloplasts or chloro-amyloplasts. The results are discussed in terms of source-sink relationships and chloroplast integrity.