ABSTRACT
The activity of the chloroplastic fonn of GS increased during the development of wheat leaves, and the increase was in line with the onset of photosynthesis and photorespiration (Tobin et at. 1985, 1988, 1989). During the dark-induced and natural senescence of wheat leaves, GS activity falls rapidly (Peeters and Van Laere 1992, 1994), and a protease capable of breaking down the enzyme protein was detected in old leaves (Froh-
lich et aI. 1994). During the senescence of rice leaves, the ch10rop1astic GS protein was lost before the cytoplasmic fonn, although the mRNA encoding the enzymes was still present (Kamachi et aI. 1991, 1992a). In tomato, a decrease in the concentration of both the ch10rop1astic GS polypeptide and mRNA was detected during senescence (Perezrodriguez and Valpuesta 1996) and in the leaves of deciduous trees (pearson and Ii 1994; Woodall et aI. 1996b). In drought-stressed tomato leaves, the total GS activity was reduced by 50%, whereas the level of the chloroplastic GS polypeptide was unchanged and a slight decrease in the mRNA abundance was detected (Bauer et aI. 1997). These results highlight the importance of comprehensive studies that include the concomitant measurement of enzyme activity, protein, and mRNA.
