sentence: Previous research also showed that N deficiency reduces the number of cells and starch granules in the endosperm , and represses the activity of enzymes regulating sucrose uptake or utilization , such as soluble and bound invertase , sucrose synthase , and AGPase ( Singletary and Below , 1990 ; Singletary et al ., 1990 ; Cazetta et al ., 1999 ) .