Genome Expression during Normal Leaf Development : I. CELLULAR AND CHLOROPLAST NUMBERS AND DNA, RNA, AND PROTEIN LEVELS IN TISSUES OF DIFFERENT AGES WITHIN A SEVEN-DAY-OLD WHEAT LEAF.

Changes in genome expression during normal cellular and plastid development in the first leaf of young (7-day-old) wheat (Triticum aestivum var. Maris Dove) were investigated by examining homogeneous populations of leaf cells and plastids of several developmental ages present in the same leaf. The cells were characterized over a period immediately following the last cell division. All of the leaf cells had cytoplasmic contents and nuclei, and between 44% (young tissue) and 54% (older tissue) of the leaf cells were mesophyll cells. Chloroplast development was complete 36 hours after the chloroplasts had ceased dividing. Extremely large changes occurred in cellular constituents over a very short period of leaf development. Maximum rates of accumulation of ribulose bisphosphate carboxylase per mesophyll cell (80 picograms/hour), chlorophyll per mesophyll cell (9 picograms/hour), and 70S ribosomes per mesophyll cell (19 x 10(5)/hour) were recorded.Total cellular DNA varied from 40 to 60 picograms/cell, reflecting the changes in nuclear and chloroplast DNA synthesis during different phases of cellular and chloroplast division. The period of maximum accumulation of protein, total RNA, and both 80S and 70S ribosomes occurred between 36 and 48 hours after the last cell division. Between 48 and 60 hours, 70S rRNA per cell and protein content per cell continued to increase as 80S rRNA per cell declined. Ribulose bisphosphate carboxylase per cell increased 20-fold between 15 and 60 hours.