Kinetin and the nucleic acid content of onion root tips

Abstract

Roots of onion ( Allium cepa) were grown for 1 2 to 48 hr in 1 ppm kinetin. Morphological analyses of the first 2 mm of the root were made. The DNA and RNA contents of the cells in this region were determined. The data indicate that kinetin has an immediate and marked effect on RNA content of the cells resulting in a doubling of RNA per cell in the region 1200 μ, to 2000 μ from the tip after 1 2 hr. The amount of RNA per cell drops after this period, reaching a plateau which is 30 per cent above the control. This plateau persists through the 12 hr period. After 24 hr the RNA content of the cells in this region drops below that of the control. Treatment of the roots with kinetin for 48 hr resulted in a decrease in RNA content per cell to less than 1 2 that of the control. This result was obtained for all cells in the root except the apical initials. The RNA content of the apical initials was not affected by any of the kinetin treatments used. The RNA content of the cells 300 μ-700 μ from the tip was affected only by the 48 hr treatment. The DNA content of the cells was not immediately affected by treatment with kinetin. After 9–12 hr treatment with kinetin there was a decrease in DNA in the cells 1000 μ-2000 μ from the tip. After 24 hr there was a decrease in DNA in the cells in the region 400 μ-1000 μ from the tip. After 48 hr the DNA content of all the cells was reduced to that of the apical initials. The DNA content of the apical initials was not affected by the kinetin treatments used. Morphological analysis of kinetin treated roots indicated no immediate effect by kinetin treatment. The 1 2 and 1 hr treatments produced no morphological changes. A decrease in average cell diameter began after 3 hr of treatment and continued through 12 hr of treatment. After this period no further reduction occurred. The average cell diameter of the epidermis and cortex showed an average reduction of 20 per cent and cells of the vascular system 33 per cent. Root cap cells distal to the apical initials showed no reduction in average diameter, while those surrounding and in front of the apical initials showed a 25 per cent reduction in average diameter. Treatment with kinetin had no effect on cell elongation.