Stimulation of Growth and Nucleic Acid Biosynthesis at Low Concentration of Abscisic Acid in Tissue Culture of Spinacia oleracea

The effect of abscisic acid on growth, ultrastructure and nucleic acid biosynthesis was studied in tissue culture of spinach (Spinacia oleracea L.). Low concentration (0.01 mg l^?1) of abscisic acid increased fresh and dry weight of calluses, whereas 1.0 mg l^?1 was inhibitory. The stimulating effect was observed only in the presence of a relatively high concentration of kinetin (1 mg l^?1). The inhibitory effect was partly overcome by the same kinetin concentration. The low concentration of abscisic acid probably accelerated the induction of callus growth after subculture and stimulated cell division in the exponential phase of growth. Electron microscopy showed the presence of numerous polysomes and rough endoplasmic reticulum in callus cells grown at the stimulating abscisic acid concentration. Control cells and cells at the inhibitory concentration had slightly hyaline cytoplasm and were more vacuolated. Incubation of callus tissue with ³²P in the presence of stimulating concentration of abscisic acid showed a significant increase in the rate of biosynthesis of all nucleic acid classes after 8 h, whereas inhibitory concentration produced a decrease in ³²P incorporation. However, when the tissue was grown in the presence of abscisic acid for 20 days, both concentrations decreased the rate of nucleic acid biosynthesis, as compared to the controls.