Synergistic and Sequential Effects of Hormones
In vitro studies on callus and suspension cultures have brought out two interesting findings. The nature and direction of differentiation is guided by the fact that whether two hormones are supplied together (synergistic effect) or in a sequence (sequential effect). Different effects result depending on whether auxins supply is followed by cytokinins and vice versa. It also matters whether only auxins and cytokinins are supplied together or whether auxins and gibberellins or similar combinations of all the five growth hormones.
The latent built in capacity to grow which resides in the mature cells of carrot or of many other angiosperms may be released by appropriate exogenous stimuli. When growth is so induced the cells respond to normal range of organic and inorganic nutrients and of vitamins if these are furnished under aseptic conditions, F.C. Steward, through his studious efforts spread over years, cultured full grown plants from free cells of carrots in suspension. It is these studies that demonstrated the synergistic and sequential effects of hormones. More of cytokinins and less of auxins stimulate shoot growth and less of cytokinins and more of auxins induce roots. Light is also needed for greening of leaves.