Differentiation of shoot is controlled by low auxin concentration. While in tissue culture auxin concentration is made high to promote rooting.

Differentiation of shoot is controlled by high cytokinin: auxin ratio.

Explanation:
Shoot differentiation refers to the process by which undifferentiated cells in the shoot apical meristem (SAM) undergo specialized growth and development to form different shoot structures such as leaves, stems, and flowers. This process is tightly regulated by the balance of plant hormones, particularly cytokinins and auxins.

1. Role of cytokinins:
Cytokinins are a class of plant hormones that promote cell division and growth. They are synthesized in actively growing tissues and are involved in various physiological processes, including shoot differentiation. Cytokinins stimulate cell division and promote the growth of shoot meristems, leading to the formation of new shoots and lateral branches.

2. Role of auxins:
Auxins are another class of plant hormones that regulate plant growth and development. They are primarily synthesized in the shoot apical meristem and are involved in cell elongation, differentiation, and organ formation. Auxins promote the differentiation of shoot cells into specialized tissues, such as leaves, stems, and flowers.

3. Role of cytokinin:auxin ratio:
The balance between cytokinins and auxins is crucial for shoot differentiation. A high cytokinin:auxin ratio promotes shoot growth and differentiation. Cytokinins stimulate cell division, while auxins promote cell differentiation. When the cytokinin:auxin ratio is high, cell division is favored over cell differentiation, leading to the formation of new shoots and lateral branches.

4. Other factors:
While the cytokinin:auxin ratio plays a significant role, other factors also influence shoot differentiation. For example, the presence of other plant hormones like gibberellins and abscisic acid can modulate the effects of cytokinins and auxins on shoot differentiation. Environmental factors such as light, temperature, and nutrient availability also influence shoot development.

In conclusion, the differentiation of shoots is controlled by a high cytokinin:auxin ratio. Cytokinins promote cell division, while auxins promote cell differentiation. When the cytokinin:auxin ratio is high, cell division is favored over cell differentiation, leading to the growth and development of shoots.