Description:
The plant growth regulator that increases the length of stem and enhances the yield of sugarcane crop when sprayed on is called gibberellin.

Explanation:
Gibberellins are a group of plant hormones that regulate various aspects of plant growth and development. They are involved in stem elongation, seed germination, flowering, and fruit development. In the case of sugarcane, gibberellins play a crucial role in increasing the length of the stem, which ultimately leads to a higher yield of sugarcane crop.

Role of Gibberellins in Sugarcane:
When gibberellins are sprayed on sugarcane crops, they stimulate cell elongation, leading to an increase in the length of the stem. This elongation of the stem allows the plant to grow taller, resulting in more leaves and more space for photosynthesis. As a result, the plant is able to produce more energy and allocate it towards the formation of larger and healthier sugarcane stalks.

Benefits of Gibberellin Application:
The application of gibberellins on sugarcane crops offers several benefits, including:

1. Increased Yield: The elongation of the stem induced by gibberellins leads to an increase in the number of internodes and ultimately increases the yield of sugarcane.

2. Better Quality: The longer stems produced as a result of gibberellin application are generally thicker and have a higher sugar content, which enhances the overall quality of the harvested sugarcane.

3. Early Harvesting: Gibberellins can also promote early maturation of sugarcane crops, allowing for an earlier harvest. This can be advantageous in areas with shorter growing seasons or in situations where early harvesting is required.

4. Resistance to Lodging: Gibberellins help in strengthening the stems of sugarcane plants, making them more resistant to lodging (falling over) under adverse weather conditions such as heavy rain or strong winds.

Conclusion:
Gibberellins are plant growth regulators that, when sprayed on sugarcane crops, promote stem elongation, leading to increased yield and improved quality of sugarcane. The application of gibberellins offers several benefits to sugarcane farmers, including higher productivity, early harvesting, and increased resistance to lodging.

Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.