In cucurbits for changing the sex forms growth regulators are sprayed at -

The correct answer is 2 - 4 true leaf stage.

• Majority of cucurbits are monoecious and sex ratio (male: female) ranges from 25-30:1 to 15:1. 
• Sex ratio is influenced by environmental factors.
• High N content in the soil, long days and high temperature favour maleness. 
• Besides environmental factors, endogenous levels of auxins, gibberellins, ethylene and abscisic acid also determine sex ratio and sequence of flowering. 
• A primordium can form either a female or a male flower and it can be manipulated by addition or deletion of auxins. 
• Endogenous application of plant growth regulators can alter sex form when applied at 2 - 4 leaf stage.