Several possible reasons can be the cause, but you should describe better flower conditions, if cut or in situ or stressed. Halevy has produced an historical review, http://mazinger.sisib.uchile.cl/repositorio/pa/ciencias_agronomicas/h2002881428ago07.pdf, but on the bench the first trial could be the treatment with an ethylene inhibitor, http://link.springer.com/article/10.1007/BF00272538#page-1

Thank you sir

Thank you sir

It may be due to stress or disease or pest problem you only have to ascertain.Probably you need to check for deficiency of any micro nutrients, for this you initially check the leaf symptoms.

Thank you