https://www.researchgate.net/post/What_is_the_simplest_method_to_detect_Proline_content

please see it

If you are only interested in (relative) differences between treatments in proline contents and not in absolute amounts, the cheap standard might be sufficient.

If absolute amounts are of real importance, you should use the reference standard or certified reference material.

thanks Mushtaq Ahmad and  C.A.(KEES)Kan

Plz tell fresh sample is better or dry one 

Fresh or dry will also depend on the question you want to answer.

If differences in dry matter content between treatments are expected or important, analyses on dry matter basis might be the way to go.

If not, fresh samples might be sufficient.

This article may be useful for you

in title:

Determination of proline concentration, an abiotic stress marker, in root exudates of excised root cultures of fruit tree rootstocks under salt stress

fresh and dry samples depends on the type of study. usually fresh samples are taken for general studies.