How about elemental analysis based on combustion?

http://www.rsic.iitb.ac.in/Chn.html

http://en.wikipedia.org/wiki/CHN_analyzer

unfortunately there is no CHN in our lab there is only spectrophotometer that i can use but i want method to know how i can deal with plant sample to determined sulfur in it by spectro photometer

Colorimetric method

http://www.sciencedirect.com/science/article/pii/0026265X59900207

Otherwise, you have to go for elemental analysis - CHN analyser or AAS

http://www.ncbi.nlm.nih.gov/pubmed/19650504

simple turbidimetric method for total sulfur,.

https://dl.sciencesocieties.org/publications/aj/abstracts/62/6/AJ0620060805

try this if you have CNS analyzer. it is very simple and accurate method. i used it during my PhD project. i have pasted it directly from thesis. it reads as below.

Carbon, nitrogen and sulphur (CNS) determination

Approximately 15-20mg of finely ground plant material was carefully weighed into a tin capsule. Five mg of vanadium pentoxide was added to each sample. Capsules were folded over using forceps and crimped tightly to avoid spillage of the sample. Reference materials (tomato leaves, cystein) with known CNS concentrations were treated similarly. One of the reference materials was chosen for calibration of the instrument while the remaining ones were run as samples. A capsule containing only vanadium pentoxide was used as a blank. CNS analysis was undertaken using a CE instruments Flash EA1112 Elemental Analyser. Samples were dropped into the combustion tube packed with approximately 25g copper oxide and 70g electrolytic copper and heated to 900oC. The resulting gas was then passed through an absorption filter packed with magnesium perchlorate to remove any water before passing through a PTFE separation column and onto a thermal conductivity detector.

Dear,

www.ncbi.nlm.nih.gov/pubmed/21639333