Thank you Telepova-Texier, my second part of the work is :what age should my plant material have

especially since I work primarily with legume trees

and what method should I follow to apply the salt stress?

 1

Normaly, the first parameter to see after water stress is the gas exchange!

Very sensible and demonstrative.

2

I proposed you to see this article, for example:

Molecular, Physiological and Biochemical Responses of Theobroma cacao L. Genotypes to Soil Water Deficit

Ivanildes C. dos Santos, Alex-Alan Furtado de Almeida , Dário Anhert, Alessandro S. da Conceição, Carlos P. Pirovani, José L. Pires, Raúl René Valle, Virupax C. Baligar

Published: December 26, 2014http://dx.doi.org/10.1371/journal.pone.0115746

Thank you Telepova-Texier, my second part of the work is :what age should my plant material have

especially since I work primarily with legume trees

and what method should I follow to apply the salt stress?

I thank you very much for your answers. What about the molecular analysis?

Before doing anything "molecular" you need good phenotype data with standard ecophysiology techniques. As mentioned, gas exchange, growth rate and allocation are a good starting point.  From there it might be useful to look at pressure-volume curves so as to get info on osmotic and elastic properties.  Finally, if you have the expertise and mass spectrometry equipment handy it can be useful to examine metabolites in leaves and roots so as to gauge if/how osmotic adjustment is occurring and maybe examine oxidate stress markers (e.g. redox state of glutathione and ascorbate)