Root volume can be good option and if you want to have an option from aboveground part you can measure leaf area and also specific leaf area (indicative of the thickness of the leaf)
Leaf area, plant height, number of side shoots, stem diameter.you could also estimate the number of stomata,and the best is to look at the effect of salinity on anatomy of leaf, root and stem.
In addition to what Dr. Muayed Fadhil Abbas and others mentioned, you might want to consider specific leaf area (leaf weight/area ratio), shoot:root ratios, degree of leaf area scorching (salt burn/necrosis).