Soak them in a bath 1 tablespoon white vinegar in 1 quart water for a couple of hours.

The osmosis will restore the cell walls. Rinse and use. If the application would have some reason not have a slight vinegar taste. Sub salt and water. Dry and proceed.
Tho, for parsnips, turnips, and others a bit of the vinegar would give it a bonus.

The vinegar tech is great for restoring 'on the edge' wilted greens--lettuce, herbs, cucumbers. etc. to use in salads where the vinegar taste is a bonus, or limp carrots cut in matchsticks soaked and used in stir fries.

Not surprising really, when you think about it. As veggies (and everything) age, they loose moisture. Seems that either the pre-blanch or the CI method would be the way to rescue old veggies, or find another way to cook them that doesn't include, high, dry heat.

I often find that a pre-roast blanching gives me better results. Sort of taking the cook's illustrated thought a step further...

raquelita, this may have to do with the age of your vegetables. root vegetables like a rutabaga will last 3-4 months in a 32-40F dark root cellar, but if you keep them in a refrigerator or at a temperature above 40F this time period drastically tapers off. a root vegetable kept in a refrigerator will keep crisp only 2-4 weeks before you start noticing differences in taste and texture. after this...they probably aren't the best to cook with anymore.

science: all vegetables store energy in the form of pectin, which is a complex carbohydrate/sugar that softens and sweetens a vegetable when roasted, which gives them that caramelized, tender loveliness that we all enjoy. pectin exists inside the cell walls on a vegetable, which allows water to be evenly distributed and the vegetable to remain crisp. as vegetables age, the pectin breaks down and vegetables begin to get soft and mushy and as a result will also roast differently. old vegetables will start mushy and get rock hard as they cook (anyone who has tried to cook old green beans can attest to this...they turn into twigs) since they don't have the pectin to release the sugars as it roasts.

if you are dead-set on trying to use vegetables that might be a little on the older side, a technique from cook's illustrated may possibly help. they roast their vegetables in two stages, the first part covered in aluminum foil (the trapped steam inside helps "pre-cook" the veggies and stabilize the pectin) followed by the second half where they remove the foil and allow the vegetables to finish roasting to their caramelized glory. the moisture trapped by the foil may help make up for their lower water content and prevent them from getting really hard. i hope this helps!