Canned fruits and vegetables use no chemical preservatives at all. The home canning process is much different than today's commercial canning process. Except for the fact that Grandma used glass jars rather than steel cans, today's commercial canned foods are naturally preserved by cooking, vacuum-sealing, then cooling. Canned fruits and vegetables are lower in nutrients because many are lost in the canned process. If a fresh fruit or vegetable is an excellent source of a nutrient, it still is an excellent source after it is canned.
Canned pumpkin is an example.
The vitamin content of canned foods may be higher than what is on the label. Manufacturers are required only to put the minimum amount they expect to remain in the can on the label. Actual content may be higher, depending on the variety used, the source of the information and the manufacturers' policy regarding labeling quantities. When comparing the nutritional value of canned versus fresh fruits and vegetables, you must compare the two after preparation for the table. But cooking at home does the same thing, sometimes to a much greater extent.