The review cited here suggests that where this has been done, nutrient concentrations have indeed declined over time. However, the cause of the decline is less clear. One factor might be modern improvements in irrigation and cultural techniques that have produced fruit and vegetables containing more water and carbohydrates than in the past and so their nutrient content is "diluted". There could also have been errors in the analytical methods employed in the past and inappropriate statistical analyses might have been used. To overcome some of these objections, older varieties of vegetables were grown side-by-side with modern ones. The cultural systems, crop environments and analytical methods were then the same for all.
In such experiments, the nutrient concentration of broccoli heads was found to decline with increasing head weight. Consequently, as modern varieties tended to produce heavier heads, they were more likely to contain lower nutrient concentrations.
It was concluded that breeding for increased yields caused the reduced mineral nutrient concentration of many vegetables.
Declining Fruit and Vegetable Nutrient Composition: What is the Evidence? by Davis (2009). HortScience 44 (1): 15-19. Members of ISHS can view HortScience from the website www.ishs.org.