A growing body of evidence suggests that the concentration of certain minerals important in human nutrition (especially calcium and copper) as well as some vitamins and proteins is lower in fruit and vegetable today than they were in the past. Research suggests that this decline in quality can be largely attributed to a “dilution effect” related to a focus on yields rather than nutritional quality (or flavor), both in plant breeding programs and in agronomy research. To assess the impact of breeding/selection practices on nutrient quality researchers have grown older low yielding cultivars of various crops alongside modern high yielding cultivars in side by side trials using identical production practices under the same environmental conditions. Although there was significant year to year and site to site variability in nutrient levels, there was also a fairly consistent trend in the findings – beneficial mineral nutrients like magnesium and iron were found in higher concentrations in the older low yielding cultivars than in the modern cultivars. This “genetic dilution effect” could be attributed to selection for fast growing, fast maturing genotypes that had a larger average cell size that rendered the harvested portion of the plant large, juicy and tender. It appears that this combination of traits has been considered more important in recent breeding programs than issues like mineral content. Davis (2009) notes that the rate of decline in nutrient levels over the past 40 years has been greater in vegetable crops than in fruit. Over the same time period, the rate of improved of yields in new vegetable varieties was double the rate of improvement achieved in fruit crops – another line of evidence suggesting that selection practices have contributed to the observed decline in the nutritional quality of fruit and vegetables. In trials conduced with varying levels of production inputs like fertilizers, irrigation or pest control, a similar dilution effect was observed – high input production practices that led to rapid crop growth and high yields also resulted in production of commodities containing reduced concentrations of various minerals and other potential beneficial nutrients. Given these two types of dilution, its is perhaps not surprising that organically grown produce tends to have higher levels of various minerals and other nutrients – as organic production systems are typically based on growing older, open-pollinated cultivars using reduced inputs. The result is a slower growing and potential lower yielding crop, but the resulting fruit and vegetables may contain higher levels of beneficial nutrients or other complex phytochemicals, including the compounds that give the crop its flavor.
Source : Crop Yield and Quality: Can We Maximize Both? A Colloquium presented at the ASHS Conference (2007). HortScience 44: 5-22. (2009)