The health benefits of following a diet rich in vegetables have been well established. However, the factors that determine exactly how “healthy” any given vegetable will be have not been as well established. For example, the Vitamin A content of carrots can vary by 5X, depending on the cultivar, the production location and the year. G. Lester (HortScience 2006) has reviewed some of the factors that can influence the levels of several important nutritional agents (vitamins and antioxidants) in fruits and vegetables. Not surprisingly, which crop you chose to eat has the greatest impact on the levels and balance of nutrients consumed. The variety or cultivar of the crop is also important. Size matters – in general, as the size of the particular item increases, so do the levels of nutrients it contains. This likely reflects the tendency for nutrient levels to increase with maturity. Once the commodity reaches full size, nutrient levels plateau and may even begin to decline as the commodity matures past prime condition. Nutrient levels tend to be highest under growing conditions that match the needs of the plant for optimum growth. For cool season crops like lettuce, nutrient levels in the harvested crop are highest when the crop is grown under relatively cool conditions. Cool conditions close to harvest time are particularly beneficial. Warmer season crops like tomato show the same trend, except at higher temperatures. In general, the more light the crop receives, the higher the concentration of nutrients found in the crop. This relationship, however can be complicated as high light levels may also lead to high temperatures and rapid growth – factors that can actually reduce nutrient concentrations in the crop. Low light levels coupled with high temperatures may explain why greenhouse-grown vegetables tend to have lower nutrient levels than field-grown crops. Crops grown on clay soils tend to have a higher nutrient content than crops from sandy soils. This reflects the greater fertility and nutrient holding capacity of the finer textured soils. Crops grown under irrigation or in areas with very abundant rainfall tend to have lower nutrient levels than crops from drier conditions. This may reflect dilution of the nutrients by the more rapid growth triggered by abundant moisture. The relative nutritional value of crops grown utilizing “organic” methods versus standard methods is a matter of considerable debate – the results seem to vary with the crop, the cropping situation and the nutrient being evaluated.
Source : G. Lester (HortScience 41: 59-64)