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For many decades plant breeders have worked to improve vegetable crops for numerous economically important traits, like host plant resistance to disease, yield, and vegetable quality. Most improvements have been made with little knowledge as to how, or if, nutritional or phytonutrient concentrations might also be indirectly altered in the process. There have been some reports suggesting that concentrations of nutrients in vegetables have been reduced over time, possibly related to introductions of new cultivars. However, for most vegetables, current evidence indicating changes in nutrient concentrations, and specifically mineral concentrations, is circumstantial at best. To effectively test whether changes may have occurred over time as new cultivars replace older ones, appropriate field studies must be conducted wherein harvested produce from “old” vs. “new” crop cultivars is analyzed by appropriate methods and compared directly. Numerous considerations and issues such as 1) the set of cultivars to be used in field tests; 2) how nutritional concentration will be expressed; and 3) the evolution, history, and consumption changes of the crop under study must be addressed in making such direct comparisons and interpreting results.

Broccoli(Brassica oleracea L. Italica Group) can contain high levels of selenium (Se) in the form of selenium methyl selenocystine. This is a relatively unique Se compound that is found in certain plant species that accumulate this element. Several recent studies have shown that high Se broccoli can inhibit the development of certain cancers (e.g., colon and mammary) in rodents and this has led to increasing interest in broccoli as a vegetable that confers chemoprotective effects. The objective of this research was to determine the relative importance of genotype vs. environment in the expression of Se concentration in broccoli heads. A set of 15 broccoli inbreds and a set of 20 hybrids were evaluated in three different environments. Mature heads were harvested from plots, heads were dried and ground, and Se concentration was determined on a dry weight basis. Overall, Se levels measured in this study were low to moderate, typically ranging from about 20 to more than 100 ng/gdw of Se per head. For both inbreds and hybrids, the effect of environment on Se head concentration was highly significant and more than 10 times greater than the effect of genotype. When analyzed across all three environments, the genotypic effect on Se concentration was significant for hybrids only. However, when assessed for individual environments, the genotypic effect was significant in just one of three of the test environments with both inbreds and hybrids. Results indicate that genetic modification of broccoli to increase selenium concentration of heads will likely be difficult to achieve.

Understanding the mechanisms that regulate xylem transport of calcium (Ca) to green bean pods could allow approaches to increase pod Ca concentrations and enhance the nutritional value of edible pods. Using the green bean cultivars `Hystyle' and `Labrador', that exhibit high and low pod Ca levels respectively, we wished to determine whether observed differences in Ca concentration of stem xylem-sap were related to differences in whole-plant water uptake and Ca import. Well-watered greenhouse-grown plants, selected at flowering and at two stages of pod development, were placed in a growth chamber at a constant light intensity. Pot weight loss was measured to determine whole-plant water use and stem xylem exudate was subsequently collected from the severed base of the shoot. `Hystyle' displayed 50% higher Ca concentration in exudate than `Labrador' during pod development. Labrador showed 35% greater total water transport through the stem than `Hystyle'. Additional plants were used to determine total, long-term Ca uptake. No significant differences in total Ca were seen between cultivars at the three harvest dates. With whole-plant Ca uptake being equivalent, the results suggest that higher water uptake in `Labrador' led to a dilution of Ca in the xylem stream and thus less total Ca was transported to developing pods, relative to that in `Hystyle'. These results reveal that green bean varieties with low whole-plant water use have the potential to yield edible pods with elevated Ca content.

Calcium is an essential human nutrient that is important in bone growth and metabolism. While dairy products in reasonable quantity can supply minimum daily requirements of calcium, the current decline in the consumption of dairy products, especially among teenagers, implies that dietary requirements must be met from other sources. Green beans, spinach and broccoli contain high concentrations of calcium, but the bioavailability of calcium from these food sources has not been determined in children. To provide accurate dietary recommendations for these foods, we have developed a recirculating hydroponic system for the growth and intrinsic labeling of plants with stable isotopes. Plants were maintained on a non-labeled nutrient solution until an appropriate developmental age and were then presented with nutrient solution containing 42-Ca. Labeled green bean pods and spinach were harvested at the proper commercial age, and were cooked, pureed and frozen until use. Thirteen teenage subjects seven girls, six boys) were recruited for a 2-week stay in the Metabolic Research Unit, and were fed 42-Ca-labeled vegetables along with 48-Ca-enriched milk; an intravenous dose of 46-Ca was also administered. Blood, urine and fecal samples were collected during the 2-week study. Calcium bioavailability and kinetics were determined using a multi-compartment model. The bioavailability of calcium averaged 28% from green beans, which was comparable to that of milk calcium. Calcium bioavailability from spinach averaged only 3%, due probably to the high oxalate content of spinach. Our results suggest that low-oxalate containing vegetables such as green beans can serve as good dietary sources of calcium. This research was funded in part by USDA-ARS Coop. Agr. No. 58-6250-1-003 and USDA-CRS Grant No. 94-347200-0605.