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Michael A. Grusak and Steven A. Abrams

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.

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Gene E. Lester and Michael A. Grusak

Commercially grown honeydew fruit (Cucumis melo Inodorus group) and netted cantaloupe fruit (C. melo Reticulatus group) in low-humidity regions of the U.S. are typically field packed, eliminating the possibility for postharvest chelated-calcium dip treatments to extend fruit shelf life. In this study, calcium treatments were applied to orange-flesh honeydew fruit commercially grown in 2001 and 2002 in Sacramento Valley, Calif. and orange-fleshed netted cantaloupe fruit commercially grown in 2002 in Imperial Valley, Calif., and Rio Grande Valley, Texas. Aminoacid-chelated calcium and mannitol-complexed calcium compounds were applied to field-grown plants at the rate of 2.3 L·ha-1 (1 qt/acre) at 0, 1, 2, or 4 total applications during growth of honeydew and cantaloupe fruit. Applications were A) at female flowering, B) within 15 days (cantaloupe) or 20 days (honeydew) after flowering, C) within 30 days (cantaloupe) or 40 days (honeydew) after female flowering, and/or D) within 3 to 5 days before abscission. One application equaled (A) or (D), two applications equaled (A + B) or (C + D) and four applications equaled (A + B + C + D). Evaluations of fully abscised fruit were exterior and interior firmness, marketability, calcium concentrations, interior soluble solids concentration (sugars), and consumer preference (taste) following harvest and up to 3 weeks commercial/retail storage. Cantaloupe fruit at both locations did not appear to benefit from preharvest plant applications of calcium when compared to fruit from plants treated with water. Honeydew fruit, however, did and the benefit was observed both years. Honeydew fruit that received four preharvest plant applications of calcium, regardless of source, were generally superior in firmness, marketability, and had a higher calcium concentration than fruit from plants receiving water or one or two applications of calcium. Fruit sugars and taste were not affected by preharvest plant applications of calcium.

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Kirk W. Pomper and Michael A. Grusak

Understanding the mechanisms that regulate xylem transport of calcium (Ca) to snap bean (Phaseolus vulgaris L.) pods could allow approaches to increase pod Ca concentration and enhance the nutritional value of edible pods. Using the snap bean cultivars Hystyle and Labrador, which exhibit high and low pod Ca levels, respectively, we wished to determine whether there were differences between the two cultivars in stem xylem-sap Ca concentration and whether any differences in sap Ca concentration were related to differences in whole-plant water uptake or Ca import between the cultivars. Well-watered greenhouse-grown plants were placed in a growth chamber at a constant light intensity for an equilibration period. 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 at flowering and at two stages of pod development. `Hystyle' displayed an exudate Ca concentration that was 50% higher than `Labrador' during pod development. `Labrador' showed 35% greater total water transport through the stem than `Hystyle'. `Labrador' plants also showed a significantly larger leaf area than `Hystyle' plants. Additional plants were used to determine total, long-term Ca influx. No difference was observed between cultivars in total Ca influx into the aerial portion of the plant. With whole-shoot Ca influx being equivalent and pod transpiration rate identical in the two cultivars, our results suggest that the higher whole-plant water uptake in `Labrador' led to a dilution of Ca concentration in the xylem stream and thus less total Ca was transported to developing pods, relative to that in `Hystyle'. Increased transpiration efficiency, enhanced root uptake of Ca, or reduced Ca sequestration in the xylem pathway of the stem could lead to an enhancement in pod Ca concentration in future cultivars of snap bean.

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Michael A. Grusak and Kirk W. Pomper

Understanding the mechanisms that regulate xylem transport of calcium (Ca) to snap bean (Phaseolus vulgaris L.) pods could allow approaches to enhance pod Ca levels, and thereby improve the value of this food source for humans. Pods of greenhouse-grown plants of `Hystyle', `Labrador', `Tendergreen', `Green Crop', `BBL94', and `Gold Crop' were examined for stomatal density and rates of pod transpiration throughout pod development. Among pods ranging from 6 to 14 mm in diameter, Ca concentration and pod stomatal density varied inversely with increasing diameter in all cultivars; Ca concentration for pods of a given diameter also varied among cultivars. To assess the influence of pod stomatal density on pod transpiration, water loss was measured from detached pods of `Hystyle' and `Labrador', which have high and low pod stomatal densities, respectively. Pod transpiration rates were similar for the two cultivars, being ≈15% the rate measured in leaves under equivalent conditions, and comparable to rates of cuticular transpiration measured in leaves with closed stomates. These results suggest that pod stomates have no role, or have only a limited role, in pod transpiration. Pods of `Hystyle' and `Labrador' were placed in enclosures that maintained constant high- or low-humidity environments throughout pod development. For each cultivar, the high-humidity environment led to lower pod Ca concentrations, demonstrating that pod transpiration does have a significant impact on pod Ca accretion. However, `Hystyle' consistently exhibited higher pod Ca concentrations, relative to `Labrador', suggesting that differences in xylem sap Ca concentration may have been responsible for cultivar differences in pod Ca concentration.

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Gene E. Lester and Michael A. Grusak

Muskmelon senescence is directly associated with a decline in hypodermal mesocarp membrane integrity and its Ca concentration, but infusing Ca into melons has been a problem. Fully ripened and abscised hybrid honeydew [Cucumis melo L. (Inodorus Group) `Honey Brew'] and netted muskmelon [Cucumis melo L. (Reticulatus Group) `Explorer'] fruit were submerged (dipped) 20 min at 25 ± 3 °C in a solution containing a Ca-chelate, a Mg-chelate, a combination of both chelates, or no mineral chelate. Following 10 or 24 days of cold storage (4 °C for `Explorer' and 10 C for `Honey Brew'), fruit were analyzed for mineral content and various senescence-related parameters. Abscised `Honey Brew' fruit dipped in either Ca-chelate or (Ca+Mg)-chelate and abscised `Explorer' fruit dipped in (Ca+Mg)-chelate, followed by 10 days cold storage, had hypodermal mesocarp Ca concentrations of at least 6.0 mg·g-1 dry weight. Maintaining hypodermal mesocarp tissue Ca concentrations at this level during postharvest storage, especially for fully ripe `Honey Brew' fruit, maintained membrane integrity and fruit firmness, and extended storage life 2.4-fold (i.e., to 24 days). The senescence regulatory effect of postharvest Ca-chelate treatments on abscised `Explorer' was highly variable, compared to `Honey Brew', which appeared to be due to the surface netting interfering with movement of Ca into the hypodermal mesocarp. Thus, postharvest Ca-chelate application to abscised `Honey Brew' fruit could delay fruit senescence in commercial storage, and open up new markets for fully ripened honeydew melons.

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Gene E. Lester and Michael A. Grusak

Commercially grown honey dew fruit [Cucumis melo (Inodorus group)] typically are harvested before abscission because fruit cut unripe have a longer storage life than fully ripe fruit. However, because fully ripe fruit contain higher concentrations of soluble solids (predominantly as sugars), an attribute that increases their preference among consumers, methods are being explored to extend the storage life of fully ripe fruit. In this study, fully abscised honey dew fruit were evaluated for tissue attributes and consumer preference following postharvest dipping in either chelated or nonchelated calcium (Ca) solutions. Calcium sources were an amino acid-chelated Ca, ethylene-diamine tetraacetic acid (EDTA)-chelated Ca, or calcium chloride (CaCl2), with each provided at three different rates. Fruit were evaluated at harvest, and after 14 or 22 days commercial storage. Evaluations were peel surface changes (color and disorders), hypodermal-mesocarp tissue Ca concentration, flesh firmness, soluble solids concentration, and consumer preference of the edible flesh. Peel color became yellowed and lighter during storage for all fruit, with higher Ca rates resulting in more intensely yellowed fruit. Hypodermal-mesocarp tissue Ca concentration was 0.90 mg·g-1 of fresh weight (900 ppm) at harvest, and declined in all fruit by 22 days storage. Peel disorders (disease and spotting) were none to slight for all fruit by 14 days storage, but by 22 days storage disease incidence ranged from none to severe, depending on the Ca source and rate. Fruit firmness declined in all fruit throughout storage, with the smallest declines measured in fruit treated with the amino acid-chelated Ca. Soluble solids concentration of fully ripe fruit was 12.3% at harvest, and showed either no decline or slight declines with storage among the treatments. Consumer preference was highest for freshly harvested fruit, but fruit were desirable even after 22 days storage across all treatments. Postharvest application of Ca at ≤0.16 m Ca in an amino acid-chelated form, versus EDTA-chelated Ca or CaCl2, slowed honey dew melon senescence so that after 22 days of commercial and retail storage the fruit were of high marketable quality, and there was no detrimental effect on consumer preference for the edible flesh.

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Mark W. Farnham and Michael A. Grusak

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.

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Mark W. Farnham, Michael A. Grusak and Min Wang

Broccoli (Brassica oleracea L., Italica Group) is a good vegetable source of Ca and Mg, two critical minerals in human nutrition. Studies have shown that bioavailability of Ca from broccoli is comparable to that from milk. Thus, broccoli is an important alternative source of Ca in segments of the population that consume limited amounts of dairy products. Essentially nothing is known about the genetic influence on variation in Ca or Mg concentration of broccoli heads. Thus, the goal of this research was to examine variation in Ca, and also Mg concentrations, in a collection of USDA inbreds and commercial F1 hybrids. In 1996 and 1997 field studies, significant differences among inbred entries and among hybrid entries were observed for Ca and Mg concentrations of broccoli heads. With hybrids and inbreds, mean head Ca concentrations were ≈3.0 mg·g-1 dry weight (DW), and entries with lowest and highest Ca concentrations differed >2-fold. Mean Mg concentrations of hybrid heads was 2.3 mg·g-1 DW (range 1.8 to 2.6) and 2.8 mg·g-1 DW (range 2.2 to 3.7) in 1996 and 1997, respectively. Inbred lines had mean head Mg concentrations of 2.0 and 2.6 mg·g-1 DW in the two respective years and ranges in concentration were similar as for hybrids. Analysis of variance indicated significant environment and entry by environment effects for Ca and Mg concentrations of hybrids. With inbreds, a significant entry by environment effect for Ca concentration and environment effect for Mg concentration was also observed. Significant environment and entry by environment effects indicate that the environmental influence on phenotypic expression of Ca and Mg concentrations may complicate genetic improvement of head mineral concentration.

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Michael A. Grusak, Brian W. Stephens and Donald J. Merhaut

Snap beans (Phaseolus vulgaris L.) are a food source that can contribute to dietary Ca requirements in humans. Factors which might enhance the concentration of Ca in snap bean pods have been investigated by measuring whole-plant net Ca influx, whole-plant Ca partitioning, and various growth parameters in two snap bean cultivars—Hystyle and Labrador—that differ in pod Ca concentration. Plants were grown hydroponically under controlled environmental conditions while being provided adequate quantities of Ca. The concentration of Ca in pods (dry weight basis) was 52% higher in `Hystyle', relative to `Labrador', but net Ca influx throughout crop development or total plant Ca content at three stages of development were similar in both cultivars, demonstrating that pod Ca concentration differences were not due to differences in total plant Ca influx. However, `Hystyle' partitioned more total plant Ca to pods, relative to `Labrador'. Calcium flux analysis also revealed that daily rates of whole-plant net Ca influx gradually declined throughout the period of pod growth in both cultivars; this decline was not related to whole-plant water influx. These results suggest that enhancements in whole-plant net Ca influx during pod growth and/or enhancements in the xylem transport of absorbed Ca to developing pods could increase the Ca concentration of snap bean pods.

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Anna L. Hale, Mark W. Farnham, Michael A. Grusak and John W. Finley

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.