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item Grusak, Michael

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2004
Publication Date: 11/1/2004
Citation: Pomper, K.W., Grusak, M.A. 2004. Calcium uptake and whole-plant water use influence pod calcium concentration in snap bean plants. Journal of the American Society for Horticultural Science. 129(6):890-895.

Interpretive Summary: We wish to understand the processes in plants that control the movement of minerals to edible organs, so that we can develop new cultivars with enhanced nutritional value. We have been studying snap bean plants to determine how calcium moves from the soil, into the roots, through the stem, and ultimately to the young pods. Two cultivars that have different pod calcium concentrations were used in our studies. We looked at total calcium uptake from the soil and total water use by the plants. We also measured water movement through developing pods, which is important for carrying calcium into the pods. We found that these two cultivars do not differ in their total whole-plant calcium uptake, or their pod water use. They do, however, differ dramatically in their whole-plant water use, such that the cultivar with low pod calcium concentration showed a higher whole-plant use of water. These results tell us that plants which use more water have a lower concentration of calcium moving in the water stream through their stem, and thus less calcium is moved to their pods. In the future, as we work to develop snap bean plants that are efficient at using water, which is good for the environment, we also should end up with plants whose pods will contain more dietary calcium. This latter characteristic is good for the bone health of consumers.

Technical Abstract: Understanding the mechanisms that regulate xylem transport of calcium (Ca) to snap bean 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', that 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 in total whole-plant Ca influx between cultivars. With whole-plant 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.