Calcium Uptake and Whole-plant Water Use Influence Pod Calcium Concentration in Green Bean Plants

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.