Stomatal Density and Calcium Concentration of Six Snap Bean Cultivars

Stomatal density of pods and leaves were determined for six commercial snap bean cultivars (Phaseolus vulgaris L. ‘Evergreen’, ‘Hystyle’, Labrador’, ‘Tenderlake’, ‘Top Crop’, and ‘Venture’) grown at three planting dates, in an attempt to find morphological traits that could be related to cultivar differences in pod Ca concentration. Snap beans were planted three times at ≈1-week intervals beginning 15 June 1995, and harvested 59 to 62 days after planting. Stomatal counts were performed using a microscope linked to a video camera, and Ca concentration determinations were made using atomic absorption spectrophotometry. Calcium concentration and stomatal density of leaf tissue was higher than that of pods. Cultivar differences for pod Ca concentration (P = 0.001) and stomatal density (P = 0.001) were observed although cultivars with higher pod stomatal density did not necessarily result in higher pod Ca concentration. Calcium concentration and stomatal density for leaves did not differ among cultivars. Stomatal density and Ca concentration of pods were positively correlated (R = 0.37), while pod maturity was negatively associated to both pod Ca concentration (R = 0.93), and pod stomatal density (R = 0.99). The effect of planting dates was absent in pod Ca concentration and significant in pod stomatal density. bean genotypes for pod Ca concentration may not be due to differences in net Ca influx at the whole plant level, but rather to Ca partitioning into fruits presumably resulting from pod transpiration (Grusak et al., 1996). Snap bean cultivars with higher pod Ca concentration levels may also have larger and/or more pod stomata which may lead to an increased rate in pod transpiration and correspondingly higher pod Ca accumulations. Soil Ca reaches the root surface mainly via mass flow (Barber, 1995). Translocation of absorbed Ca in the xylem occurs through mass flow of free Ca and some organically complexed Ca, and by movement along Ca exchange sites in the xylem walls (Clarkson, 1984). Calcium appears immobile in the phloem (Peel, 1972) and is consequently not redistributed within the plant (Biddulph et al., 1959). Thus, the Ca needed during reproductive growth must be supplied directly by uptake (Barber, 1995). Since Ca appears to enter snap bean pods as a result of pod transpiration rates, genotypes with high pod Ca concentration levels may have proportionally higher stomatal densities. The objectives of this investigation were to determine the 1) association between stomatal density and Ca concentration in snap bean pods and leaves, and 2) effects of planting date and pod maturity on the Ca concentration and stomatal density of snap bean pods. Materials and Methods EXPERIMENTAL DESIGN AND STATISTICAL ANALYSIS. This study was conducted at the University of Wisconsin, Agricultural Research Station, Hancock. Soil at the Hancock Station was classified as Plainfield loamy sand (sandy, mixed, mesic, Typic Udipsamment) with 1.2% organic matter. Soil analysis taken at preplant revealed a pH of 6.1, 88 mg·L P2O5, 107 mg·L K2O, and 580 mg·L Ca. The experiment was arranged as a randomized complete block repeated at each of the three planting dates. Each planting date consisted of two blocks with six snap bean cultivars each. Comparisons among cultivars were done using a LSD test (Snedecor and Cochran, 1991). Analyses of variance (ANOVA) for the data were performed using the GLM procedure Received for publication 12 July 1999. Accepted for publication 8 Sept. 2000. Research conducted at the University of Wisconsin, Madison WI. We appreciate the valuable aid of Erik V. Nordheim with statistical analyses, Michell E. Sass and Olufunmilayo Ewumi with technical assistance, and the constructive comments of Brent H. McCown and John C. Stier. From a PhD dissertation by J.M. Quintana. Research supported by the Graduate Research Committee at the Univ. of Wisconsin, Madison, the College of Agricultural and Life Sciences, and Hatch Act Funds. Use of trade names does not imply endorsement of the products named or criticism of similar ones not mentioned. Former graduate research assistant. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Professor and corresponding author: e-mail [email protected]. Campbell-Bascom professor. 4 Associate professor. Research plant breeder. Professor. Snap bean (Phaseolus vulgaris) is a vegetable with important economic value and rising popularity, with per capita consumption in the United States having increased 45% in the last 10 years (National Agricultural Statistical Service, 1997). Beans also have notable nutritional value, with high levels of protein, Fe, K, vitamin B (Ensminger et al., 1994), and Ca (USDA, 1984). Due to low levels of inhibitory compounds such as phytates and oxalates, snap beans posses relatively high Ca bioavailability levels (Vazquez Oderiz et al., 1994) and have been the focus of numerous studies investigating ways to augment pod Ca accumulation in pods. Initial screening experiments demonstrated genetic diversity for Ca concentration in snap bean pods and suggested that breeding might be considered the most attainable approach to pod Ca enhancement at the present time (Quintana et al., 1996). Several recent studies have explored whether Ca fertilizer applications might increase pod Ca levels and have consistently found this not to be the case (Miglioranza et al., 1997; Quintana et al., 1999). Early research demonstrated that Ca moves into snap bean pods primarily by pod transpiration (Mix and Marschner, 1976a, 1976b). Recent work has indicated that differences between snap