Enhancing Tuber Calcium Concentration May Reduce Incidence of Blackspot Bruise Injury in Potatoes

Our previous research has provided evidence that in-season calcium applications can increase tuber calcium and improve tuber quality with reduced internal defects. To determine if increasing the tuber calcium concentration also mitigates tuber bruise incidence, five commercially relevant potato (Solanum tuberosum L.) cultivars ( Russet Burbank , Atlantic , Snowden , Superior , and Dark Red Norland ) were grown during three seasons, 1999–2001. Three split applications of a calcium/nitrogen water soluble blend totaling 168 kg hacalcium were made starting at hilling. All plots, including controls, received an equal amount of total nitrogen in a season. Tubers were allowed to be bruised during normal machine harvest standard to commercial production in Wisconsin. Over 100 tubers from each replication (5–10 replications/treatment) were cut and examined for the incidences of bruise and internal brown spot. Paired samples of medullary tissue were taken for measuring calcium concentration. As expected, tuber tissue calcium concentration increased significantly, in all cultivars and in all years, with in-season calcium application. Bruise incidence varied among cultivars and seasons. Although tuber calcium concentration varied among seasons, Atlantic and Snowden consistently had the lowest calcium concentration, whereas Superior and Dark Red Norland consistently had the highest calcium concentration. Meta-analysis of pooled data for three years showed that blackspot bruise incidence was significantly reduced with calcium application in Atlantic , Burbank , and Snowden . On the other hand, Dark Red Norland and Superior had low incidence of bruise and were unaffected by calcium applications. Regression analyses of pooled data from all cultivars for three years revealed a significant quadratic relationship between blackspot bruise and tuber tissue calcium as well as between blackspot bruise and internal brown spot. A linear to plateau plot of medullary calcium concentration versus blackspot bruise incidence revealed that bruise incidence is minimized between 200 and 250 mg/kg (dry wt) tuber calcium concentration. To our knowledge, ours is the first study providing evidence for reducing bruise by improving tuber calcium. Variations in the bruise incidences among cultivars generally followed tuber calcium concentration suggesting a genetic control. Given the role of calcium in improved membrane health and enhanced wall structure, and as a modulator of physiological responses, it is not surprising that internal brown spot and bruise incidences are reduced by in-season application to calcium-deficient cultivars. Machine harvest, handling, and storage of potato tubers can impart pressure and impact injury resulting in tuber blackspot bruise and reduced crop value. Tuber injury can promote premature aging (Workman and Holm, 1984) or elevated levels of respiration (Pisarczyk, 1982) leading to losses in processing quality, decreased seed piece performance, or susceptibility to soft rot (Bartz and Kelman, 1986). Factors that influence bruise incidence and severity include cold tuber temperatures and flaccidity during harvest or handling (Thornton and Timm, 1989) and harvesting practices such as use of padded rollers or optimizing soil conditions for minimal bruise (Brook, 1996). Tuber maturity at the time of harvest may also influence bruise rates with immature tubers showing less susceptibility (Pavek et al., 1985). Mineral nutrition during the growing season may also influence tuber blackspot bruise susceptibility. Potassium deficiency was found to be associated with a higher blackspot bruise incidence and increased respiration levels in early studies (Mulder, 1949, 1955). The influence of potassium is not fully understood because Schenk (1981) tested several cultivars and concluded that higher potassium tended to reduce blackspot bruise but that it was not sufficient to explain incidence. Potassium deficiency has been associated with increased specific gravity, which in turn can render tubers more susceptible to blackspot bruising (Ophius et al., 1958). Nitrogen in excess, on the other hand, is associated with reduced dry matter and delayed maturity (Reeve et al., 1973), which renders tubers less susceptible to blackspot bruising. Early studies established that specific gravity and blackspot bruising are highly correlated (Massey et al., 1952). However, the role of specific gravity in bruise potential is thought to be limited in that it is probably more an indicator of tuber maturity than of blackspot bruise potential (Brook, 1996). Although some studies have implicated possible roles for minor nutrients such as copper (Kertesz, 1952) and zinc in bruise incidence (Mulder, 1956; Monday and Chaudra, 1981), these elements are not commercially recommended for reducing bruise incidence. We have recently reported that by increasing tuber calcium, one can expect a reduction in the internal defects such as internal brown spot and hollow heart (Kleinhenz et al., 1999; Ozgen et al., 2006). During blackspot bruising, impact disrupts cells under the periderm allowing phenols to come in contact with polyphenoloxidase to ultimately produce the dark pigment melanin (Corsini et al., 1992). Because internal brown spot and bruise involve discoloration of tuber tissue, it follows that localized calcium deficiencies may also contribute to heightened bruise potential. In support of this, tyrosine concentration and phenolase (an enzyme whose activity is associated with bruise) activity were also found to be decreased by increases in tuber tissue calcium (Mapson et al., 1963). Localized deficiencies in calcium have also been associated with tissue necrosis such as brown center or internal brown spot (Bangerth, 1979; Collier et al., 1980; Levitt, 1942). The objective of the present study was to investigate the potential for in-season supplemental calcium to reduce the incidence of blackspot bruise in field grown, machineharvested potatoes. Tubers from five commercially significant cultivars that vary in bruise susceptibility were subjected to standard field-harvesting practices. By examining large numbers of tubers for blackspot bruising and internal defect and by obtaining paired samples for tissue calcium concentration analysis, we attempted to relate the incidence of blackspot bruise to tissue calcium concentration. Using a three-season study, we were able to assign the variability in blackspot bruise that can be explained by tuber calcium concentration in five cultivars. Data analyses also allowed us to predict the critical tuber calcium concentrations necessary to minimize blackspot bruising. Because both blackspot bruise and internal brown spot involve discoloration of the tuber tissue, we explored possible relationships Received for publication 7 Mar. 2006. Accepted for publication 13 Apr. 2006. This research was supported by the College of Agricultural and Life Sciences and by the Wisconsin Potato and Vegetable Growers’ Association. To whom correspondence should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 41(5) AUGUST 2006 1213 JOBNAME: horts 41#5 2006 PAGE: 1 OUTPUT: July 15 02:02:29 2006 tsp/horts/118440/01494