Materials and Methods

Seven bogs of ‘Early Black’ cranberries intended for dry harvest were selected before bloom in 2005 for the 2-year experiment. Dry-harvested bogs were selected so that upright density would be similar among bogs, because dry harvesters prune uprights as they are removing fruit if a Furford or Western picker is used (DeMoranville and Sandler, 2000). All sites were located in southeastern Massachusetts (Table 1) and were owned by a single cranberry company and managed similarly. Latitudes and longitudes of bog locations were: Rochester: 41.73°N, 70.82°W; West Wareham: 41.79°N, 70.75°W; and Carver: 41.88°N, 70.76°W.

Table 1. Total flavonols, total anthocyanins, and total phenolics measured in fruit harvested during 2005 and 2006 from seven southeastern Massachusetts farms planted with ‘Early Black’ cranberry vines (n = 3)^z.

View Fullscreen

Hobo dataloggers (Onset Computer Corp., Bourne, MA) containing thermocouples with radiation shields were nestled into the canopy of uprights at a randomly selected vine location on each bog beginning on 20 May 2005 and 26 June 2006. Air temperature readings were initiated at midnight and then recorded every 3 h for the duration of the season. Loggers were downloaded as needed.

To determine the impact of air temperature for different periods during the growing season, the season was divided into the following phenological stages: bloom (20 May to 25 June 2005; bloom data not collected in 2006 as a result of equipment malfunction), fruit set (26 June to 20 July 2005; 26 June to 20 July 2006), fruit growth (21 July to 20 Aug. 2005; 21 July to 20 Aug. 2006), and preharvest (21 Aug. to 5 Sept. 2005; 21 Aug. to 10 Sept. 2006). Within each of the four phenological periods (bloom, fruit set, fruit growth, and preharvest), data from each bog within each year were used to calculate daily average, average high, average low, average night, and average day temperatures as well as the average difference between the low and the high temperature for each 24-hr period (Table 2). Growing degree day (GDD) accumulations (base 4.4 °C) were calculated for each bog during each phenological stage (Table 3).

Table 2. Average night temperature (AN), average day temperature (AD), daily minimum temperature (MIN), daily maximum temperature (MAX), and daily average temperature (AVG) in °C through bloom, fruit set, fruit growth, and preharvest periods for 2005 and 2006 on seven southeastern Massachusetts farms planted with ‘Early Black’ cranberry vines.

View Fullscreen

Table 3. Growing degree day accumulations (base 4.4 °C) through bloom, fruit set, fruit growth, and preharvest periods for 2005 and 2006 on seven southeastern Massachusetts farms planted with ‘Early Black’ cranberry vines.

View Fullscreen

Fruit were harvested from all bogs on the same date (6 Sept. 2005; 11 Sept. 2006), which was determined by the first commercial harvest date at one of the sites. Three random samples of ≈500 g each were picked from throughout the canopy (i.e., top through bottom) using a hand scoop within 3 m of the datalogger. Fruit were sorted to remove rotted berries, and a random subsample of ≈150 g was frozen in liquid nitrogen for further analysis. Quantification of TAcy, TF, and TP followed the method of Onayemi et al. (2006); however, TP was calculated as gallic acid equivalents as opposed to catechin equivalents, and a Hitachi U-1100 (Hitachi America Ltd., Brisbane, CA) spectrophotometer was used.

All temperature means were correlated using PROC CORR (SAS Institute, Cary, NC) with TAcy, TF, and TP concentrations measured for each bog in each year with data combined across bogs and years. Differences in TAcy, TF, and TP among bogs were determined by analysis of variance (using SAS PROC MIXED), and means were separated with Tukey's honestly significant difference (P = 0.05).

Results

TAcy and TF differed among bogs in both years, and TP differed among bogs in 2005 but not 2006 (Table 1). TAcy ranged from 7.63 to 20.54 mg/100 g fresh weight in 2005 and from 12.58 to 29.07 mg/100 g fresh weight in 2006. TF ranged from 11.59 to 22.73 mg/100 g fresh weight in 2005 and from 14.88 to 20.42 mg/100 g fresh weight in 2006. TP ranged from 7.07 to 11.50 mg·g⁻¹ dry weight in 2005 and did not differ significantly in 2006 ranging from 4.03 to 7.25 mg·g⁻¹ dry weight. In general, trends were similar between years with Bog 1 and Bog 2 producing fruit with the lowest and highest TAcy and TF concentrations, respectively. TAcy was positively correlated with TF (r = 0.506, P = 0.036), but not with TP (P = 0.801). TF and TP were not significantly correlated (P = 0.064).

Average night temperature through bloom (Table 2) was positively correlated with TAcy concentration in the harvested fruit (Fig. 1A). Similar to TAcy, TF in harvested fruit was positively correlated with average night temperature through bloom and fruit set (Fig. 1B), whereas average night temperature through fruit set, fruit growth, and preharvest was positively correlated with TP concentration in harvested fruit. The only significant correlation between fruit chemistry and average day temperature occurred with TP in harvested fruit (Fig. 1C).

View Figure

• Download as Powerpoint
• Download Figure

Air temperature over a 24-h period (midnight to midnight) (Table 2) through the growing season impacted fruit composition at harvest. TAcy concentration in harvested fruit was positively correlated with daily minimum temperature and negatively correlated with temperature differential through bloom and fruit set (Fig. 2A). Average daily temperature was positively correlated with TAcy concentration through fruit set (Fig. 2A). After fruit set, temperature had no effect on TAcy (Fig. 2A). Daily minimum temperature through bloom, fruit set, and fruit growth was positively correlated with TF in harvested fruit, whereas temperature differential through bloom and fruit set was negatively correlated with TF at harvest (Fig. 2B). Average daily temperature during fruit growth was positively correlated with TF measured in harvested fruit (Fig. 2B).

View Figure

• Download as Powerpoint
• Download Figure

Although TAcy and TF concentrations were most affected by temperatures early in the growing season, TP was most affected later in the season. Daily minimum temperature for the fruit growth and preharvest periods was positively correlated with TP in harvested fruit, and daily maximum temperature and average temperature late in the season were positively correlated with TP in harvested fruit (Fig. 2C).

Discussion

The data suggest a large variation in phenolic concentration of ‘Early Black’ cranberries growing in Massachusetts within a given season, even on bogs located geographically close to one another and grown using similar production practices. Interestingly, Bog 1 was generally lowest in phenolic concentration; however, it was the first bog harvested by the company in both years of the study. Commercial harvest occurred in early September in both years before significantly cold temperatures on the bog, thereby limiting our ability to determine the impact of cold fall temperatures and mild frost events on phenolic concentration of berries.

There are several possible explanations for differences in fruit phenolic concentrations among bogs, the most likely being microclimatic differences resulting from a number of factors, including proximity to the ocean, elevation, and surrounding topography. Although the furthest distance between bogs was ≈20 km, the microclimates at each bog differed a great deal in temperature with average air temperatures through a given period differing by as much as 3.0 °C (Table 2), whereas at any given time, temperatures differed by as much as ≈6 °C (data not shown). Growing degree day accumulations during the defined phenological stages also varied (Table 3). Although not measured in this study, it is likely that light microclimate in the canopy was similar among bogs, because dry harvest machines prune the uprights to a given density (DeMoranville and Sandler, 2000), and the same dry harvest machines were used on all bogs in the study.

Differences in phenolic concentration of fruit among bogs may be the result of differences in fruit size, which was not recorded in this study. Fruit size impacts phenolic concentrations when quantified on a whole-berry basis through changes in the surface-to-volume ratio (Sapers et al., 1986; Vorsa and Welker, 1985), and fruit size can be impacted by air temperature (DeMoranville et al., 1996). Another possible explanation for the wide variation noted in phenolic production could be the existence of differing genotypes (both on a single bog and among bogs), because studies have revealed significant genotypic variation within the same cranberry cultivar (Novy and Vorsa, 1995; Novy et al., 1996). However, all fruit were visually determined to have ‘Early Black’ features and ripened according to the characteristics associated with the cultivar (DeMoranville, 2004).

Anecdotal information regarding cranberry suggests that cooler night temperatures (or large temperature differentials or a light frost) before harvest increase TAcy concentrations, but this study suggests otherwise. Average night temperatures through bloom were positively correlated to TAcy in harvested fruit (Fig. 1A) suggesting that higher temperatures, not lower temperatures, actually improve TAcy (as well as TF and, later in the season, TP). Temperature had no effect on TAcy and TF later in the growing season (Fig. 1), indicating that high temperatures before harvest do not detrimentally impact TAcy in ‘Early Black’ cranberries as they do in grapes (Spayd et al., 2002). It is not clear how warmer temperatures through bloom are positively influencing TAcy and TF in harvested fruit, because the fruit are not yet formed at that time. One hypothesis is that higher temperatures through bloom accelerated plant development (Table 3), resulting in advanced fruit maturity at those locations. This hypothesis is supported by the GDD data demonstrating that Bog 1, which was lowest in TF, TAcy, and TP in both years when compared with the other bogs, had the lowest GDD accumulations through fruit set, growth, and preharvest in 2005. Additionally, the correlation between high temperatures at bloom and increased phenolic production could be a function of temperature impacting photosynthesis and hence carbon availability in vegetative tissues for precursors. The relationship between carbon availability and phenolic production in cranberry is not clear, because TAcy has been negatively correlated with nonstructural carbohydrate concentrations in the uprights, whereas TF has been positively correlated with nonstructural carbohydrate concentrations (Onayemi et al., 2006). In addition, the effects of temperature on photosynthesis of cranberry are unclear, and high temperatures have demonstrated little detrimental impact on carbon exchange (Kumudini, 2004; Vanden Heuvel and Davenport, 2005). Another possibility is that higher temperatures may lead to increased fungal pressure resulting in formation of phenolic compounds (or their precursors) as a stress response (Vvedenskaya and Vorsa, 2004) early in the season.

Minimum daily temperature was also positively correlated to TAcy and TF early in the season and to TP later in the season, suggesting that low temperatures may detrimentally impact phenolic production. Temperature differential (maximum–minimum) was negatively correlated to TAcy and TF through bloom and through fruit set, again in contrast to the anecdotal information existing in the industry. This may again be a function of carbon availability or a stress response for formation of anthocyanin or flavonol precursors. Studies have suggested that low temperature induces anthocyanin accumulation by inducing expression of anthocyanin biosynthetic genes in apple (Ubi et al., 2006) and that high night temperatures hinder expression of anthocyanin biosynthetic genes in at an early stage of ripening in grape (Mori et al., 2005), although the low temperatures in both of those studies were not nearly as low as those recorded in this study on the bogs.

The correlation coefficients between temperatures and TAcy, TF, or TP often changed dramatically through the growing season (for example, the relationship between minimum temperature and TAcy demonstrated in Fig. 2A). Numerous potential explanations exist for this observation, the most likely being that the chemistry of the berry changes throughout the season, and so temperature could have impacted fruit composition through indirect avenues (i.e., multiple precursors). Additionally, influences on plant development may be confounding the results as well.

This study confirms the observation by Onayemi et al. (2006) that production of anthocyanins and flavonols can be regulated independently of one another and proposes that anecdotal information suggesting the relationship of cool night temperatures before harvest on increased anthocyanin production in cranberry is misleading. Warmer temperatures early in the season (around bloom and fruit set) had the most positive impact on phenolic production in ‘Early Black’ growing in southeastern Massachusetts, although it is not clear why. This phenomenon may be cultivar-dependent.