Effect of Planting and Harvesting Dates on Strawberry Fruit Quality under High Tunnels

ABSTRACT

There is an increasing demand in the market to improve strawberry quality by promoting human-health compounds content, as these may play a significant role in the prevention of chronic diseases. Strawberry cultivars, environmental conditions, and agronomical conditions have an effect on fruit characteristics; therefore, it is necessary to constantly generate information about the cultivar response to different production areas and cultural practices. The goal of this work was to evaluate the effect of two planting dates, two harvest dates, and four strawberry cultivars on total phenolic acid (gallic acid equivalent), ascorbic acid (vitamin C), and soluble solid content∙titratable acidity-1 (SSC∙TA-1) ratio in Huelva, Spain. Sixteen treatments resulted from the combination of four cultivars, two planting dates, and two harvest dates. Strawberry ‘Camarosa’, ‘Sabrosa’, ‘Aguedilla’, and ‘Fuentepina’ were selected for both seasons and planted on 7 Oct. (early planting) and 21 Oct. (late planting). Mid-February and mid-April were considered early and late harvest, respectively. Nutraceutical and organoleptic responses were specifics for each cultivar. The highest SSC∙TA-1 ratio was observed in ‘Fuentepina’ planted late and harvested early. ‘Aguedilla’ resulted in the highest phenolic content when harvested in the late season. Additionally, phenolic content was significantly higher when using late planting combined with late harvest in most of the cultivars. Late harvested ‘Camarosa’ showed the highest TA concentration, whereas ‘Sabrosa’ resulted in the highest SSC regardless of harvesting date. Additionally, ‘Aguedilla’ and ‘Sabrosa’ showed the highest ascorbic content. Strawberry quality could be improved by selecting the adequate planting and harvesting dates according to specific cultivars.

KEYWORDS:

• Fragaria × ananassa
• cultural practices
• strawberry cultivars
• nutraceutic characteristics

Introduction

Strawberry (Fragaria × ananassa Duch) is well known for its exceptional taste, deep color, intense sweetness, and nutrition characteristics. Strawberries are a good source of natural antioxidants (Heinonen et al., 1998; Robards et al., 1999; Wang et al., 1996; Wang and Lin, 2000); vitamin C; and polyphenolic constituents, such as anthocyanins, flavonoids, and phenolic acids (Buendía et al., 2010; Espín and Tomás-Barberan, 2001; Hannum, 2004; Hansawasdi et al., 2006; Heinonen et al., 1998; Laugale and Bite, 2006; Rice-Evans and Miller, 1996; Seeram, 2008). Organoleptic and nutraceutical quality of berries has been related to genotype, fruit maturity, pre- and post-harvest conditions, and environmental conditions (Correia et al., 2011; Giamperi et al., 2012; Pelayo-Zaldivar et al., 2005; Singh et al., 2011). Throughout the years, knowledge about the crop response to different growing areas has led breeding programs to put effort into development of cultivars especially suited to particular growing environments. Besides the common effort of improving strawberry yield and disease resistance, there is an increasing demand from consumers to improve strawberry quality through increasing the content of human-health-promoting compounds in the fruit, as these may play a significant role in the prevention of chronic diseases (Capocasa et al., 2009; Scalzo et al., 2005; Tulipani et al., 2008).

Fruit consumption plays an important role in improving the antioxidant defenses of the human body helping to counter the detrimental effects of reactive oxygen species, which are related to degenerative human diseases (Hung et al., 2004; Nagai et al., 2003; Sun et al., 2002; Wang et al., 1996; Wresburger, 2002). Chandler et al. (2003) noticed variations throughout the years in fruit acidity across different cultivars, suggesting that warm weather could have affected sugar production within the fruits. Del Pozo-Insfran et al. (2006) compared 22 genotypes and two harvest dates on soluble solids and antioxidant phytochemical levels finding low concentration of polyphenolic and high soluble solid content (SSC) and ascorbic acid content in early harvest dates, whereas a late harvest date resulted in high polyphenolic content and low concentration of ascorbic acid and SSC. There were also differences among cultivars in interaction with years, attributable partially to solar radiation and air temperature variations.

It has been established before that cultivars, environmental conditions, and agronomical conditions have an effect on fruit characteristics, and it is necessary to constantly generate information about the response of new cultivars to different production areas and cultural practices. Therefore, the goal of this work is to evaluate the effect of two planting dates, two harvest dates, and four strawberry cultivars on total phenolic acid (gallic acid equivalent), ascorbic acid (vitamin C) content, and SSC∙TA^–1 relation in Huelva, Spain.

Materials and methods

Field trials were conducted during the 2008 and 2009 seasons at the Instituto de Investigación y Formación Agraria y Pesquera (Agricultural and Fishery Research and Education Institute [in English]) at the experimental station “El Cebollar” located in Moguer, Huelva, Spain (37° 14′ N, 6° 48′ W, and 66 m altitude). Soil at the experimental site was a sandy soil with less than 1% organic matter and a pH of 6.5. The soil was solarized and biofumigated (Medina et al., 2009) before planting. Raised beds were covered with plastic mulch at the same time one drip tape line (T-Tape TSX 508-20-500, T-Tape Systems International, San Diego, CA, USA) was laid down on top of the bed, directly under the polyethylene film. Bare-root transplants (Nava de Arévalo, Castilla y León, Spain), were planted 25 cm apart in double rows with 25 cm between rows (i.e., plant density of 70,000 plant∙ha^–1). In mid-November, plants were covered with a polyethylene-covered high tunnel (transparent thermic polyethylene 150 μ) for frost protection. High tunnel dimensions were 6.6 m wide with 3.5 m high and 70 m long. Plots were fertilized based on soil analysis conducted before planting resulting in nutrient rates of 175 kg∙ha^–1 of nitrogen, 77 kg∙ha^–1 of phosphorus (P₂O₅), 185 kg∙ha^–1 of potassium (K₂O), 85 kg∙ha^–1 of calcium, and 14 kg∙ha^–1 of magnesium for each season. Fertilizer was applied through the irrigation system, and equally split over the season from 1 Nov. trough 30 May during both years. Total irrigation water applied during each season totaled 4200 m³∙ha^–1.

Sixteen treatments resulted from the combination of four cultivars, two planting dates, and two harvest dates. Strawberry ‘Camarosa’, ‘Sabrosa’, ‘Aguedilla’, and ‘Fuentepina’ were selected for both years of experimentation. ‘Camarosa’ and ‘Sabrosa’ (cultivar name Candonga) were two of the major cultivars grown during 2008–09 and 2009–10 seasons in Huelva, Spain. During the 2008–09 season, about 45% of the total area was planted with ‘Camarosa’, whereas 35% corresponded to ‘Sabrosa’. Similarly, during the 2009–10 season about 70% of the total area was planted with the two cultivars. Additionally, Camarosa cultivar is important in Greece and South-America, while ‘Sabrosa’ cultivar is commonly used in South Italy. Aguedilla and Fuentepina are two cultivars obtained from breeding programs carried out in Huelva, Spain, which are very well adapted to the environmental conditions from the area. The bare-root transplants were planted on 7 Oct. and 21 Oct., which are considered standards for early and late planting dates for strawberry planting in Huelva (López-Aranda, 2008). Two harvest dates were selected throughout the season for fruit analysis. Mid-February was considered an early harvest, while mid-April was considered late harvest. Both dates correspond to main periods during the strawberry season; the early harvest ensures a high price for grower due to earliness of fruits in the market, whereas the last corresponds to fruit peak production.

Treatments were established in a split plot design with planting date in the main plot and cultivar in the sub-plot, with three replications. Plots were 50 plants long. For fruit analysis, samples of 250 g of ripe fruits were randomly chosen per plot and homogenized. The blends obtained from the samples were used immediately or frozen and stored at –20 °C for further determination of nutraceutical and organoleptic traits depending of the variable. SSC (°Brix) was evaluated before freezing the samples with a refractometer (Atago PR-32α, Atago USA Inc., Bellevue, WA, USA) by adding a few drops of sample into the lens (Domínguez-Romero et al., 2004). Titratable acidity (TA) was measured with an automatic titrator (Titroline Easy, Schott North America, Inc., Elmsford, NY, USA). The results of TA analysis were expressed in g of citric acid per 100 g of fresh weight (Hancock, 1999). The SSC∙TA^–1 ratio was obtained by dividing SSC (°Brix) by TA (g citric acid∙100 g^–1 FW) previously computed and expressed as an absolute value. SSC∙TA^–1 ratio is considered by several authors as a good indicator of the taste quality of strawberry fruits (Alavoine and Crochon, 1989; Haffner and Vestrheim, 1997; Moore and Janick, 1983).

Phenolics were analyzed using the Folin-Ciocalteu spectrophotometric method as modified by Slinkard and Singleton (1977), which is a common method used to measure total phenolic content. The method differs from high performance liquid chromatography (HPLC) on the way that the first provides the total amount of phenolic compounds, while the second allows identification and classification of the phenolic compounds into separate classes. Quantifications using the spectrophotometric method were calculated through a calibration curve prepared with pre-known concentrations of gallic acid (GA), and results were expressed as ml of GA equivalents per 100 g of fresh weight of strawberry (mg GAE∙100g^–1 FW). Ascorbic acid (vitamin C) content was calculated with a digital reflectometer (Rqflex 10 Merck; Reflectoquant, EDM Millipore Corp., Darmstadt, Germany) diluting 1 g of sample in 10 ml of distilled water. Results were expressed as ml of ascorbic acid per 100 g of fresh weight of strawberry (mg ascorbic acid∙100 g^–1 FW). To identify differences among means of each trait a preliminary analysis of variance (ANOVA) was carried out using Statistix 9.0. (Analytical Software, Tallahassee, FL, USA). In case significant differences were found, means were separated using Fisher’s least significant difference analysis.

Results and discussion

Mean, maximum, and minimum temperature, and relative humidity inside the high tunnel during both seasons are shown in Table 1. Monthly mean temperatures were similar during both years with slight variations between the two seasons, but following a pattern of cooler temperatures early on in the season and warmer temperatures as the season progressed (Table 1). The highest value for maximum temperature was observed during late season with 27.1 and 25.9 °C (Table 1), whereas minimum temperature inside the high tunnel was always above 5 °C during both seasons (Table 1), providing adequate conditions for plant growth and development (Rowley et al., 2010). Additionally, relative humidity was always above 60% during both seasons, reaching the highest mean values from December to February with about 80% and 86% during the 2008–09 and 2009–10 seasons, respectively. Season by treatment interaction was non-significant; hence, data from two seasons was combined for analysis. Cultivar, planting, and harvest date interaction was significant for strawberry phenolic content and SSC∙TA^–1 ratio. ‘Camarosa’ ‘Sabrosa’, and ‘Fuentepina’ showed the highest phenolic content when planted on 21 Oct. and combined with late harvest in the season (Mid-April), averaging 136.61 mg GAE∙100g^–1 FW (Table 2). Aguedilla showed its highest phenolic content when harvested late in the season; however, concentrations for this cultivar were 10% lower than cultivars: Camarosa, Fuentepina, and Sabrosa (Table 2). Across cultivars, phenolic content was significantly higher when using late planting in combination with late harvest (Table 2). Mattila et al. (2006) reported phenolic acid levels for seven strawberry cultivars to be between 10 and 18 mg∙100 g^–1 FW, extracted by the Mattila and Kumpulainen method (2002) as modified by Mattila and colleagues. Mattila et al. (2006) also mentioned the general inconsistency in reports regarding phenolic acid content in berries, mainly due to differences in analytical methodology and natural variation in sample material. This statement is consistent with our data, which is about eight times higher than the average reported by Mattila et al. (2006). It is important to underline that, both analytical methods are widely accepted for determination of phenolic acid content (Blainski et al., 2013; Olkowski et al., 2003). Consequently, the difference in phenolic acid content between reports could be related to the selective genetic improvement in newer cultivars of their nutraceutical characteristic.

Table 1. Environmental conditions inside the high tunnel during 2008–09 and 2009–10 seasons in Moguer, Huelva, Spain.

	2008–09				2009–10
	T° mean^z	T° Max.^y	T° Min.^x	HR mean	T° mean	T° Max.	T° Min.	HR mean
Month	(°C)			(%)	(°C)			(%)
November	11.7	19.0	5.4	61.7	11.9	19.5	5.7	63.5
December	10.5	17.8	5.8	79.9	10.1	18.2	5.5	82.5
January	10.3	17.5	5.5	80.0	9.9	17.2	4.8	81.7
February	11.7	19.8	5.6	74.8	8.4	15.0	3.1	86.1
March	15.2	24.0	9.3	66.8	11.5	19.5	5.3	76.7
April	15.3	23.5	8.7	65.7	15.8	24.0	8.8	70.8
May	18.9	27.1	12.1	60.5	17.8	25.9	11.0	70.6

^zMean temperature.

^yMaximum temperature.

^xMinimum temperature.

Table 2. Effects of cultivar, planting date, and harvest date in phenols and soluble solids content-titratable acidity ratio in strawberry production during 2008–09 and 2009–10 growing seasons in Moguer, Huelva, Spain.

Treatments^z
Cultivar (CV)	Planting date (PD)	Harvest (HVST)^y	Phenols^x (mg∙100 g^–1 FW)	SSC^w∙TA^–1v ratio
Camarosa	7 Oct.	Early	106.03efg	9.41e
		Late	116.12cde	7.27f
	21 Oct.	Early	116.12cde	9.55e
		Late	136.45a	7.30f
Sabrosa	7 Oct.	Early	105.37efg	12.49c
		Late	138.78a	8.64e
	21 Oct.	Early	112.70cde	11.23d
		Late	142.12a	9.29e
Aguedilla	7 Oct.	Early	107.53ef	9.52e
		Late	119.70bcd	7.53f
	21 Oct.	Early	112.37de	9.60e
		Late	123.62bc	6.87f
Fuentepina	7 Oct.	Early	95.70fg	14.83b
		Late	95.37g	12.20cd
	21 Oct.	Early	105.78efg	16.32a
		Late	131.28ab	11.84cd
Significance (P < 0.05)^z			*	*
CV × PD × HVST			*	*

^zValues followed by the same letter do not differ according to Fisher’s-protected least significant difference at the 5% level.

^yEarly and late harvests correspond to mid-February and mid-April, respectively.

^xExpressed as mg of gallic acid per 100 g of fresh weight.

^wSoluble solids content.

^vTitratable acidity.

*P < 0.05.

With regards to SSC∙TA^–1 ratio, Fuentepina was more responsive to planting and harvest date than the rest of the cultivars, showing the highest SSC∙TA^–1 ratio among all cultivars regardless of planting date and harvested early on the season (Table 2). Late harvest in ‘Fuentepina’ significantly decreased SSC∙TA^–1 ratio in 23%, equating ‘Sabrosa’ planted on 7 Oct. and harvested early in the season (Table 2). In general, all cultivars showed significantly higher SSC∙TA^–1 ratio when harvested early, regardless of planting date (Table 2). SSC∙TA^–1 has been reported as a strong indication of sensory panel rating with high ratios associated with increased sweetness (Jouquand et al., 2008). In an evaluation of 12 cultivars and advanced selections in Florida (USA) month-to-month variation of SSC, TA, and SSC∙TA^–1 ratio showed a decrease of 30%, 20%, and 13%, respectively, from February to March harvest (Whitaker et al., 2011). These results are consistent with ours, where SSC∙TA^–1 decreased in 23% from early harvest to late harvest. However, that was not the case for SSC and TA.

Cultivar by harvest interaction was significant for TA, SSC, and vitamin C quality traits. The highest TA value was observed in ‘Camarosa’, when harvested late in the season with 1.02 g∙100 g⁻¹ FW (Table 3), followed by ‘Sabrosa’ and ‘Aguedilla’, when harvested late in the season averaging 0.83 g∙100 g^–1 FW (Table 3). While early harvest in Fuentepina showed the lowest TA value among all cultivars (Table 3). With regards to SSC, Sabrosa showed the highest SSC content among cultivars regardless of harvest date with 8.23 °Brix in average (Table 3). The rest of cultivars showed significantly lower SSC regardless of harvest date with values ranging between 6.72 and 7.36 °Brix (Table 3). Del Pozo-Insfran et al. (2006) reported slightly lower values for ‘Camarosa’ planted in October in Florida and harvested on 23 Feb. with an average of 5.13 ± 0.43. However, a similar trend of higher SSC in late harvest was observed. Vitamin C content was significantly affected by cultivar and harvest date. The highest Vitamin C content was observed in Aguedilla and Sabrosa cultivars when harvested early in the season averaging 43.41 mg∙100 g^–1 FW of ascorbic acid content (Table 3). By contrast, the lowest Vitamin C content was observed in late harvested ‘Camarosa’ and ‘Fuentepina’ averaging 32.49 mg∙100 g^–1 FW of ascorbic acid content (Table 3). This trend was also observed by Del Pozo-Insfran et al. (2006), where L-ascorbic acid levels were higher during early harvest (January) compared to late harvest (February) for six out of nine cultivars in Dover, Florida. Additionally, planting by harvest date interaction was significant for vitamin C content. Planting on 21 Oct. in combination with an early harvest resulted in the highest vitamin C content among all cultivars with 35.79 mg∙100 g^–1 FW of ascorbic acid content (Table 4), whereas the 7 Oct. planting date showed the second highest values, regardless of harvest date, averaging 36.22 mg∙100 g^–1 FW of ascorbic acid content (Table 4).

Table 3. Effects of cultivar and harvest date in titratable acidity, soluble solids content, and vitamin C content in strawberry production during 2008–09 and 2009–10 growing seasons in Moguer, Huelva, Spain.

Treatments^z
Cultivar (CV)	Harvest (HVST)^y	Titratable acidity^x (g∙100 g^–1 FW)	Soluble solids content (°Brix)	Vitamin C^w (mg∙100 g^–1 FW)
Camarosa	Early	0.71d	6.72c	36.08c
	Late	1.02a	7.36bc	33.83cd
Sabrosa	Early	0.72cd	8.49a	42.66ab
	Late	0.90b	7.98ab	40.16b
Aguedilla	Early	0.76c	7.22bc	44.16a
	Late	0.94b	6.74c	35.08c
Fuentepina	Early	0.46f	7.21bc	40.41b
	Late	0.64e	7.61b	31.16d
Significance (P < 0.05)		*	*	*
CV × HVST		*	*	*

^zValues followed by the same letter do not differ according to Fisher’s-protected least significant difference at the 5% level.

^yEarly and late harvests correspond to mid-February and mid-April, respectively.

^xExpressed as g of citric acid per 100 g of fresh weight.

^wExpressed as mg of ascorbic acid per 100 g of fresh weight.

*P < 0.05.

Table 4. Effects of cultivar and harvest date in vitamin C content in strawberry production during 2008–09 and 2009–10 growing seasons in Moguer, Huelva, Spain.

Treatments^z
Planting date (PD)	Harvest (HVST)^y	Vitamin C^x (mg∙100 g^–1 FW)
7 October	Early	36.66b
	Late	35.79b
21 October	Early	45.00a
	Late	34.33c
Significance (P < 0.05)^z		*
PD × HVST		*

^zValues followed by the same letter do not differ according to Fisher’s protected least significant difference at the 5% level.

^yEarly and late harvests correspond to mid-February and mid-April, respectively.

^xExpressed as mg of ascorbic acid per 100 g of fresh weight.

*P < 0.05.

Each cultivar showed individual trends of response with regards to the measure variables. However, some responses were similar across cultivars. Epidemiologic studies have shown a correlation between an increased consumption of antioxidants and a reduced risk of cardiovascular disease and certain types of cancer (Shiow and Zheng, 2001). Flavonoids are consider the most important group of phenolics in plants, and its concentration has been related to antioxidant activity against free oxygen radicals. Therefore, there is a positive correlation between antioxidant activity and total phenolic content. High temperature has been related to enhance free oxygen radicals absorbance, which increases phenolic content of strawberry fruits (Shiow and Zheng, 2001). Thus, warm temperatures inside the high tunnel during the season is a possible explanation to the general increase in phenolic content from early harvest to late harvest across cultivars in our study. A contrary trend was shown for ascorbic acid content, where the lowest values were collected during late harvest. However, this was not constant for all cultivars. Del Pozo-Insfram et al. (2006) stated that variations in solar radiation and air temperature are factors known to impact photochemical biosynthesis, and this response can vary across cultivars. Higher growth temperatures caused development of fruit color more rapid than that at lower growth temperatures. Hence, fruits reach maturity faster than under lower temperatures, giving less time for synthesis and accumulation of compounds.

Similar is the case for SSC and TA concentration. All cultivars resulted in the highest value for SSC∙TA^–1 ratio regardless of the planting date when harvested early, while the highest values for TA concentration were shown during late harvest across cultivars. Cultivar responses showed the highest SSC∙TA^–1 ratio for Fuentepina planted late and harvested early. However, this response seems to be related to lower TA concentrations rather than high SSC. ‘Sabrosa’ resulted in the highest SSC. Nutraceutical and organoleptic responses were specific for each cultivar. However, some general patterns where observed, which can be helpful to breeding programs dedicated to consumer acceptability and high concentration of antioxidant in strawberry fruits.

Acknowledgments

The authors are grateful to R. Villalba for technical assistance.

Funding

The authors gratefully acknowledge the INIA project RTA2007-00012, the INIA agreement CME10-46, and the European project EUBerry and FEDER funds.

Additional information

Funding

The authors gratefully acknowledge the INIA project RTA2007-00012, the INIA agreement CME10-46, and the European project EUBerry and FEDER funds.