Introduction
Phenology, the study of the timing of recurrent life cycle events and the causes of their temporary interrelation with biotic and abiotic factors, is now acquiring great relevance for the planning of cropping systems. Phenological information can be used to determine the proper time to carry out activities such as pruning, application of chemicals or planning the harvest date (Bisognin et al., 2015; Rahman et al., 2016). In addition, plant phenology is getting environmental interest as more evidences show that chronology of stages like dormancy release, flowering and harvest in some crop species are being altered due to climate change (Bethere et al., 2016; Funes et al., 2016; Guo et al., 2015). In this regard, the chronology of phenological phases in plants based on growing degree days (GDD) seems more practical and accurate than that based on natural days, as the former involves air temperature in the computations; thus, resulting in a better analysis of plant phenology (Elnesr y Alazba, 2016; Parra-Coronado et al., 2015). Phenology based on GDD can be used to generate prediction models either for precision horticulture or cropping systems in a specific region affected by climate change (Bethere et al., 2016; Caffarra et al., 2012; Parthasarathi et al., 2013); nevertheless, these models require enough phenological data from different regions to be constructed and to render trustable outputs, and this information is rarely available (Caffarra et al., 2012; Grab and Craparo, 2011; Mendoza et al., 2017; Olesen, 2011). Phenological records based on GDD become important to the fruit crops not only to schedule activities during the growing cycle, but also to project policies related to the regional distribution of species in a future affected by climate change.
Phenological analyses are essential for protected horticulture due to manipulation of environmental factors around the crop and the search for maximum and faster returns from the system. Regarding strawberry cultivation, there is an evident interaction between temperature and photoperiod that affects not only floral induction, but also the periods of leaf and fruit development, thus, modifying the harvest time (Krüger et al., 2012; Sønsteby and Heide, 2006); despite the fact that this interaction has been addressed in several reports, there is not enough studies quantifying the GDD at the beginning of phenological phases in this crop (Bethere et al., 2016; da Costa et al., 2014; Diel et al., 2017a; Krüger et al., 2012; Rosa et al., 2011).
Previous research has shown that organic substrates could affect strawberry phenology (Diel et al., 2017a), but not much is known about plant density and inorganic substrates affecting phenology and GDD requirements in this crop. Considering the importance of phenological records based on thermal needs of strawberry under protected systems, this research aimed to determine the effect of volcanic red rock and plant density on foliar phenology (phyllochron) and reproductive phenology of the Festival strawberry growing under a polyethene macro-tunnel while comparing the days after planting (DAP) to GDD as chronological methods for phenology.
Materials and methods
Plant material
Strawberry plants (Fragaria × ananassa Duch.) cv. Festival from a commercial nursery in central Mexico were planted on August 26, 2016 inside a polyethylene macro-tunnel (34.0 m length, 5.0 m width, 2.9 m height) located at 19o 20’ N, 98o 53’ W and 2250 masl. Temperature records inside the tunnel are shown in Figure 1. Plants were chilled at 4 oC for one month and then planted on beds (20.0 m length, 1.1 m width, 0.35 m height) filled with either red volcanic rock or local agricultural soil. Beds were previously disinfected with Metalaxil-M for soil-borne diseases control following commercial recommendations. There were three rows of plants per bed with 30 cm spacing between rows and 20 or 30 cm between plants, resulting in plant densities of 15 and 10 plants m-2, respectively.
Cultivation system
A Steiner mineral solution (Steiner, 1984) adjusted to 50 % of its original concentration, pH 5.5 to 6.0 and electric conductivity (EC) of 1.2 to 2.0 dS m-1 was used in a fertigation system with three drip lines and compensated emitters (3.5 mL min-1) at 20 cm separation. The system was set to allow five events per day from 9:00 am to 6:00 pm for a total of 16 min dripping for the red volcanic rock substrate and one event of 10-12 min every other day at 11:00 am for the soil. Volcanic red rock was < 1.0 cm particle diameter and came from local mines; physical and chemical properties for this substrate have been reported by Trejo-Tellez et al. (2013). Local agricultural soil was clay loam with pH 7.5, EC 0.5 dS m-1, bulk density 1.23 g cm-3 and CaCO3 15.8 %. Pests and weeds were controlled following commercial recommendations for strawberry.
Phenological stages
Temperatures were recorded every hour from the day of planting by using a sensor placed 50 cm above ground level and connected to a Dataloguer (HOBO®, Onset Computer Corporation, Bourne, Massachusetts, USA). Daily growing degree days (GDD) after planting date were computed with temperature data and the cumulative GDD requirements for phenological stages, as decribed by Enz and Dachler (1997) for strawberry, were determined: early balloon stage (first flowers with petals forming a hollow ball), onset of anthesis (first flowers open), flowers fading (majority of petals fallen), receptacle protruding from sepal whorl, seed clearly visible on receptacle tissue, beginning of ripening (most fruits white in color), first fruits with cultivar-specific color. Daily GDD were computed using the single sine method (Equation 1) with lower and upper threshold temperatures of 3.0 and 30 °C, respectively (Elnesr and Alazba, 2016; Krüger et al., 2012;), fitting inside the range every temperature found outside the thresholds.
Where GDD: daily growing degree days, Tmax: maximum temperature for the day, Tmin: minimum temperature for the day, Tbase: 3 °C.
Experimental design and statistical analysis
The experiment was conducted under a completely randomized design and split-plot arrangement with factor substrate (red volcanic rock and soil) as the main plots and factor plant density (10 or 15 plants m-2) as the split plot with three replications (beds). Distribution of treatment resulted from dividing three beds corresponding to each substrate into two equal sections and randomly assigning plant density to each section. Twenty plants from the central part of each replication were labeled and used to collect data. Resulting data were submitted to analysis of variance by using the GLM procedure of the SAS v9.0 program and mean separation by the LSD test (P ≤ 0.05) to compare treatment means. Linear regression was performed to predict the phyllochron and the DAP in every phase of reproductive phenology using the GDD. Prior to the regression analysis, natural logarithm transformation of the independent variable was performed to estimate the best linear fit between the involved variables (Ott and Longnecker, 2001).
Results and discussion
Growing degree days (GDD) vs. days after planting (DAP)
High correlation coefficients (r > 0.964) between GDD and DAP were found for each phenological stage considered in this study (Table 1). Confidence interval (95 % confidence) for the slope was under a 20-day range. The decision to choose one or the other method to count the time to reach a specific phenological phase will depend on the benefits of each one. Parra-Coronado et al. (2015) found that GDD can reliably predict the days to the onset of various phenological stages in feijoa [Acca sellowiana (O. Berg) Burret] grown in different natural environments. Chronology to beginning of the phenological phases of cultivated fruit species has traditionally been reported in calendar days; however, because of the influence of temperature on plant phenology, these values could vary significantly for the same cultivar grown in different environments. In contrast, GDD-based phenology becomes important in determininig the onset of phenological stages by providing a more reliable tool in scheduling activities in a specific crop cycle (Krüger et al., 2012; Parra-Coronado et al., 2015; Parthasarathi et al., 2013; Rahman et al., 2016).
Variable | Equation | Standard error of the slope | Confidence interval of slope (α = 0.05) | R-Square | p-Value |
Early balloon stage† | Y = 152.33X - 1009 | 29.13 | 144.3 to 160.3 | 0.978 | < 0.0001 |
Onset of anthesis† | Y = 164.3X - 1100 | 25.05 | 157.5 to 171.1 | 0.985 | < 0.0001 |
Flowers fading | Y = 169.9X - 1143 | 25.42 | 163.2 to 176.6 | 0.973 | < 0.0001 |
Receptacle protruding from sepal whorl | Y = 174.0X - 1175 | 24.26 | 167.6 to 180.4 | 0.978 | < 0.0001 |
Seed clearly visible on receptacle tissue | Y = 170.3X - 1147 | 23.68 | 164.1 to 176.5 | 0.978 | < 0.0001 |
Beginning of fruit ripening (fruits white in color) | Y = 176.2X - 1193 | 32.48 | 1667.7 to 184.7 | 0.968 | < 0.0001 |
First fruits with cultivar specific color | Y = 183.3X - 1246 | 37.42 | 173.4 to 193.1 | 0.964 | < 0.0001 |
†Values only for plants grown in soil. Y: days after planting, X: growing degree days.
Leafing phenology (Phyllochron)
The analysis of this process becomes relevant since the productivity of the Festival strawberry is highly correlated with its foliar development (Menzel and Smith, 2014) and the data from these type of analysis might be used to generate models that facilitate detailed scheduling of activities, thus increasing productivity. In this experiment, leaf formation was not different between treatments up to 163 DAP (February 4, 2017); afterwards, plants grown in soil formed more leaves than those grown on red volcanic rock (Figure 2). Compared to soil, red volcanic rock has low water retention and low water use efficiency (fruit mass per volume of water) (Ojodeagua et al., 2008; Trejo-Téllez et al., 2013). It seems that water availability was not a limiting factor for plants of any treatment during most of the growing season; however, during the last third of the season, as the plant accumulated more leaves, the water on red volcanic rock was clearly insufficient to maintain the same leafing rate compared to the soil, even though there were no obvious symptoms of water stress in plants of both substrates throughout the growing season. During the experiment, plant density had no effect on the number of leaves per plant and there was interaction between factors (substrate × plant density; P = 0.003) only on the last sampling date (March 22, 2017) resulting in more leaves (12.9) in the plants grown in soil at low plant density (10 plants m-2).
Treatments had no significant effect on phyllochron in this experiment (Figure 3A). There was a requirement of 597 GDD for the onset of leafing and the second leaf to be evident. Other studies have demonstrated that phyllochron in strawberry may depend on the cultivar, the origin of the plant and the planting date or the interaction among these factors (Rosa et al., 2011; Tazzo et al., 2015; Thiesen et al., 2018); furthermore, the chilling treatment to the crown before planting may or may not have a significant effect on this variable (da Costa et al., 2014; Diel et al., 2017b); nevertheless, all these studies agreed on a high correspondence between air temperature and phyllochron, which was also found in this experiment (R2 = 0.9754; Figure 3B). All these previous studies indicate that every period between leaves in the strawberry crown is highly genetically influenced yet significantly modulated by air temperature; in addition, crown carbohydrates, either stored or newly synthesized, are important for this modulation (da Costa et al., 2014; Rosa et al., 2011). Though the strawberry phyllochron and all the regulating factors should be carefully investigated, this experiment shows that despite the fact that cultivation on red volcanic rock could decrease the final number of leaves on the strawberry crown compared with cultivation in soil, its phyllochron based on GDD remains relatively constant.
Reproductive phenology
All phenological phases studied in this research are also considered in the BBCH code for strawberry (Enz and Dachler, 1997). Literature on strawberry phenology that considers the BBCH code and that is based on GDD is scarce. Data from this experiment are in the range reported in literature for strawberry Elsanta, Korona and Clery grown at different latitudes in Europe, from Italy to Norway, by Krüger et al. (2012), either for DAP to the onset of flowering and harvest (red fruit) or for GDD accumulated during the period between these two stages (Table 2). Strawberry reproductive phenology is genetically regulated and environmentally modulated by temperature, photoperiod, humidity, and plant nutrition (Diel et al., 2017a; Krüger et al., 2012; Rahman et al., 2016; Sønsteby and Heide, 2006; Wan et al., 2018); as a result, activities at the place of origin of the plantlets and during their cultivation could modify the reproductive phenology and the phyllochron while growing in substrates under plastic cover (Diel et al., 2017a; 2017b). In this study there was no significant effect of the substrate (soil vs. red volcanic rock) on reproductive phenology of Festival strawberry, meaning that plants had equivalent conditions before and after planting or at least not significant enough to generate differences between treatments for most of the registered variables.
Factor | Level | Early balloon stage | Onset of anthesis | Flowers fading | Receptacle protruding from sepal whorl | Seed clearly visible on receptacle tissue | Beginning of fruit ripening (fruits white in color) | First fruits with cultivar specific color | Flower to fruit period† |
Days after planting (DAP) | |||||||||
Substrate (s) | Soil | 128.2 | 134.9 | 140.6 | 142.5 | 148.2 | 152.3 | 153.4 | 32.6 |
Red volcanic rock | 135.1 | 141.7 | 146.3 | 147.4 | 149.1 | 149.5 | 151.1 | 36.2 | |
Significance | NS | NS | NS | NS | NS | NS | NS | * | |
Plant density (d) | 15 plants m-2 | 140.6 | 147.4 | 151.3 | 149.9 | 153.2 | 155.5 | 155.5 | 35.2 |
10 plants m-2 | 123.1 | 129.7 | 136.2 | 140.6 | 144.9 | 147.8 | 150.2 | 33.4 | |
Significance | * | * | NS | NS | NS | NS | NS | NS | |
s × d interaction | NS | NS | NS | NS | NS | NS | NS | NS | |
Growing degree days (GDD) | |||||||||
Substrate (s) | Soil | 1809.2 | 1891.8 | 1958.6 | 1982.4 | 2052.5 | 2098.8 | 2109.6 | 385.6 |
Red volcanic rock | 1892.6 | 1972.1 | 2018.6 | 2024.2 | 2062.3 | 2063.7 | 2079.7 | 427.2 | |
Significance | NS | NS | NS | NS | NS | NS | NS | * | |
Plant density (d) | 15 plants m-2 | 1957.1 | 2039.4 | 2087.2 | 2070.1 | 2112.8 | 2138.4 | 2136.5 | 418.9 |
10 plants m-2 | 1749.6 | 1829.7 | 1906.1 | 1961.1 | 2012.7 | 2043.9 | 2069.6 | 392.6 | |
Significance | * | * | NS | NS | NS | NS | NS | NS | |
s × d interaction | NS | NS | NS | NS | NS | NS | NS | NS |
†Period from onset of anthesis to fruit red in color (harvest). NS: non-significant, *: statistical significatance (LSD test, P ≤ 0.05) within the respective factor (n = 3).
Regarding plant density, there was a significant delay of 17 days to reach the early balloon stage and the onset of anthesis in plants at high density (15 plants m-2) compared to plants at low density (10 plants m-2), but this difference disappeared thereafter (Table 2). Nitrogen is a mineral that can affect floral differentiation in strawberry (Wan et al., 2018), and when applied three weeks after the beginning of short-day conditions it can advance flowering by eight days (Woznicki et al., 2018). The plants in this experiment were exposed to short days just one month after the planting date; thus, plants at high densities could have a stronger competition for nitrogen than those at low densities, which affects the first two reproductive stages considered in this study. The lack of effect of plant density on the later reproductive stages is probably due to the similar foliar development between treatments during most of the growing period and to the similar de novo synthesis of carbohydrate which apparently is determinant for reproductive phenology in strawberry (Menzel and Smith, 2014).
Conclusions
Growing Festival strawberry on red volcanic rock had no effect on plant phyllochron or its reproductive phenology compared to plants grown in soil. Plant density also had no effect on the strawberry phyllochron; however, high densities can delay the onset of the early balloon stage and the onset of anthesis. Growing degree days (GDD) and days after planting (DAP) are highly correlated and GDD may predict DAP within a high-confidece interval. Phyllochron is highly correlated with air temperature and GDD can reliably predict leaf protrusion.