Abstract
The mean daily temperature effects on plant development rates and quality of compact container-grown pepper were evaluated. Compact pepper cultivars Fresh Bites Yellow and Hot Burrito were grown in greenhouses at 18 to 26 °C (Expt. 1) and 20 to 30 °C (Expt. 2) under supplemental high-pressure sodium lighting and a 16-hour photoperiod. The number of days to first open flower, to first ripe fruit, and from flower to ripe fruit were measured and the development rates calculated by taking the reciprocal (e.g., 1/day). Temperature effects were predicted by fitting a nonlinear exponential function that included the base temperature (Tmin) and maximum developmental rate (Rmax) parameters. Plant quality attributes were measured during Expt. 2. As the temperature increased, the times to flower and fruit decreased (i.e., developmental rates increased) for both cultivars. The estimated Tmin was 13.3 °C for ‘Fresh Bites Yellow’, and that for ‘Hot Burrito’ was 9.3 °C, whereas the Rmax was similar between cultivars (averages of 0.0488 at flower, 0.0190 at fruit, and 0.0252 from flower to fruit). ‘Fresh Bites Yellow’ and ‘Hot Burrito’ grown at ≈25 °C had a relatively short crop time, compact canopy, large fruit size, and high number of fruits per plant at finish. Compact peppers are new crops being grown by greenhouse floriculture operations for their ornamental and edible value, and the information from this study can help growers schedule these crops to meet critical market windows and determine the impacts of changing the growing temperature on crop timing and quality.
Greenhouse floriculture operations schedule the production of container crops to finish during specific market windows, and this requires knowledge of the crop time needed to produce a marketable plant (Vargo and Faust 2022). Empirical models have been developed to help growers predict crop time and quality under various environmental conditions for economically important container crops, including chrysanthemum [Chrysanthemum ×grandiflorum (Ramat.) Kitam.] (Larsen and Persson 1999), Easter lily (Lilium longiflorum Thunb.) (Erwin and Heins 1990), poinsettia (Euphorbia pulcherrima Willd. ex Klotz) (Liu and Heins, 2002), potted rose (Rosa L.) (Steininger et al. 2002), and a range of annual bedding plant and herbaceous perennial species (Blanchard and Runkle 2011).
The crop time is determined by the rate at which plants develop, such as the unfolding of leaves or the production of flowers and fruit, which is largely influenced by the integrated mean daily temperature (MDT) (Roberts and Summerfield 1987; White and Warrington 1984, 1988). The time required for plants to complete a stage of development (i.e., flowering, fruiting) can be converted into a development rate by calculating the reciprocal of time (e.g., 1/d). The plant development rate increases as the MDT increases between the base temperature (Tmin, low temperature at which the rate is zero) and optimal temperature (Topt, temperature corresponding to the maximum rate). As the MDT increases past Topt, the development rate decreases until it reaches the maximum temperature (Tmax), where the rate is zero from heat stress. Tmin, Topt, and Tmax are species-specific (Blanchard et al. 2011), and sometimes they are cultivar-specific. The estimation of these parameters requires quantifying plant development rates across a wide range of MDT values.
Relationships between plant development rates and MDT have been characterized using a wide range of different functions, including linear, quadratic, cubic, and exponential (Blanchard and Runkle 2011; Landsberg 1977; Larsen 1990). In addition, the development rate response to the MDT has been described as either a symmetrical (Pearson et al. 1995; Volk and Bugbee 1991) or an asymmetrical (Brøndum and Heins 1993; Faust and Heins 1993) shape around Topt, depending on the species and cultivar. A practical outcome of characterizing these relationships is the estimation of Tmin and Topt (Blanchard and Runkle 2011). The temperature range resulting in the highest crop quality often falls between Tmin and Topt, and Tmin can be used as a reference when reducing greenhouse temperatures to slow growth and achieve greater energy efficiency (i.e., reduced heating).
Compact pepper (Capsicum sp.) is a popular container crop that is grown increasingly by floriculture operations in response to an expanding container edibles and home gardening market (Cruz et al. 2023). Compared with traditional garden cultivars, new compact peppers are short-statured and tend to have pungent fruits in different colors and sizes, thus providing both ornamental and edible values to consumers (Cruz et al. 2023; Dwyer 2021; Fortunato et al. 2019; Lillywhite et al. 2013; Stommel and Bosland 2007). Compact peppers are considered marketable when plants have at least one ripe and ready-to-harvest fruit in addition to multiple unripe fruits, open flowers, and emerging flower buds (i.e., future harvests). A fruit is considered ripe when 90% of that fruit is the characteristic color for that cultivar (US Department of Agriculture 2016).
We hypothesized that plant development rates would differ between the stages of flowering and fruiting for compact container-grown pepper, and that increasing temperatures would reduce fruit weight and yield. The first objective was to investigate the effects of the MDT on crop time and flowering and fruiting rates of two compact pepper cultivars. The second objective was to evaluate the effects of temperature on the final plant canopy size, fruit weight, and yield potential (i.e., numbers of fruits and flowers).
Materials and Methods
Expt. 1: Compact pepper cultivars evaluated between 18 and 26 °C
Seeds of the sweet pepper (Capisicum annuum L.) cultivar Fresh Bites Yellow and hot pepper (Capisicum frutescens L.) cultivar Hot Burrito (Pan American Seed, West Chicago, IL, USA) were sown in plug trays [128-cell (12-mL volume)] with ProMix BX substrate (Premier Tech, Quebec City, Canada) and placed in a polycarbonate climate-controlled greenhouse at the University of Arkansas in Fayetteville, AR, USA (36.0627°N, 94.1606°W). These cultivars were selected for their compact growth habit in containers and market popularity. After germination, seedlings were irrigated as needed with a commercial 13N–0.9P–10.8K (J.R. Peters, Allentown, PA, USA) water-soluble fertilizer mixed at 100 mg·L−1 N in tap water that contained electrical conductivity (EC) of 0.3 mS·cm−1 and bicarbonate alkalinity <60 mg·L−1.
Seedlings were ready for transplant 35 d after sowing (each cultivar had three to four leaves per transplant); at this time, 20 plants per cultivar were randomly assigned to each temperature treatment and placed in adjacent greenhouse compartments with day and night air temperature setpoints of 18, 20, 22, 24, and 26 °C. Seedlings were transplanted into 15.4-cm-diameter containers with ProMix BX substrate (Premier Tech, QC, Canada). Within each greenhouse, cultivars were grouped based on height, and plants within each group were placed at a density of 36 plants per m2 and were later spaced to 9 plants per m2. Plants were top-irrigated as needed with a 17N–2.2P–14.1K (J.R. Peters) water-soluble fertilizer mixed at 150 mg·L−1 N in tap water. Individual plants were irrigated with fertilizer solution to container capacity once the substrate dried to ≈50% moisture content, which was determined visually using the method described by Healy (2008). The experiment started with the transplant of seedlings on 10 Feb 2022.
Expt. 2: Compact pepper cultivars evaluated between 20 and 30 °C
The cultivars and cultural methods were the same as those of Expt. 1, except there were six plants per cultivar and treatment, and temperature setpoints were 20.0, 22.5, 25.0, 27.5, and 30.0 °C. The experiment began on 6 Mar 2022 (experimental run 1), and it was replicated on 3 Apr 2022 (experimental run 2). Plants were sub-irrigated with fertilizer solution and supplied with 150 mg·L−1 N and 80 mg·L−1 N at each irrigation during the first and second experimental runs, respectively. The fertilizer N concentration was lowered for the second experimental run to reduce excess vegetative growth (leaves and stems). The applied N concentration did not interact with temperature effects on flower and fruit development rates or plant quality and yield potential during this study.
Environmental monitoring.
To achieve the temperature treatments, each greenhouse compartment was individually controlled with an environmental control system (QCom Greenhouse Controls, Temecula, CA, USA) programmed to target each setpoint and record temperature data at the height of the plant canopy. The MDTs (±1 SD) from transplant to first open flower (anthesis), from transplant to first ripe fruit, and between first flower and first fruit for both experiments are shown in Tables 1 and 2. High-pressure sodium lamps suspended above the greenhouse benches provided a 16-h photoperiod and photosynthetic photon flux density of ∼100 μmol·m−2·s−1. A line quantum sensor (SQ-301X-SS; Apogee Instruments, Logan, UT, USA) connected to the environmental control computer and placed at canopy height in two greenhouse compartments (20 and 24 °C setpoints for Expt. 1; 22.5 and 27.5 °C setpoints for Expt. 2) measured the photosynthetically active radiation every 60 s and was used to estimate the mean daily light integral (MDLI). The estimated MDLIs at first flower and ripe fruit for plants in Expt. 1 were (mean ± SD) 7.0 ± 2.2 and 9.9 ± 3.3 mol·m−2·d−1, respectively. In Expt. 2 (data from both experimental runs were pooled), the estimated MDLIs were 13.0 ± 0.5 and 16.0 ± 1.4 mol·m−2·d−1, respectively.
Mean daily temperature (MDT) at first flower and ripe fruit from transplant and from first flower to first ripe fruit for pepper cultivars grown at temperature setpoints of 18, 20, 22, 24, and 26 °C during Expt. 1. Data represent the least square means of at least 18 plants (observational units) per cultivar and temperature setpoint treatment ±1 SD.
Mean daily temperature (MDT) at first flower and ripe fruit from transplant and from first flower to first ripe fruit for pepper cultivars grown at temperature setpoints of 20, 22.5, 25.0, 27.5, and 30.0 °C during Expt. 2. Data represent the least square means of six plants (observational units) per cultivar and temperature setpoint treatment ±1 SD.
Data collection and analysis.
The dates of the first open flower and first ripe fruit were recorded for each plant. The open flower was defined as the point of anthesis, and ripe fruit was defined as the point where one fruit per plant had completely transitioned from a green color to either yellow (Fresh Bites Yellow) or red (Hot Burrito), depending on the cultivar. The number of days (i.e., time) from transplant to flowering and fruiting stages and the days between flower and fruit were determined. These data were then converted to flowering and fruiting rates by calculating the reciprocal (e.g., 1/d to flower or fruit).
The function in Eq. [1] has been used to model the rates of leaf unfolding and flowering in a range of containerized and flowering crops (Blanchard and Runkle 2011; Larsen 1988, 1989; Larsen and Hidén 1995; Larsen and Persson 1999) and is appropriate when the relationship between the MDT and temperature follows an exponential curve and when Tmin < MDT < Topt. The Rmax has biological meaning and refers to the maximum rate of plant development at Topt. Tmin and MDT are measured in °C, and C1 defines the skew.
Parameter estimates Tmin, Rmax, and C1 for Eq. [1] were estimated using the nonlinear regression procedure (PROC NLIN) in SAS (SAS 9.4; SAS Institute, Cary, NC, USA). Initial parameter estimates were obtained visually by inspecting the graphs of the observed data. Parameter estimates were generated from 153 (‘Fresh Bites Yellow’) and 160 (‘Hot Burrito’) observational units, where each individual plant was one observational unit. Psuedo-R2 values were determined by performing a linear regression analysis of the observed data and the data predicted by the parameter-fit functions, as recommended by Maceina and Pereira (2007).
After collecting the crop timing data of Expt. 2, all plants remained in the greenhouse to allow for at least three fruits to ripen per plant, which occurred within 10 d from the first ripe fruit per cultivar. When an individual plant developed three ripe fruits, the following additional data were collected: plant canopy width, canopy height from the substrate surface, individual ripe fruit weight, and the total numbers of fruit (ripe and unripe), opened flowers, and unopened flowers. Canopy width was determined as the average of two perpendicular measurements per plant. Each ripe fruit weight measurement consisted of the average weight of the three ripened fruits per plant. The total numbers of fruit (ripe and unripe) and opened and unopened flowers were evaluated as a percentage of the total reproductive plant organs per plant. Data were pooled between Expt. 2 replications and analyzed using PROC GLIMMIX in SAS; thereafter, pairwise comparisons between treatments were performed using Tukey’s honestly significant difference test at α = 0.05.
Results and Discussion
The MDTs at first flower, first ripe fruit, and between first flower and fruit for ‘Fresh Bites Yellow’ and ‘Hot Burrito’ were near the target greenhouse temperature setpoints for Expts. 1 and 2 (Tables 1 and 2). The MDT at first flower was lower than the 26 °C setpoint in Expt. 1 (Table 1) because the greenhouse had difficulty maintaining heat during early Spring 2022. Expt. 2 started later in Spring 2023, and, in contrast, the MDT was greater than the 20 °C setpoint because of the warmer outdoor temperatures (data not shown) and limitations of greenhouse cooling (Table 2).
Increasing the greenhouse temperature resulted in fewer days to flowering, fruiting, and between flowering and fruiting for both pepper cultivars (Fig. 1). The time to flower ranged from 29 to 52 d for ‘Fresh Bites Yellow’, and from 25 to 40 d for ‘Hot Burrito’, whereas the time to first fruit ranged from 71 to 128 d, and from 65 to 122 d, respectively. The time between flower and fruit ranged from 42 to 76 d for ‘Fresh Bites Yellow’, and from 40 to 82 d for ‘Hot Burrito’. Flowering and fruiting rates increased for both pepper cultivars as temperature increased from ∼18 to 30 °C (Fig. 1). The flowering rate ranged from 0.020 to 0.039 for ‘Fresh Bites Yellow’, and from 0.025 to 0.042 for ‘Hot Burrito’ (Fig. 1A, 1B), whereas the fruiting rate ranged from 0.008 to 0.015 for both cultivars (Fig. 1C, 1D). The flowering to fruiting rate ranged from 0.014 to 0.024 for ‘Fresh Bites Yellow’, and from 0.012 to 0.023 for ‘Hot Burrito’ (Fig. 1E, 1F).
The effect of the mean daily temperature (°C) on the flowering and fruiting rates of compact ‘Fresh Bites Yellow’ and ‘Hot Burrito’ peppers using Eq. [1] and parameter estimates from Table 3. Regression slopes were nonsignificant (P ≥ 0.05) between Expts. 1 and 2 and replications (Expt. 2); therefore, data were pooled (●) for statistical analysis. Flowering and fruiting rates were calculated by taking a reciprocal of the number of days to first open flower, first ripe fruit, and between flower and fruit. Each symbol represents the treatment means, and error bars represent 95% confidence intervals. Trendlines are the predicted rates as a function of temperature.
Citation: HortScience 59, 9; 10.21273/HORTSCI17903-24
The estimated maximum rates (Rmax) for flowering, fruiting, and from flowering to fruiting were statistically similar between cultivars even though Fresh Bites Yellow and Hot Burrito are sweet and hot pepper cultivars, respectively (Table 3). Similar Tmin values were estimated between the flowering rate (12.9 ± 2.0 °C), fruiting rate (13.3 ± 1.9 °C), and flowering to fruiting rate (13.8 ± 2.4 °C) equations for ‘Fresh Bites Yellow’, where the average overall Tmin was 13.3 °C (Table 3). Estimated Tmin values were also statistically similar but varied more between flowering (11.7 ± 2.5 °C), fruiting (7.4 ± 3.7 °C), and flowering to fruiting rate (8.7 ± 4.1 °C) equations for ‘Hot Burrito’, with an average of 9.3 °C (Table 3).
Parameter estimates for the nonlinear equation (Eq. [1]) relating flowering and fruiting rates to the mean daily air temperature of three compact container pepper cultivars using data from Expts. 1 and 2. Parameter estimates were used to generate Fig. 2. Base (Tmin) temperature refers to the temperature at which the flowering and/or fruiting rate is zero, and Rmax refers to the maximum rate of development. C1 defines the skew for the function in Eq. [1]. Tmin, Rmax, and C1 are followed by columns containing their asymptotic 95% confidence interval (ACI).
The coefficients of determination generated for the nonlinear equations (R2) ranged from 0.491 to 0.846 for ‘Fresh Bites Yellow’ and ‘Hot Burrito’ (Table 3), and reflect the accuracy of Eq. [1] and the parameters in Table 3 for predicting temperature effects on flowering and fruiting times. The variability in time to flowering and fruiting increased at higher temperatures, especially for the second experimental run in Expt. 2 (Fig. 1). Overall, the nonlinear R2 values in Table 3 were comparable to the linear (r2) and nonlinear (R2) model coefficients of determination generated for 36 annual bedding plant species evaluated by Vaid and Runkle (2013) and Blanchard and Runkle (2011), which ranged from 0.33 to 0.94 between studies. In our study, the variability in timing of flowering and fruiting was partially attributed to the occurrence of flower and young fruit abortion, which is common for pepper (Wubs et al. 2009) and was observed with both cultivars.
Increasing the temperature setpoint in Expt. 2 resulted in greater canopy height, particularly for ‘Fresh Bites Yellow’ (Table 4), but the canopy width was unaffected. The 25 °C setpoint resulted in the greatest individual fruit weight for both ‘Fresh Bites Yellow’ (20.9 g/fruit) and ‘Hot Burrito’ (14.7 g/fruit), and fruit weight decreased above and below this temperature. The fruits from plants grown at 20 °C did not elongate as much during development compared with those grown at warmer temperatures (data not shown), which was a trend consistent with previous observations of smaller and more compact fruit developing when these cultivars were grown at ≤20 °C (Dickson R, unpublished data).
Effects of temperature on canopy width, height, individual fruit weight, total number of fruit (ripe and unripe), number of open flowers, and number of unopened flowers per plant for three compact pepper cultivars during Expt. 2. Data were pooled between replications. L = linear; Q = quadratic.
Increasing the temperature from 20 to 30 °C decreased the number of fruits per finished plant from 31 to 8 (74% decrease) for ‘Fresh Bites Yellow’, and from 34 to 25 (27% decrease) for ‘Hot Burrito’ (Table 4). In contrast, increasing the temperature from 20 to 30 °C increased the total numbers of flowers (unopened and opened) from 15 to 131 (773% increase) for ‘Fresh Bites Yellow’, and from 9 to 85 (844% increase) for ‘Hot Burrito’ (Table 4); the majority were unopened flowers at each temperature setpoint. Figure 2 shows how the proportions of reproductive organs per finished plant shifted from mostly fruits to mostly unopened flowers for both cultivars as temperature increased.
Temperature setpoint effects on the percentage of fruit (lower; dark gray bars), open flowers (middle; light gray bars), and unopened flowers (upper; white bars) per plant for two pepper cultivars during Expt. 2. There were three to four ripe fruit per plant at data collection, with the remainder being unripe and green in color. Means within a category with the same letter are not significantly different according to Tukey’s honestly significant difference test at α = 0.05.
Citation: HortScience 59, 9; 10.21273/HORTSCI17903-24
The abortion of flowers and young fruits was visually observed with both cultivars and at each temperature setpoint in Expt. 2, and the severity of abortion appeared to increase at greater temperature setpoints (data not collected). The periodic abortion or “flushing” of reproductive organs is a known phenomenon among sweet bell and hot pepper (Wubs et al. 2009), and correlations between increasing temperatures (especially night temperatures) and increased abortion of buds, flowers, and young fruit have been documented (Aloni et al. 1991; Bakker 1989; Erickson and Markhart 2001, 2002; Huberman et al. 1997; Marcelis et al. 2004). Marcelis et al. (2004) found 100% abortion of sweet pepper buds and flowers at a constant temperature of 33 °C, which was just a few degrees above the highest average daily temperature setpoint (30 °C) in this study. Wubs et al. (2009) also reported the period of greatest sensitivity to abortion was within 10 d and 14 d after anthesis for sweet and hot pepper, respectively, which may help explain the lower fruit set per finished plant despite the greater numbers of buds and open flowers at higher temperatures (Table 4, Fig. 2). For indeterminate cultivars, higher temperatures promoted a shift toward more vegetative growth and less reproductive growth (i.e., fruit development) in pepper (Oh and Koh 2019), and even short-term exposure to >30 °C has been shown to decrease pepper pollen viability and fruit set (Erickson and Markhart 2002). Based on past research and the results in Table 4 and Fig. 2, it seems likely that the higher temperatures had negative effects on flower viability and fruit set, resulting in lower fruits per plant for both cultivars at finish.
An exponential function adequately described the effect of temperature on pepper flowering and fruiting rates because the temperature range in this study (18 to 30 °C) was between Tmin and Topt for these cultivars, and the responses were curvilinear. However, the 30 °C temperature setpoint was likely near the optimum temperature (Topt), and previous studies have shown that increasing temperatures above the optimum often causes a rapid decrease in plant development rate (Blanchard et al. 2011; Brøndum and Heins 1993; Cave et al. 2013). Therefore, predictive equations may be inaccurate at temperatures >30 °C. Other environmental conditions such as the daily light integral (DLI) and photoperiod also influence development rates of container crops (Erwin and Warner 2002; Faust et al. 2005). The predictive equations developed for pepper assume a DLI ≥7 mol·m−2·d−1 and 16-h photoperiod and may not be valid under lower DLI conditions, which can limit the plant development rate (Faust and Heins 1993), or when cultivars with a facultative long-day flowering response are grown under a shorter photoperiod (Vaid and Runkle 2013).
The greenhouse environmental controls were occasionally adjusted to target the specific mean 24-h temperature treatments outlined in Tables 1 and 2; therefore, the plants experienced positive day/night temperature differentials ranging from ≈1 to 8 °C, which depended on the treatment and the point during the study (data not shown). Bakker (1989) evaluated the effects of 12 day/night temperature regimes on the flowering, fruit set, and fruit development of glasshouse sweet pepper and found that the day/night temperature differential was of minor importance when compared with the 24-h mean temperature. For example, day/night temperature differentials from −5 to 13 °C had no effect on the number of fruits, fruit length, pericarp thickness, patterns of flowering and fruit set, and the time from flowering to harvest (Bakker 1989), whereas each of these variables were affected by the MDT. The total number of flowers per plant, the percentage fruit set, and fruit weight were significantly related to both the MDT and day/night differential (Bakker 1989). Regarding sweet pepper seedlings, Si and Heins (1996) also found that changes in plant growth and morphology were primarily functions of the MDT, with minimal to no effect from day/night temperature differentials ranging from −12 to 12 °C. Therefore, based on previous research, it appeared possible that canopy height, canopy width, and individual fruit weight in Table 4 may have been influenced partially by day/night temperature differentials. However, the total fruits set per plant and the timing to flowering and fruiting were likely most influenced by the MDT.
Plant species can be subjectively categorized in terms of cold tolerance according to the estimated Tmin (Vaid and Runkle 2013); cold-sensitive floriculture container species can be associated with Tmin ≥8 °C, as shown for both pepper cultivars (Table 3). Floriculture container species that are cold-sensitive typically show a greater delay in flowering (i.e., longer crop time) when grown at cooler temperatures compared with cold-tolerant species (Tmin ≤4 °C) (Blanchard and Runkle 2011; Vaid and Runkle 2013), meaning they are often less suitable for lower-temperature production for the purpose of increasing energy efficiency and cost-savings as a result of reduced greenhouse heating (Blanchard 2009). The results of this study suggest that compact containerized pepper should be grown at a relatively warm temperature of ≈25 °C to achieve a nearly optimal combination of production attributes, including a relatively short crop time, low canopy height, high fruit weight, and high number of fruits per plant (Figs. 1 and 2, Table 4).
Conclusions
This study quantified the effects of the MDT on the growth and development of two compact container-grown pepper cultivars (Fresh Bites Yellow and Hot Burrito) in controlled greenhouse environments. As the temperature increased from 18 to 30 °C, the developmental rates for flowering and fruiting increased exponentially. The results suggested that ‘Fresh Bites Yellow’ and ‘Hot Burrito’ grown at ≈25 °C would result in a nearly optimum combination of a relatively short crop time, compact canopy, large fruit size, and high number of fruits per plant at finishing. The nonlinear equations that characterized the relationship between temperature and crop timing in this study are, to our knowledge, the first developed for compact container-grown pepper. These equations also incorporated Tmin and Rmax, which have biological meaning and can be used to draw inferences regarding pepper cold tolerance and optimal growing temperatures. Compact container-grown peppers are relatively new crops that are being grown by greenhouse floriculture operations for both ornamental and edible values. Information from this study can be used to help growers schedule compact peppers alongside other containerized ornamental crops to meet critical market windows as well as determine the effects of changing the growing temperature on crop timing and quality.
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