Abstract
Bromeliads are important ornamental foliage plants, but until now, their daily water use during production was unknown. Using a canopy closure model developed for container-grown woody ornamental plants, in this study we investigated actual evapotranspiration (ETA) of Guzmania ‘Irene’ and Vriesea ‘Carly’ from tissue-cultured liners grown in 15-cm containers to marketable sizes in a shaded greenhouse. The mean daily ETA of Guzmania ‘Irene’ ranged from 4.02 to 66.35 mL per plant, and the mean cumulative ETA was 16.66 L over a 95-week production period. The mean daily ETA of Vriesea ‘Carly’ varied from 3.98 to 59.89 mL per plant, and the mean cumulative ETA was 15.52 L over the same production period as the Guzmania cultivar. The best-fit models for predicting daily ETA of the two bromeliads were developed, which had correlation coefficients (r2) of 0.79 for Guzmania ‘Irene’ and 0.68 for Vriesea ‘Carly’. The success in the model of ETA for both bromeliads suggested that the canopy closure model was equally applicable to container-grown ornamental foliage plants produced in greenhouse conditions. The daily ETA and cumulative ETA values represent research-based information on water requirements, and, when applied, could improve irrigation practices in bromeliad production. This study also showed that roots per se of the two epiphytic bromeliads were able to absorb water and nutrients from a peat-based container substrate and support their complete life cycles.
The Bromeliaceae family comprises more than 3000 species across 58 genera that are largely native to tropical North and South America (Stevens, 2013). Their exotic appearance, graceful symmetry, and potential for year-round flowering make bromeliads among the most sought-after plants in the world (Henny and Chen, 2003). Among them, Aechmea Ruiz & Pav., Cryptanthus Otto & A. Dietr., Guzmania Ruiz & Pav., Neoregelia L.B. Smith, Nidularium Lem., Tillandsia L., and Vriesea Lindl. are important ornamental foliage plants (Chen et al., 2005). It is estimated that nearly 60 million potted bromeliad plants are sold annually worldwide, of which 60% are Guzmania (Vanhoutte et al., 2016) and 15% are Vriesea cultivars (Vanhoutte et al., 2017).
Bromeliads have many unique characteristics, including epiphytism, water-absorbing leaf trichome, tank habit, and crassulacean acid metabolism (CAM), that enable them to survive under various harsh environmental conditions (Leroy et al., 2016). More than 60% of bromeliads are epiphytes (Zotz, 2013). Epiphytic bromeliads have two life forms: tank bromeliads, whose leaves spiral and arrange to form a rosette cup or tank where they accumulate water and organic debris, and atmospheric bromeliads, whose leaves are narrow and do not serve as a reservoir (Benzing, 2000; Givnish et al., 2007). Leaf hair or foliar trichomes are specialized for water and nutrient absorption and are called absorptive trichomes (Zotz, 2013).
Both Guzmania and Vriesea are tank bromeliads and considered to be C3 plants (Pierce et al., 2001) with exception of some Guzmania that are facultative CAM species (Freschi et al., 2010). Their growth and development under protected culture may be different from their wild habitats. Commercially, they are produced in soilless substrates under climate-controlled greenhouses and supplied with ample water and nutrients. A recent study showed that roots of two cultivars of Vriesea are able to absorb much more water than trichome in commercial production conditions (Vanhoutte et al., 2017). Roots of Vriesea have been shown to possess root hairs, a vascular system, and an endodermis (Proenca and Sajo, 2008; Reinert and Mereilles, 1993), implying that the roots are capable of taking up water and nutrients. Gas exchange and plant growth of Guzmania lingulata (L.) Mez. fertigated through roots were comparable with those fertigated through tank (Silva et al., 2017).
With current commercial production practices, irrigation of bromeliads has been primarily based on growers’ experience. A common problem with the production of container-grown plants has been overirrigation, resulting in not only poor aeration in container substrates but also nutrient leaching (Chen et al., 2001). This holds true in bromeliad production, where daily water requirements have not been quantified for any bromeliad species. To increase irrigation efficiency, Beeson (2004) proposed a canopy closure model for estimating ETA of container-grown woody ornamental plants as a basis for precision irrigation. Based on the relationships of ETA, potential evapotranspiration (ETO), and projected canopy area (PCA) of a given species, a water need index (WNI) or plant factor (Kjelgren et al., 2016) is calculated, which is a function of canopy closure of a group of plants, relating individual plant ETA to plant size and canopy ventilation and radiation (Beeson, 2005). Daily ETO was calculated using the Campbell Scientific version of the Penmen–Monteith equation, whereas ETA was determined by an autonomous weighing lysimeter system (Beeson, 2011). The model has been used to quantify daily ETA of several woody ornamental plants including Ligustrum japonicum (Beeson, 2004), Viburnum odoratissimum (Beeson, 2010a, 2010b), Rhaphiolepis indica (Beeson, 2012), as well as foliage plants Asplenium nidus L. and Chamaedorea elegans Mart. (Chen and Beeson, 2013), and Calathea G. Mey. ‘Silhouette’ and Stromanthe sanguinea Sond. (Beeson and Chen, 2018). Irrigation of container-grown plants based on daily water use has been documented to reduce nursery runoff volume and nutrient load without reducing plant growth (Hagen et al., 2014; Pershey et al., 2015).
The objectives of this study were to determine ETA of Guzmania and Vriesea from tissue-cultured liners grown in 15-cm containers to marketable sizes in a shaded greenhouse and to develop models to predict daily ETA rates. It is anticipated that such effort could provide research-based information for improving irrigation efficiency during bromeliad production.
Materials and Methods
Experimental location.
This study was carried out in an 80% shaded greenhouse at the University of Florida’s Mid-Florida Research and Education Center (MREC) in Apopka. A Florida Automated Weather Network (FAWN, https://fawn.ifas.ufl.edu/) station at MREC is about 46 m east of the shaded greenhouse. The station provided readings of air temperatures at three elevations (0.6, 1.8, and 9.1 m), soil temperature, wet bulb temperature, dew point, relative humidity, rainfall, and wind speed every 15 min daily as well as daily evapotranspiration outside the greenhouse. The outside ETO was calculated based on the Campbell Scientific Program (Campbell Scientific, 1991) which uses the full ASCE Penman–Monteith equation (Jensen et al., 1990).
Data collection system.
The miniature weighing lysimeter system described by Beeson (2011) was used in this study. The system was composed of a control/data collection board connected to mini lysimeters. The control board consisted of a CR10X data logger, SDM-AM16-32 multiplexer, and SDM-CD16AC relay control module (Campbell Scientific Inc., Logan, UT) for receiving and storing data from the mini-lysimeters. Each mini-lysimeter included a load cell (SSM-50-AJ; Interface Inc., Scottsdale, AZ) suspended from a miniature tripod with a plant support suspended from the load cell. All load cells were calibrated with a seven-point curve using known masses. The data logger program recorded the mass of each lysimeter every half hour and stored it for later retrieval. At midnight, the program determined that day’s ETA for each lysimeter as the difference between mass at 0500 hr and midnight. There was no transpiration between midnight and 0500 hr. ETA was calculated by hand using the half hour lysimeter data as described by Beeson (2006) when needed.
Plants were watered through a drip irrigation system to allow a 10% leachate fraction. Water was supplied through self-fashioned rings of pressure compensated drip tubing (Netafilm USA, Fresno, CA). As supplied, pressure compensating emitters were arranged in 30.5-cm intervals along the length of the tubing. Lengths containing four emitters were cut and rolled to form two loops, which were joined by a T-barb. These fit snugly into the top of each container. Loops were connected to a 19-mm polyethylene tubing using equal lengths of 6 mm tubing. The Christensen’s Coefficients of Uniformity were determined for 15 loops from plants on each bench. The mean coefficient was 0.94 and ranged from 0.93 to 0.96. Typical application rates per bench were 187 mL·min−1.
Bromeliad plant production.
Tissue cultured liners of Guzmania ‘Irene’ and Vriesea ‘Carly’ were transplanted singly into 15-cm containers (1.6 L) filled with a peat-based substrate composed of 60% Canadian peat, 20% vermiculite, and 20% perlite by volume. Plants were fertilized by top dressing 5 g of a controlled-released fertilizer (CRF) (Osmocote 19N–2.18P–7.47K, 8–9 months; The Scotts Co., Marysville, OH) per container 3 weeks after potting. The same amount of the CRF was applied 42 weeks later. The experiment was arranged as a completely randomized block design with four replications. In each block, 15 plants of a species were arranged in five rows with three plants per row in an offset pattern and spacing of 30 cm between pots. The center plant was placed in a suspension-weighing lysimeter, and the four closest plants to the lysimeter plant were designated as the interior plants for repeated canopy measurements. Plants were grown in a shaded greenhouse under a maximum photosynthetic active radiation of 200 μmol·m−2·s−1.
Air temperature, relative humidity, wind, and solar radiation required for calculation of reference evapotranspiration (ETO) were collected with an automated Weatherhawk weather station (Campbell Scientific, Inc.) inside the middle of the greenhouse 50 cm above the lysimeter tripods. The algorithm used to calculate ETO was the same as that used in more robust outside weather stations based on Campbell Scientific equipment (Campbell Scientific, 1991). This was installed to evaluate the hypothesis that although the use of the Penman–Monteith equation inside a greenhouse violates the basis of Penman–Monteith, the thermodynamic associations of heat and random movement of water molecules within leaves and plant canopies would still mimic that of leaves outside nonetheless.
Data collection.
Data collection was initiated within a week after transplanting. Canopy measurements of widest width, width perpendicular to the widest width, and average height were recorded every 3 weeks on the lysimeter plants and adjacent four plants for each replication. The two widths of each plant were multiplied to estimate the two-dimensional PCA (i.e., canopy footprint). When PCA was multiplied by the average height, canopy volume, or growth index, was estimated (Henny et al., 2009), assuming the three-dimensional canopy resembled a rectangular box. Plant water-use efficiency was calculated as total dry matter produced (g) by actual amount of water used (ETA) (Stanhill, 1987).
Modeling plant water use.
ETA, ETO, and PCA were used to evaluate the suitability of the percent canopy closure (%CC) model (Beeson, 2004, 2010a, 2010b) for determining water use of greenhouse-grown bromeliads. To summarize, %CC at each measurement was calculated by adding half the PCA of each of the four border plants to the PCA of the lysimeter plant and dividing the sum by allocated bench space for each plant (929 cm2 in this study). Because plants were not respaced, canopies could become overlapped with plant growth. Thus, overlapping could result in calculation of %CC greater than 100%. For each lysimeter plant, ETA was recorded for 7 d and converted to a depth by dividing with its average PCA (cm2). ETA (cm) was then normalized by dividing with its corresponding ETO (cm) each day, and then averaged over the 7 d to calculate WNI using the equation of [WNI = (ETA/PCA)/ETO] for each lysimeter plant at each measurement date. Calculated WNI values of the four lysimeter replicates for each date were plotted against their corresponding %CC values. The plot was fitted to a three-parameter exponential decay curve using SigmaPlot (Version 10; SPSS Inc., Chicago, IL). An equation for the nonlinear line was derived using a three-level inverse polynomial equation (Version 10; SPSS Inc.).
Results
Reference evapotranspiration.
The ETO (ETO-GH) values in the shaded greenhouse were ≈12% of that outside the shaded greenhouse as measured by the FAWN station. Solar radiation in the shaded greenhouse was around 15% that of outdoor conditions, and relative humidity was normally higher inside the greenhouse. Although there was no measurable wind, air movement occurred during most afternoons from early spring until late fall due to operation of evaporative cooling fans. Temperatures in the shaded greenhouse were also more moderate than outside conditions, with minimums set at 18.3 °C for heating and 32.2 °C for evaporative cooling. ETO was highest in April and May (days 90 to 150), declined the rest of the period with the onset of summer rains, then shorter days. In Central Florida, April and May tend to have the highest vapor pressure deficits (VPD) due to mostly clear skies and low humidifies resulting from the near absence of rainfall. Evaporative cooling during this period often is insufficient to maintain low VPD.
Root growth.
To assess root growth of two bromeliad species watered through drip irrigation, plants were pulled from containers 3 months after potting. Both species had well-established roots systems. Many roots had penetrated the substrate, with white, healthy roots appearing in the interface of container wall and substrate, and some had root hairs (Fig. 1).
Shoot growth.
Guzmania ‘Irene’ was potted in Nov. 2006 and harvested in Sept. 2008, when they had reached a size suitable for commercial marketing. Leaves were glossy green with a mean length of 40 cm. Canopy heights and widths of Guzmania ‘Irene’ increased linearly (data not shown), but its growth index increased polynomially (Fig. 2A). Guzmania ‘Irene’ produced 10 to 15 fuchsia–magenta bracts, which are modified leaves. At harvest, mean canopy height and width were 45.6 cm and 71.9 cm, respectively. It had a mean of 33.3 leaves and a total leaf area of 4,067.4 cm2. Shoot and root fresh weights were 176.2 g and 12.7 g, and corresponding dry weights were 29.1 g and 2.4 g, respectively. Guzmania ‘Irene’ had a water use efficiency of 1.9 (Table 1).
Plant growth measurements at harvest by species. Plants were harvested when they had reached common commercial canopy sizes along with flowers.z
Vriesea ‘Carly’ was potted concurrently with Guzmania ‘Irene’ in Nov. 2006 and harvested in Sept. 2008 when a marketable size was attained. ‘Carly’ has green and stiff leaves with a mean length of 16 cm. Canopy heights of Vriesea ‘Carly’ increased linearly, and widths increased polynomially (data not shown). Growth index increased linearly (Fig. 2B). Vriesea ‘Carly’ has yellow and red flower spikes. Mean canopy height and width at the time of harvest were 22 cm and 26. 3 cm, respectively. ‘Carly’ produced 28.9 leaves with a total leaf area of 1583.5 cm2. Shoot and root fresh weights were 48.7 g and 9.6 g; shoot and root dry weights were 8.5 g and 2.0 g, respectively. Water use efficiency of Vriesea ‘Carly’ was 0.7 (Table 1).
Actual evapotranspiration.
The mean cumulative ETA for Guzmania ‘Irene’ was 16.66 L per plant during the entire production period. At the beginning of the experiment, mean ETA ranged from 5.91 to 17.74 mL per plant per day (Fig. 3A). Beginning in Feb. 2007, mean ETA increased more than 2-fold over the next 50 d, ranging from 23.66 to 35.49 mL per day. During the summer months of 2007, mean ETA averaged around 29.57 mL per day but ranged from 6.21 to 51.37 mL per day during this period. The variability in ETA is likely due to highly fluctuation in cloud cover during the summer rainy season. Mean ETA began declining in August (day 300) to reach a low in Dec. 2007, comparable with that of the first December in 2006. Mean ETA again began increasing from late January 2008 to the summer of 2008. Mean ETA was quite variable during this period, ranging from 4.02 to 66.35 mL per day. The decline in Aug. 2008 mirrored that measured the previous year.
The mean cumulative ETA value was 15.52 L for Vriesea ‘Carly’. Initial daily ETA for individual plants ranged from 7.64 mL to 32.93 mL (Fig. 3B). This declined quickly to 4.48 to 11.83 mL per day by the middle of December (day 45). Beginning in Feb. 2007, mean ETA increased 2.5-fold over the next 50 d to a range from 23.66 to 35.49 mL per day, similar to that shown for Guzmania ‘Irene’ (Fig. 3A). During the summer months of 2007, mean ETA averaged around 29.57 mL per day but ranged from 7.5 to 59.89 mL per day during this period. Mean ETA began declining in August (day 300) to reach a low in Dec. 2007, comparable with that of the first December and ‘Irene’. Mean ETA again began increasing in late January 2008 to a median mean ETA of 32.53 mL per day during the summer of 2008. Mean ETA was variable during this period, ranging from 3.98 mL to 47.32 mL per day. The sharp decline in Aug. 2008 also mirrored that measured the previous year.
Data analysis and modeling.
Application of the %CC model (Beeson, 2010a) was successful for both bromeliads with the best fit model presented in Table 2. The significance of r2 values (0.79 and 0.68 for Guzmania and Vriesea, respectively) for both equations suggested that the two species exhibited strong relationships between WNI and the level of canopy closure. As derived, equations predict the water use of individual plants. If means could have been derived from these individual plant measurements, such that the relationships were derived for populations of plants, the r2 would have been higher and predictions of overall crop water use would be more certain.
Best-fit models for predicting daily ETA values of Guzmania ‘Irene’ and Vriesea ‘Carly’ grown in 15-cm containers from tissue-cultured liners to marketable sizes.
For both Guzmania ‘Irene’ and Vriesea ‘Carly’, the WNI coefficients declined with increasing plant growth, indicated by the increase in %CC (Fig. 4A and B). In brief, this decline in WNI was due to increases in canopy boundary layer resistance as plant foliage expanded and filled in the gaps between containers. At near 100% canopy closure, transpiration of all but the upper leaves become decoupled from the air above the canopy, resulting in 40% deceases in whole plant transpiration outdoors (Beeson, 2010a). Conversely random removal of ≈33% plant canopy coverage had been shown to increase individual plant transpiration by 40% (Beeson, 2010a).
Discussion
Agricultural use of fresh water has been under rigorous scrutiny due to irrigation, accounting for up to 62% of freshwater use, including 68% of groundwater and 29% of surface water withdrawals in the United States (Kenny et al., 2009). Container-grown plant production is an extensive water user within the realm of agriculture (Beeson, 2011; Chen et al., 2001, 2003). Plants are grown in confined volumes filled with artificial substrates, which are watered frequently to avoid drought stress. Irrigation frequency and quantity, however, is largely based on grower’s intuition or experience. As a result, plants are often overirrigated (Belayneh et al., 2013). Overirrigation not only reduces substrate aeration but also results in excess runoff and leaching of nutrients, primarily nitrogen (N) and phosphorus (P). Movement of N and P in waterways could potentially contaminate ground and/or surface water (Chen et al., 2001). Thus, irrigation based on plant growth requirements is increasingly important for sustainable production of container-grown ornamental plants.
The present study shows that plants differ almost daily in water requirements. ETA values varied from 4.02 to 66.35 mL for Guzmania ‘Irene’ and 3.98 mL to 59.89 mL for Vriesea ‘Carly’ over a 95-week production period. The variation, however, does exhibit an identical pattern regardless of plant species: a slight increase in ETA in Nov. 2006 that was due to plant establishment after transplanting and then a decrease in the winter; a substantial increase in early Feb. 2007 and a decrease in late August; and an increase again in late Jan. 2008 and decreased in late August (Fig. 3). The pattern generally coincided with the seasonal changes in Central Florida, suggesting that the growth of these two bromeliads was synchronized by weather conditions, even though they grew in a shaded greenhouse. A difference between the two species is that the daily ETA values of Guzmania from Mar. to Aug. 2008 were much higher than those of Vriesea. This could be related to the emergence of bracts of Guzmania, where 10 to 15 modified fuchsia–magenta leaves appeared, which might have much high transpiration than a compact single spike of Vriesea.
The cumulative ETA values of Guzmania ‘Irene’ (16.66 L) and Vriesea ‘Carly’ (15.52 L) were comparable with those our previous studied Asplenium nidus (7.95 L) and Chamaedorea elegans (6.43 L) (Chen and Beeson, 2013) as well as Calathea G. Mey. ‘Silhouette’ (4.84 L) and Stromanthe sanguinea Sond. (6.81 L) (Beeson and Chen, 2018) if considering the production time. The production time for the two bromeliads was 665 d compared with 294 d for both A. nidus and C. elegans, 224 d for Calathea, and 231 d for Stromanthe. Commercial production of Guzmania and Vriesea in 15-cm containers usually takes less than 250 d, during which ethylene is used to force flowering. In our study, we did not use ethylene; plants flowered naturally, which resulted in a prolonged production time. Another note is that Guzmania ‘Irene’ and Vriesea ‘Carly’ varied greatly in leaf areas, shoot, and root dry weights (Table 1), but their cumulative ETA only differed by 1.1 L. This could be due in part to the faster growth rate of Guzmania ‘Irene’, as illustrated in Fig. 1, and higher water use efficiency (1.9) (Table 1), whereas Vriesea ‘Carly’ growth was slower with a water use efficiency of 0.7. Another reason is that leaves of Guzmania ‘Irene’ were much longer than Vriesea ‘Carly’ (40 cm vs. 16 cm). Guzmania ‘Irene’ reached canopy closure much quicker (Fig. 4A) than Vriesea ‘Carly’ (Fig. 4B). The continuous growth of Guzmania ‘Irene’ caused increased overlapping of leaves, resulting reduced transpiration and the demand for water, as shown in Fig. 4A. In contrast, the appearance of 10 to 15 bracts after May 2008 increased transpiration; thus, ETA of Guzmania ‘Irene’ was higher compared with Vriesea ‘Carly’ during the same period. Nevertheless, the application of the canopy closure model was successful for both bromeliads, and the best-fit model is presented in Table 2. The r2 values for the WNI as a function of %CC were high, 0.79 for Guzmania ‘Irene’ and 0.68 for Vriesea ‘Carly’. Results from this study, along with the previous reports (Beeson and Chen, 2018; Chen and Beeson, 2013) suggest that the canopy closure model and WNI developed for woody ornamental plants are suitable for modeling daily water requirements of container-grown foliage plants in greenhouse conditions. As far as we know, this is the first report on the daily water use of bromeliads. This study provides important information for commercial production of both Guzmania and Vriesea and probably other bromeliads.
This study also showed that roots of both Guzmania ‘Irene’ and Vriesea ‘Carly’ were functional. Since plants were fertilized by topdressing a CRF on substrate and watered via drip irrigation and not through tank, water and nutrients must have be taken up through roots. As shown in Fig. 1, both Guzmania ‘Irene’ and Vriesea ‘Carly’ indeed had well-developed root systems. Interestingly, root systems of many tank bromeliads, such as Tillandsioideae species in general, often are classified as nonabsorptive and mainly serving to anchor these plants (Benzing, 2000), whereas foliar trichomes are essential for water and nutrient absorption from the tank solution (Inselsbacher et al., 2007). Our results showed that epiphytic bromeliads, when cultivated in a substrate, emitted new roots that are functional. Vanhoutte et al. (2017) also showed that roots of Vriesea are important for water and nutrient absorption. The importance of trichomes in water and nutrient absorption could be due to the evolutionary adaptation of epiphytic bromeliads in the wild. The ability of cultivated bromeliad roots to take up water and nutrients may suggest great physiological plasticity of epiphytic bromeliads in adaptation to commercial cultivation. Breeding of bromeliads for ornamental purposes also may select plant roots not only for anchoring plants but also for effective absorption of water and nutrient for commercial production.
In conclusion, the canopy closure model and WNI developed for woody ornamental plants are suitable for modeling daily water requirements of container-grown foliage plants in greenhouse conditions. The daily ETA established for Guzmania and Vriesea could be used as references for developing optimal irrigation recommendations for commercial bromeliad production. Development of WNI models for other groups of bromeliads would like conserve water and reduce nutrient leaching. In addition, Guzmania and Vriesea develop functional roots and can be irrigated via roots or roots and tanks in commercial bromeliad production.
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