Reducing Substrate Moisture Content during Greenhouse Production of Poinsettia Improves Postproduction Quality and Economic Value

Abstract

<jats:p>The objective was to determine the effect of substrate moisture content (SMC) during poinsettia (<jats:italic>Euphorbia pulcherrima</jats:italic>) greenhouse production on plant quality, postproduction longevity, and economic value. Two experiments were conducted, one in 2016 with ‘Freedom Red’ and the other in 2017 with ‘Christmas Eve Red’. Treatments included two SMC levels (20% or 40%) applied in four timing of application combinations. Total production (TP) time was 14 (2016) or 12 (2017) weeks in which vegetative production (VP) occurred from week 33 (2016) or 35 (2017) to week 39 and reproductive production (RP) continued from week 40 to 47. The four timing of application treatments were 40/40 = TP at 40% SMC; 20/40 = VP at 20% + RP at 40%; 40/20 = VP at 40% + RP at 20%; 20/20 = TP at 20% SMC. After simulated shipping in the dark, plants were evaluated in a simulated retail environment with two packaging treatments: no sleeve covering or plastic perforated plant sleeves covering container and plant. At the end of greenhouse production, plants grown in 20% SMC during RP (20/20 and 40/20) had shorter bract internode length, stem length, and smaller growth index (GI), decreased shoot and root dry weight (DW), and bract and leaf surface area compared with those in 40% SMC during RP (40/40 and 20/40). Photosynthetic rate was higher when plants were watered at 40% SMC regardless of production stage compared with those in 20% SMC. Leaf thickness, petiole thickness, total bract and leaf number were unaffected by SMC treatments. Plants in 20% SMC during RP (20/20 or 40/20) had earlier bract coloring despite days to anthesis being the same for all SMC treatments. Compared with 40/40, 40/20, and 20/20 could save 44.2% or 43.6%, respectively, irrigation and fertilizer usage, and 39.1% and 47.8%, respectively, labor time. During postharvest, ethylene concentration was unaffected by packaging method. Sleeved plants, regardless of SMC treatment, received lower light intensity in the middle of the plant canopy, causing plants to have lower total leaf number due to abscission and SPAD reading at the end of postproduction. The 40/40 treatment abscised more bracts during five weeks (in 2016) of postproduction and with no sleeve had higher number of bracts with bract edge burn (BEB). In summary, reducing SMC to 20% during TP or RP reduced water usage during production and produced more compact plants with increased postproduction quality.</jats:p>

Developing a Water-stress Index for Potted Poinsettia Production

“…There are opportunities for additional research to evaluate whether the responses we discovered are consistent across other growing media and how PGR, which were not used in our trial but are commonly sprayed on poinsettias, affect transpiration. Although our study differs in many aspects (e.g., experimental design and physiological measures) from the Guo et al (2018) research, our results provide complementary evidence supporting the idea of decreasing poinsettia irrigation scheduling by 50% from 0.4 to 0.2 m 3 • m -3 VWC. This is not the first time research could influence radical changes to poinsettia production.…”

“…Our results indicate that poinsettia irrigation schedules that attempt to maintain 0.4 m 3 • m -3 VWC are applying excessive volumes of water, resulting in wasted resources and potential sources of pollution. It has been suggested that a standard commercial production irrigation schedule allows the substrate to dry down to 0.4 m 3 • m -3 VWC before rewatering to container capacity (Guo et al, 2018). Our results suggest that it was not until <0.2 m 3 • m -3 VWC that most of the weekly treatment plants began expressing water stress symptoms.…”

“…For ornamental plants such as poinsettias, it is essential to improve irrigation efficiency and achieve water conservation benefits without sacrificing plant quality. Guo et al (2018) demonstrated how irrigation thresholds at 0.2 m 3 • m -3 VWC for a peat-based substrate could create stunting without degrading aesthetics as a chemical-free alternative to ap-plications of PGR such as chlormequat and paclobutrazol. Their research also pointed to reduced disease pressure for plants watered with deficit irrigation schedules (Guo et al, 2018), which has also been found in other deficit irrigated nursery systems (Dumroese and James, 2005).…”

“…Although water is inexpensive in many production regions, it can represent a large cost for nursery and greenhouse plant production in arid regions, such as California and the southwestern United States (Pitton et al, 2018). Nonoptimized production practices, and overwatering in particular, are wasteful and deleterious in terms of increased poinsettia postproduction disease pressure (Guo et al, 2018) and decreased plant quality (Morvant et al, 1998). Excessive irrigation is known to promote the spread of waterborne plant pathogens (Redekar et al, 2019;Weiland et al, 2018) that cause millions of dollars in horticultural crop losses every year.…”

“…Therefore, to optimize irrigation scheduling, it is essential to study plant yield and physiological responses to substrate moisture deficit. For herbaceous crops, leaf transpiration has been used as a nondestructive sensor-based assessment of plant water use (Guo et al, 2018;Kim et al, 2013;Nackley et al, 2014;Nambuthiri et al, 2017). Sap-flow sensors measure xylem flux, which is affected by the physiological interactions between the available water in the soil and the atmospheric demand and provide another nondestructive measurement of plant water use.…”