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  • Author or Editor: Uttara C. Samarakoon x
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Uttara C. Samarakoon and James E. Faust

Vegetatively propagated unrooted cuttings typically are grown in equatorial locations and shipped via airfreight to propagators located in temperate climates. Cutting quality, defined as the resistance to external forces, such as physical damage and pathogen infection, impacts postharvest durability during shipping and propagation. During our previous studies, foliar application of calcium (Ca) in the form of Ca chloride was effective at increasing leaf mechanical strength of poinsettia (Euphorbia pulcherrima) and zonal geranium (Pelargonium ×hortorum). Calcium chloride applied at ≥800 mg·L−1 Ca caused phytotoxicity symptoms in poinsettia; therefore, in the current work, we investigated the use of chelated Ca by providing Ca at 40, 80, or 160 mg·L−1 and salicylic acid (SA) at 150 or 300 mg·L−1 to increase the mechanical strength of poinsettia leaves. Mechanical strength of leaves was assessed using a force-displacement graph generated from a texture analyzer using a ball probe to penetrate a unit area of a clamped leaf. The peak force to fracture the leaf and work-of-penetration, defined as the area under the force-displacement curve, were used as indicators of mechanical strength. Calcium concentration in the leaves increased by 27% with increased application of Ca from 0 to 160 mg·L−1. Peak force was 26% greater in treatments with Ca at 80 or 160 mg·L−1 compared with the untreated control. Work-of-penetration was 24% and 29% greater for treatments with Ca at 80 and 160 mg·L−1, respectively, compared with the control. Foliar application of SA did not affect leaf mechanical strength. Chelated Ca applied at 160 mg·L−1 Ca caused visual phytotoxicity symptoms; thus, applications of 80 mg·L−1 Ca are recommended to improve resistance to physical damage for poinsettia leaves.

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James E. Faust, Pamela C. Korczynski and Uttara C. Samarakoon

During the production of ornamentals in commercial greenhouses, hanging baskets are often grown above the bench or floor space to maximize production. These hanging baskets impact the light environment delivered to the crop underneath. An experiment was conducted to quantify the effect of hanging basket density (determined by number of lines of containers per greenhouse bay and container spacing per line), container content (with plants vs. no plants), and container color (white vs. green) on photosynthetic photon flux (PPF) transmission and red (R) and far-red (FR) light measurements at the greenhouse floor under the hanging basket treatments. Interception of PPF was calculated as a proportion of the treatment with no hanging baskets. Interception of PPF increased as hanging basket density increased, from 5.3% interception at 0.21 containers/yard2 to 25.5% interception at 2.57 containers/yard2. Green containers intercepted 36.1% more radiation than the white containers. Presence of plants in the containers resulted in 62.3% greater PPF interception than containers without plants. R:FR was reduced from 1.15 measured under hanging basket treatments without plants to 1.07 under hanging basket treatments containing plants.

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Uttara C. Samarakoon, James E. Faust and John M. Dole

Vegetatively propagated unrooted cuttings are typically imported to the United States from Central America. Death or damage of cuttings during shipping and propagation can be reduced if cuttings can be made more resistant to external forces, such as physical damage or pathogen infection. However, strategies to develop durable cuttings via treating stock plants have not been previously quantified in controlled studies. During the current study, mechanical strength of leaves and resistance to infection by Botrytis cinerea were evaluated after weekly applications of calcium chloride (CaCl2) as a foliar spray to stock plants that delivered calcium (Ca) at the concentrations of 0, 400, or 800 mg·L−1. A texture analyzer quantified the peak force required to fracture the leaf and the work of penetration,or area under the force–displacement curve, and these measurements were indicators of mechanical strength. For poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) cuttings at the time of harvest from the stock plant, work of penetration increased by 10% with the application of 800 mg·L−1 Ca compared with the control, whereas peak force by 9%. For zonal geranium (Pelargonium ×hortorum Bailey), work of penetration increased 15% with the application of 800 mg·L−1 Ca compared with the control. Calcium concentration in the leaves increased from 1.2% to 2.0% in geranium and from 1.0% to 1.6% in poinsettia with increasing application from 0 to 800 mg·L−1 Ca. In poinsettia, disease incidence in response to inoculation with B. cinerea spores was 55% and 15% less with CaCl2 applications compared with controls with water and surfactant, respectively, whereas CaCl2 application to geranium did not affect disease incidence.

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Joaquin A. Chong, Uttara C. Samarakoon and James E. Faust

Poinsettia stock plants consist of a dense canopy of competing shoots, and the growth and development of these individual shoots have not been previously quantified. The effects of air temperature, daily light integral (DLI), and canopy density (CD) were investigated on poinsettia (Euphorbia pulcherrima Willd. ex. Klotsch) ‘Freedom Red’ shoot development in a stock plant canopy. Plants were grown at two constant temperatures (20.3 or 25.7 °C), five CD (43, 86, 129, 172, or 215 shoots/m2), and three DLI treatments (2.6, 4.4, or 7.7 mol·m−2.d−1 for the September planting and 4.0, 6.0, or 10.6 mol·m−2.d−1 for the January planting). Shoot position at the final data collection was used to assign shoots to different levels within the canopy; Level 1 = the four highest shoots, Level 2 = the next four highest shoots, and so forth for Levels 3, 4, and 5. Temperature did not significantly affect leaf unfolding rate (LUR), shoot fresh mass (FM), or shoot caliper, whereas DLI and CD affected shoot growth and development. LUR and FM increased as DLI increased from 2.6 to 10.6 mol·m−2.d−1, whereas LUR and FM decreased on the uppermost shoots in the canopy, e.g., Level 1 shoots, as CD increased from 43 to 129 shoots/m2. Therefore, higher CD required higher DLI to achieve similar LUR and FM. Shoot caliper on Level 1 shoots increased from 6.3 to 7.4 mm as CD decreased from 129 to 43 shoots/m2; and shoot caliper increased from 5.8 to 7.6 mm as DLI increased from 4.0 to 10.6 mol·m−2.d−1. The DLI environment needs to be managed to accommodate greater CD, to sustain growth and development of individual shoots within the canopy of poinsettia stock plants.

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Uttara C. Samarakoon, Keith A. Funnell, David J. Woolley and Edward R. Morgan

The time to harvest maturity of flowering shoots and the extent and source of variability in maturity dates differed among cultivars of gentian (Gentiana sp.), with a wider spread in time to harvest maturity in Showtime Starlet (41 days) than Showtime Diva (35 days) and Showtime Spotlight (29 days). Cultivars also differed by more than twice in their plant-to-plant variability in time to harvest. Although later-emerging shoots reached harvest maturity more quickly than earlier-emerging shoots, the use of growing degree-days (GDD) for this field-grown cut flower did not account for differences. For ‘Showtime Diva’, 77% of outliers reached harvest maturity at the beginning of the season (i.e., before the 10th percentile). For ‘Showtime Spotlight’, only 20% of shoots classified as outliers flowered early with the remaining 80% emerging late (i.e., after the 90th percentile). Strategies to control the spread in time to harvest maturity in late-maturing cultivars such as Showtime Starlet should focus on uniform shoot emergence and controlling temperature during growth. Although strategies to achieve uniform shoot emergence should also be targeted for ‘Showtime Diva’, controlling temperature during the growing season would not appear to offer significant control of the spread in time of harvesting floral shoots. However, in earlier-maturing cultivars such as Showtime Spotlight, strategies will primarily require a greater understanding of the factors influencing the variability in maturation of shoots within individual plants before, and after, emergence.

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Uttara C. Samarakoon, David J. Woolley, Ed R. Morgan and Keith A. Funnell

Cut flower productivity and quality of gentian is associated with growth and development of crown buds. Experiments were carried out with the gentian cultivar Showtime Diva to identify the response to treatments that break dormancy [cold temperature (chilling), gibberellic acid (GA3)] applied at different stages of development of crown buds (plants with nonemerged crown buds, shoots recently emerged, or shoots emerged and elongated). The comparative growth potential of crown buds within the cluster was also investigated. At the stages of development examined, the application of GA3 (100 ppm) increased emergence of crown buds as shoots, leading to development of more flowering shoots. A similar response was observed with exposure to cold, but only on plants with nonemerged crown buds. Shoot emergence increased in response to increased duration of cold from 0 to 42 days (5 °C). Both chilling and GA3 could potentially be used to reduce the duration to, and spread of, harvest maturity if applied before shoot emergence. The hierarchical relationship of buds in crown bud clusters led to differential responses to application of GA3. Buds ontogenetically positioned at the proximal end of the bud cluster took a similar duration to reach shoot emergence or harvest maturity. For buds located at the distal end there was a positive correlation between ontogenetic bud position and the duration to reach shoot maturity. Shoot length and number of nodes at harvest maturity showed slight negative correlations with the position of the bud in the bud cluster. The results provide an explanation for possible sources of the variation in quality and quantity of floral shoots, and spread in time to harvest maturity within a single plant, and with development stage.

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Uttara C. Samarakoon, Keith A. Funnell, David J. Woolley, Barbara A. Ambrose and Ed R. Morgan

Shoot productivity and overwintering survival of gentians (Gentiana sp.) are determined by the initiation and subsequent development of crown bud clusters. Understanding of the anatomical features and origins of crown buds and bud clusters, and plant ontogeny, the morphological features of crown buds, and their associated development is required to achieve manipulation of bud initiation, emergence, and development. Anatomical features of the crown bud clusters were examined using both light and confocal microscopy using hybrids of Gentiana triflora × G. scabra. The initiation of bud clusters presented characteristics typical of adventitious buds in terms of their origin and presence of external vascular connection to the parental tissue. In contrast, crown buds forming subsequently within the cluster developed as axillary buds within that initial bud, collectively forming on a compact stem with minimal internode elongation. Stem elongation within the cluster after application of gibberellic acid enabled identification of a hierarchical arrangement of buds within the cluster with one bud at each node and arranged spirally at 90°. Arrangements of buds within the cluster were different from the opposite decussate phyllotaxis in floral shoots with two axillary buds at each node. Based on the current study, a crown bud cluster originated from a first bud initial, which was adventitious followed by development of subsequent crown buds within the cluster as axillary buds from this first bud initially with a single bud developing at each node.