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  • Author or Editor: Nihal C. Rajapakse x
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Low-temperature storage potential of rooted cuttings of garden chrysanthemum [Dendranthema ×grandiflorum (Ramat.) Kitamura] cultivars and its relationship with carbohydrate reserves were evaluated. Storage of chrysanthemum cuttings at -1 and -3 °C resulted in freezing damage. Visual quality of rooted cuttings stored at 0 or 3 °C varied among cultivars. Quality of `Emily' and `Naomi' cuttings was reduced within a week by dark storage at 0 or 3 °C due to leaf necrosis, while `Anna' and `Debonair' cuttings could be held for 4 to 6 weeks without significant quality loss. In `Anna' and `Debonair', low-temperature storage reduced the number of days from planting to anthesis regardless of storage duration. However, flowers of plants grown from stored cuttings were smaller than those of nonstored cuttings. At the beginning of storage, `Emily' and `Naomi' had lower sucrose, glucose, and fructose (soluble sugars) content compared to `Anna' and `Debonair'. Regardless of temperature, leaf soluble sugar was significantly reduced by dark storage for 4 weeks. In stems, sucrose and glucose were reduced while fructose generally increased during low-temperature storage probably due to the breakdown of fructans. Depletion of soluble sugars and a fructan-containing substance during low-temperature dark storage was greater in `Emily' and `Naomi' than in `Anna' and `Debonair'. Low irradiance [about 10 μmol·m-2·s-1 photosynthetically active radiation (PAR) from cool-white fluorescent lamps] in storage greatly improved overall quality and delayed the development of leaf necrosis in `Naomi'. Cuttings stored under light were darker green and had a higher chlorophyll content. Leaf and stem dry weights increased in plants stored under medium and high (25 to 35 μmol·m-2·s-1 PAR) irradiance while no change in dry weight was observed under dark or low light. Results suggest that the low-temperature storage potential of chrysanthemum cultivars varies considerably, and provision of light is beneficial in delaying the development of leaf necrosis and maintaining quality of cultivars with short storage life at low temperatures.

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Abstract

Potted foliage and floricultural species were evaluated for transpiration rates under low-light conditions. Environmental conditions during the experiment were 25° ± 2°C, 75% ± 10% RH, and 20 μmol·s−1 ·m−2 (400−700 nm) light intensity. Leaf cuticular and stomatal morphology were characterized with scanning electron micrographs. Coleus had the highest transpiration rate, Chrysanthemum was intermediate, and Ficus, Peperomia, and Epipremnum had the lowest transpiration rates. Abscisic acid (ABA) treatment reduced the daytime transpiration, which eliminated the diurnal fluctuation of transpiration in all species but had no effect on night transpiration, except in Coleus. Assuming complete stomatal closure at night with ABA treatment, cuticular transpiration accounts for 43% to 80% of the total transpiration rate under low-light conditions. This result points to the importance of leaf cuticular and stomatal characteristics in controlling water use of plants under low light or dark-storage. Species differed in cuticular characteristics, stomatal frequency and size, and leaf area. Stomatal frequency correlated well with transpiration rates, except in those species with unique stomatal morphologies, such as Ficus, with sunken stomates surrounded by a protruding ridge. Coleus and Chrysanthemum developed less epicuticular wax than the other species. Epicuticular and stomatal characteristics were correlated with transpiration rates of these species.

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The effects of carbon dioxide enrichment on growth, photosynthesis, and postharvest characteristics of `Meijikatar' potted roses were determined. Plants were grown in 350, 700, or 1050 μl CO2/liter until they reached 50% flower bud coloration and then were placed into dark storage for 5 days at 4 or 16C. Plants grown in 700 or 1050 μl CO2/liter reached the harvest stage earlier and were taller at harvest than plants produced in 350 μl CO2/liter, but there were no differences in the number of flowers and flower buds per plant among CO2 treatments. Plants grown in early spring were taller and had more flowers and flower buds than plants grown in late winter. Shoot and root growth of plants grown in 700 or 1050 μl CO2/liter were higher than in plants produced in 350 μl CO2/liter, with plants grown in early spring showing greater increases than plants grown in late winter. Immediately after storage, plants grown in 350 μl CO2/liter and stored at 4C had the fewest etiolated shoots, while plants grown in 1050 μl CO2/liter and stored at 16C had the most. Five days after removal from storage, chlorophyll concentration of upper and lower leaves had been reduced by ≈50% from the day of harvest. Carbon dioxide enrichment had no effect on postharvest leaf chlorosis, but plants grown in early spring and stored at 16C had the most leaf chlorosis while plants grown in late winter and stored at 4C had the least leaf chlorosis.

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Plants grown in far red (FR) light deficient environments are typically shorter because of short internodes, resembling plants treated with GA biosynthesis inhibitors. The role of GAs in the reduction of stem elongation of `Bright Golden Anne' chrysanthemum [Dendranthem ×grandiflora (Ramat.) Kitam. (syn. Chrysanthemum ×morifolium Ramat.)] grown in FR light deficient (-FR) environment was investigated by following the response of chrysanthemums grown in - FR environment to exogenous application of GA1, GA19, or GA20, and the metabolism of GA12 and GA19 in -FR or +FR environment. FR light deficient environment resulted in 25% to 30% shorter plants than in +FR environment. Final height of GA1- and GA20-treated plants followed a quadratic pattern while that of GA19 treated plants followed a linear pattern as the dosage increased from 0 to 50 μg/apex. The response to GA1 was the greatest followed by GA20 and GA19, regardless of the light environment. Application of GA1 (50 μg/apex) increased final height by 65% compared with no GA (0 μg/apex) application under either +FR or -FR light environment, suggesting the response to GA1, which is the active form, remained the same. Responses to GA19 and GA20 declined under -FR light. [14 C]GA12 and [14C]GA19 metabolized slowly in the -FR environment suggesting that the turnover of GAs may have caused in part the lower response to GA19. Although metabolism of GA1 under -FR environments was not investigated, observations with GA1 application experiments support that -FR environment may have enhanced inactivation of GA1. Chemical name used: gibberellic acid (GA).

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Storage systems for tissue-cultured plants offer versatility in managing labor to meet market availability. Storage systems that minimize growth and yet sustain photosynthetic and regrowth potential require temperature, light quality, and light intensity to be manipulated for plantlet quality during and after storage. Broccoli (Brassica oleracea L. Botrytis Group `Green Duke') plantlets were cultured photoautotrophically (without sugar) or photomixotrophically (with sugar) on cellulose plugs in liquid medium in vitro for 3 weeks at 23°C and 150 μmol·m–2·s–1 photosynthetic photon flux (PPF). To determine the conditions that yield a zero carbon balance, plantlets were subsequently stored for 3 days under different temperatures (1°C, 5°C, 10°C, 15°C), different light intensities (1.6 PPF, 4.1 PPF, 8.6 PPF), and different light spectra (white, blue, red). Plantlets stored under 5 PPF and 5°C maintained a zero carbon balance. Subsequently, plantlets were stored for 4, 8, or 12 weeks at 5°C under darkness or 5 PPF of white, red or blue light. Stem elongation was observed for plantlets stored under blue light. Plantlets stored under red light were characterized by increased chlorophyll, increased specific leaf mass (leaf dry mass per unit leaf area, SLM), increased starch in leaf tissue, and increased total soluble sugars in leaf and stem tissue. Plantlets grown with sucrose were characterized by increased dry mass, regardless of light treatment. After 8 weeks, plantlets grown with or without sucrose and stored in darkness did not survive acclimatization to greenhouse.

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Broccoli (Brassica oleracea L. Botrytis group `Green Duke') seeds were cultured in vitro photoautotrophically (without sugar in the medium) or photomixotrophically (with sugar in the medium) for 3 weeks at 23 °C and 150 μmol·m-2·s-1 photosynthetic photon flux (PPF). Vessels were then stored at 5 °C under 1.6, 4.1, or 8.6 μmol·m-2·s-1 of white (400-800 nm), red (600-700 nm), or blue (400-500 nm) light. Concentrations of CO2 inside the vessels were monitored until equilibrium was reached. Light compensation point was reached at 3.5 μmol·m-2·s-1 for photoautotrophic seedlings and at 6.5 μmol·m-2·s-1 for photomixotrophic seedlings. Therefore, in the long-term storage experiment, seedlings were stored for 4, 8, or 12 weeks at 5 °C in darkness or under 5 μmol·m-2·s-1 (average light compensation point) of white, red, or blue light. Illumination during storage was necessary to maintain dry mass, leaf area, and regrowth potentials of in vitro seedlings. All seedlings stored in darkness were of poor quality and died when transferred to the greenhouse. Red light during storage increased seedling dry mass and chlorophyll content and improved overall appearance, whereas blue light decreased chlorophyll content and increased stem elongation. The addition of 2% sucrose to media increased dry mass and leaf area and maintained overall seedling quality during illuminated storage. However, plantlets stored for more than 4 weeks did not survive poststorage greenhouse conditions, regardless of light treatment.

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Broccoli (Brassica oleracea L. Botrytis group `Green Duke') seeds were cultured photoautotrophically (without sugar) or photomixotrophically (with sugar) in vitro for 3 weeks at 23 °C and150 μmol·m-2·s-1 photosynthetic photon flux (PPF). In vitro seedlings were stored for 0, 4, 8, or 12 weeks at 5 °C in darkness or under 5 μmol·m-2·s-1 of white (400–800 nm), blue (400–500 nm), or red (600–700 nm) light. Photosynthetic ability and soluble sugar contents were determined after removal from storage. Photomixotrophic seedlings contained approximately five times more soluble sugars than did photoautotrophic seedlings. Dark storage reduced soluble sugars in both photoautotrophic and photomixotrophic plants, but photosynthetic ability was maintained for up to 8 weeks in the latter whereas it decreased in the former. Illumination in storage increased leaf soluble sgars in both photoautotrophic and photomixotrophic seedlings. Soluble sugars in stems decreased during storage regardless of illumination, but remained higher in illuminated seedlings. Red light was more effective in increasing or maintaining leaf and stem soluble sugars than was white or blue light. Regardless of media composition or illumination, storage for more tan 8 weeks resulted in dramatic losses in quality and recovery, as well as photosynthetic ability. Seedlings stored for 12 weeks comletely lost their photosynthetic ability regardless of media composition or illumination. The results suggest that carbohydrate, supplied in the media or through illumination, is essential for maintenance of photosynthetic ability during low-temperature storage for up to 4 or 8 weeks.

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Daily light integral (DLI) describes the rate at which photosynthetically active radiation is delivered over a 24-hour period and is a useful measurement for describing the greenhouse light environment. A study was conducted to quantify the growth and flowering responses of bedding plants to DLI. Eight bedding plant species [ageratum (Ageratum houstonianum L.), begonia (Begonia ×semperflorens-cultorum L.), impatiens (Impatiens wallerana L.), marigold (Tagetes erecta L.), petunia (Petunia ×hybrida Juss.), salvia (Salvia coccinea L.), vinca (Catharanthus roseus L.), and zinnia (Zinnia elegans L.)] were grown outdoors in direct solar radiation or under one of three shade cloths (50, 70 or 90% photosynthetic photon flux (PPF) reduction) that provided DLI treatments ranging from 5 to 43 mol·m–2·d–1. The total plant dry mass increased for all species, except begonia and impatiens, as DLI increased from 5 to 43 mol·m–2·d–1. Total plant dry mass of begonia and impatiens increased as DLI increased from 5 to 19 mol·m–2·d–1. Impatiens, begonia, salvia, ageratum, petunia, vinca, zinnia, and marigold achieved 50% of their maximum flower dry mass at 7, 8, 12, 14, 19, 20, 22, and 23 mol·m–2·d–1, respectively. The highest flower number for petunia, salvia, vinca, and zinnia occurred at 43 mol·m–2·d–1. Time to flower decreased for all species, except begonia and impatiens, as DLI increased to 19 or 43 mol·m–2·d–1. There was no consistent plant height response to DLI across species, although the shoot and flower dry mass per unit height increased for all species as DLI increased from 5 to 43 mol·m–2·d–1. Guidelines for managing DLI for bedding plant production in greenhouses are discussed.

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The purpose of this review is to promote a discussion about the potential implications of herb production in controlled environments, focusing on our recent works conducted with feverfew. Research suggests that the content of secondary metabolites in medicinal plants fluctuates with changing environmental conditions. Our studies with feverfew (Tanacetum parthenium [L.] Schultz-Bip., Asteraceae) lend support to this hypothesis. Feverfew plants exposed to different water and light conditions immediately before harvest exhibited changes in content of some secondary metabolites. The highest yield of parthenolide (PRT) was in plants that received reduced-water regimes. Phenolics concentration however, was higher in plants receiving daily watering. Light immediately before harvest enhanced accumulation of PRT, but reduced the phenolic content. Notably, PRT decreased at night whereas total phenolics decreased during the photoperiod and increased at night. PRT also increased with increased plant spacing. UV light supplementation increased PRT only in plants that had undergone water stress, whereas phenolics increased when UV was applied to continuosly watered plants. Clearly, production of medicinal plants under greenhouse conditions is a promising method for controlling levels of phytochemicals through manipulation of light and water as discussed here, and possibly other environmental factors such as temperature and daylength. However, better understanding of how the environment alter secondary metabolite levels is needed as it was revealed that manipulating the environment to favor increased accumulation of one group of phytochemicals could result in a decline of other key metabolites.

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Experiments were conducted to correlate the response of chrysanthemum [Dendrathema ×grandiflorum (Ramat.) Kitamura] plants to light environment based on various quantitative light quality parameters by growing plants under 6% or 40% CuSO4 and water spectral filters. Using a narrow band width (R = 655-665 and FR = 725-735 nm) or a broad band width (R = 600-700 and FR = 700-800 nm) for R: FR ratio calculation, 6% CuSO4 filter transmitted light with a higher R: FR ratio than 40% CuSO4 or water filters. Light transmitted through 40% CuSO4 and water filters had similar narrow band R: FR ratios (≈1.2), but the broad band R: FR ratio (2.0) of 40% CuSO4 filter was higher than that of water filters. The estimated phytochrome photoequilibrium (ϕ) value varied considerably with the photochemical properties of phytochrome used for estimations. Final height and internode length of plants grown in 6% or 40% CuSO4 chambers was ≈30% less than of plants in corresponding control chambers. Leaf and stem dry weights were reduced by light transmitted through CuSO4 filters. The results suggest that broad band R: FR ratio correlated more closely to above plant responses than the narrow band R: FR ratio. Blue (B): R and B: FR ratios (not absolute amount of blue wavelengths) correlated well with plant response, suggesting that involvement of blue light should not be ignored in expressing plant response to light transmitted through CuSO4 filters. At present, the presentation of complete spectral data would be the most useful in explaining plant response to light environment.

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