Electronic dimming of high-intensity discharge lamps offers control of photosynthetic photon flux (PPF) but is often characterized as causing significant spectral changes. Growth chambers with 400-W metal halide (MH) and high-pressure sodium (HPS) lamps were equipped with a dimmer system using silicon-controlled rectifiers (SCR) as high-speed switches. Phase control operation turned the line power off for some period of the alternating current cycle. At full power, the electrical input to HPS and MH lamps was 480 W (root mean squared) and could be decreased to 267 W and 428 W, respectively, before the arc was extinguished. Concomitant with this decrease in input power, PPF decreased by 60% in HPS and 50% in MH. The HPS lamp has characteristic spectral peaks at 589 and 595 nm. As power to the HPS lamps was decreased, the 589-nm peak remained constant while the 595-nm peak decreased, equaling the 589-nm peak at 345-W input, and the 589-nm peak was almost absent at 270-W input. The MH lamp has a broader spectral output but also has a peak at 589 nm and another smaller peak at 545 nm. As input power to the MH lamps decreased, the peak at 589 diminished to equal the 545-nm peak. As input power approached 428 W, the 589-nm peak shifted to 570 nm. While the spectrum changed as input power was decreased in the MH and HPS lamps, the phytochrome equilibrium ratio (Pfr: Ptot) remains unchanged for both lamp types.
David L. Bubenheim, Raman Sargis, and David Wilson
Venkat K. Reddy and Nihal C. Rajapakse
The response of `Bright Golden Anne' chrysanthemum plants grown under CuSO4 spectral filters to exogenous GA3 application was evaluated to determine the relationship between gibberellins (GAs) and carbohydrate levels. The CuSO4 filters removed far red (FR) wavelengths of light and increased red: far red (R:FR), blue: far red (B:FR), blue: red (B:R) ratios, and phytochrome photoequilibrium (Ø) values of transmitted light compared to water (control) filter. Plant height, internode length, and leaf and stem dry weights were significantly reduced by light passing through CuSO4 filters in spring and summer seasons. Weekly applications of exogenous GA3 reversed the reduction in height and internode length induced by CuSO4 filters. Plants grown under CuSO4 filters responded more to exogenous GA3 application with respect to height and internode length, suggesting that the sensitivity to GA was not lowered. Light passing through CuSO4 filters reduced the carbohydrate levels, but the response varied with the season. Weekly GA3 application increased the carbohydrate levels, but did not totally reverse the reduction in carbohydrate levels under the CuSO4 filters. Although GA3 application increased the carbohydrate levels partially in CuSO4 filter-grown plants, the inhibition of GAs may not be solely responsible for reduction of carbohydrate levels under CuSO4 filters, showing that exogenous GAs and carbohydrate levels are not well correlated under CuSO4 spectral filters.
David L. Bubenheim, Raman Sargis, and David Wilson
Electronic dimming of high intensity discharge lamps offers control of photosynthetic photon flux (PPF) but is often characterized as causing significant spectral changes. Growth chambers with 400 W metal halide (MH) and high pressure sodium (HPS) lamps were equipped with a dimmer system using silicon controlled rectifiers (SCR) as high speed switches. Phase control operation turned the line power off for some period of the AC cycle. At full power the electrical input to HPS and MH lamps was 480 W (RMS) and could be decreased to 267 W and 428 W, respectively, before the arc was extinguished. Concomitant with this decrease in input power, PPF decreased by 60% in HPS and 50% in MH. The HPS lamp has characteristic spectral peaks at 589 and 595 nm. As power to the HPS lamps was decreased the 589 nm peak remained constant while the 595 nm peak decreased, equalling the 589 nm peak at 345 W input, and was almost absent at 270 W input. The MH lamp has a broader spectral output but also has a peak at 589 nm and another, smaller peak, at 545 nm. As input power to the MH lamps decreased the 589 nm peak diminished to equal the 545 nm peak. As input power approached 428 W the 589 nm peak shifted to 570 nm. While a spectral change was observed as input power was decreased in both MH and HPS lamps, the phytochrome equilibrium ratio (Pfr/Ptot) remain unchanged for both lamp types.
Heather. H. Friend, Arne Sæbø, and Dennis R. Decoteau
Previous research has demonstrated that watermelon plants are sensitive to changes in light quality, as suggested by an individual treatment of plants with 15-min of end-of-day (EOD) red (R)and far-red (FR) light. FR-induced growth responses (i.e., petiole elongation, internode elongation, reduced petiole angles) were reversed by immediately following the FR light treatment with R light implicating phytochrome as the light quality perception mechanism. The objective of the present experiment was to determine the influence of individual and multiple FR light treatments (each treatment of 15-min duration) during the light and dark phase of the photoperiod on photomorphogenic growth responses of young watermelon plants. Light regulated growth responses of watermelon were influenced by the timing and the number of light quality exposures during the light or dark phase of the photoperiod. Individual FR treatments during the light phase except for the EOD exposure did not affect plant growth responses. In contrast, individual FR treatments at selected intervals during the dark period affected plant development. The most effective individual FR treatment to induce growth responses was at the beginning of the dark period, with decreasing responses as the FR treatment was delayed into the dark period. Multiple exposures of FR during the dark slightly increased growth responses as compared to a single EOD FR treatment.
Timothy M. Spann, Jeffrey G. Williamson, and Rebecca L. Darnell
Experiments were conducted with V. darrowi and two cultivars of southern highbush blueberry, `Sharpblue' and `Misty,' to test whether V. darrowi and cultivars derived from it are photoperiodic with respect to flower bud initiation. Plants of each cultivar were grown under three different photoperiod treatments [long days (LD) = 16-hour photoperiod; short days (SD) = 8-hour photoperiod; and short days + night interrupt (SD-NI) = 8-hour photoperiod with 1-hour night interrupt] at constant 21 °C for 8 weeks. Vegetative growth was greatest in the LD plants of both cultivars. Flower bud initiation occurred only in the SD treatments, and the lack of flower bud initiation in the SD-NI treatment indicates that flower bud initiation is a phytochrome mediated response in Vaccinium. Previously initiated flower buds on the V. darrowi plants developed and bloomed during the LD treatment, but bloom did not occur in the SD and SD-NI treatment plants until after those plants were moved to LD. These data indicate that flower bud initiation in both V. darrowi and southern highbush blueberry is photoperiodically sensitive, and is promoted by short days, while flower bud development is enhanced under long days.
Timothy M. Spann, Jeffrey G. Williamson, and Rebecca L. Darnell
Experiments were conducted with `Misty' southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) to test the effects of high temperature on flower bud initiation and carbohydrate accumulation and partitioning. Plants were grown under inductive short days (SDs = 8 hour photoperiod) or noninductive SDs with night interrupt (SD-NI = 8 hour photoperiod + 1 hour night interrupt), at either 21 or 28 °C for either 4 or 8 weeks. Flower bud initiation occurred only in the inductive SD treatments and was significantly reduced at 28 °C compared with 21 °C. The number of flower buds initiated was not significantly different between 4- and 8-week durations within the inductive SD, 21 °C treatment. However, floral differentiation appeared to be incomplete in the 4-week duration buds and bloom was delayed and reduced. Although plant carbohydrate status was not associated with differences in flower bud initiation between SD and SD-NI treatments, within SD plants, decreased flower bud initiation at high temperature was correlated with decreased whole-plant carbohydrate concentration. These data indicate that flower bud initiation in southern highbush blueberry is a SD/long night phytochrome-mediated response, and plant carbohydrate status plays little, if any, role in regulating initiation under these experimental conditions.
Teresa A. Cerny, Nihal C. Rajapakse, and Ryu Oi
A research collaboration between Clemson Univ. and Mitsui Chemicals, Japan, has been established to develop and test photoselective greenhouse covers that can filter out far-red (FR) light and control plant height with minimal use of chemicals. The effects of polymethyl methacylate (PMMA) filters containing FR-intercepting dyes were evaluated on watermelon, pepper, chrysanthemum, and tomato to select an optimum dye concentration. As the dye concentration increased, FR interception increased, photosynthetic photon flux (PPF) decreased, and phytochrome photoequilibrium increased from 0.72 to 0.82. Light transmitted through photoselective filters reduced plant height effectively in all species tested. However, watermelon was the most responsive (50% height reduction) and chrysanthemum was the least responsive (20% height reduction) to filtered light. Tomato and peppers had an intermediate response. In watermelons, total shoot dry weight was reduced over 25% compared to the control plants, with a progressive decrease in shoot weight as the dye concentration increased. The specific stem dry weight was gradually reduced as the dye concentration increased. Specific leaf dry weight was slightly reduced under filters, suggesting that smaller plants as opposed to a reduction in dry matter production primarily caused total dry weight reduction. Light transmitted through filters reduced percentage dry matter accumulation into stems from 27% to 18% and increased dry matter accumulation into leaves from 73% to 82%. Photoselective filters are effective in controlling height similarly to chemical growth regulators. Considering the PAR reduction by increase in dye concentration, a dye concentration that gives a light reduction of 25% or 35% may be optimum for commercial development of photoselective films.
Erik S. Runkle and Royal D. Heins
For many plants, light quality has a pronounced effect on plant morphology; light with a low red (R, 600 to 700 nm) to far-red (FR, 700 to 800 nm) ratio promotes stem elongation and a high R: FR, or blue light (B, 400 to 500 nm), suppresses it. In addition, FR light is required for rapid flowering in some species, particularly for long-day plants. Our objective was to quantify how flexible spectral filters, which selectively reduce FR, B, or R, influence plant height and flowering of the quantitative long-day plants Pisum sativum L. `Utrillo' and Viola ×wittrockiana Gams. `Crystal Bowl Yellow'. Plants were grown at 20 °C with reduced FR, B, or R environments or with a neutral density control (C) filter. Calculated phytochrome photoequilebria were 0.78, 0.73, 0.71, or 0.46 for the altered FR, B, C, or R environments, respectively. All filter treatments transmitted a similar photosynthetic photon flux. Sixteen-hour photoperiods were created with natural daylight supplemented with high-pressure sodium lamps positioned above filters. Viola grown under the FR filter never reached 100% flowering within 8 weeks, and visible bud appearance was delayed by at least 17 days compared to all other filters. The R and B filters enhanced peduncle length by at least 25% compared to the C or FR filters. In Pisum, average internode length was 2.2, 2.9, 3.4, and 3.7 cm under the FR, C, B, and R filters, respectively, all statistically different. Fresh and dry shoot weights were similar under the C and FR filters but were at least 35% greater under the B filter and 35% lower under the R filter.
Gokhan Hacisalihoglu and Anwar A. Khan
The effects of chemical or physical factors during pregermination imbibition phase, or on dry seeds, on embryo growth potential (EGP) was studied in lettuce (Grand Rapids and Mesa 659) and tomato (H-9889) seeds in relation to dormancy, invigoration, and vigor loss. Embryos were excised from treated seeds (washed if imbibed in chemical solutions) and their growth rate (a measure of EGP) followed at 25°C at high magnification (X55). Treated seeds were also germinated at 25°C. In lettuce seeds, dormancy inducing treatments, i.e., a 2-day dark soak at 25°C with 50–100 μM tetcyclacis (TCY) or a 2-day dark soak in water at 35°C, reduced the subsequent embryo growth and germination rate at 25°C. The reduction was prevented by 1 mM GA4+7 or irradiation applied during dormancy induction. A -d osmoconditioning (OC) at 15C with -1.2 MPa PEG-8000 solution in light or in dark with added GA4+7 enhanced the EGP; addition of TCY reduced the EGP and the TCY inhibition reversed by GA4+7. A progressive reduction in EGP occurred with increase in vigor loss. In tomato seeds, a soak with 100 μM TCY in light or dark for 2 days at 30°C induced a dormancy, but had little effect on EGP. Application of GA4+7 plus TCY prevented dormancy induction without affecting EGP. A 4-day matriconditioning (MC) at 25°C in light or dark with moist Micro-Cel E enhanced the EGP; TCY and/or GA added during MC, had little effect on EGP. EGP progressively decreased as the aging period increased. Thus, in lettuce, the EGP is coupled with the reversible –GA/+GA or phytochrome-controlled dormancy induction/release process, enabling germination, its inhibition, or its enhancement. In tomato, the EGP is not subject to light or GA control. Reduction in EGP, accompanying vigor loss in both seeds, is independent of light or GA action.
Erik S. Runkle, Royal D. Heins, Arthur C. Cameron, and William H. Carlson
Intermediate-day plants (IDP) flower most rapidly and completely under intermediate photoperiods (e.g., 12 to 14 h of light), but few species have been identified and their flowering responses are not well understood. A variety of experiments was conducted to determine how light controls flowering and stem extension of Echinacea purpurea `Bravado' and `Magnus'. Both cultivars flowered most completely (79%) and rapidly and at the youngest physiological age under intermediate photoperiods of 13 to 15 h. Few (14%) plants flowered under 10- or 24-h photoperiods, indicating E. purpurea is a qualitative IDP. Plants were also induced to flower when 15-h dark periods were interrupted with as few as 7.5 min of low-intensity lighting (night interruption, NI). Flowering was progressively earlier as the NI increased to 1 h, but was delayed when the NI was extended to 4 h. Stem length increased by 230% as the photoperiod or NI duration increased, until plants received a saturating duration (at 14 h or 1 h, respectively). At macroscopic visible bud, transferring plants from long days to short days reduced stem extension by up to 30%. Flowering was inhibited when the entire photoperiod was deficient in blue or red light and was promoted in a far-red deficient environment, suggesting that phytochrome and cryptochrome control flowering of E. purpurea. Because of our results, we propose the flowering behavior of IDP such as E. purpurea is composed of two mechanisms: a dark-dependent response in which flowering is promoted by a short night, and a light-dependent response in which flowering is inhibited by a long day.