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Tomohiro Jishi, Ryo Matsuda, and Kazuhiro Fujiwara

Cos lettuce was grown under different spectral photon flux density distribution (SPFD) change patterns with blue- and/or red light-emitting diode (LED) irradiation with a 24-hour cycle. Twelve treatments were designed with a combination of four relative SPFD (RSPFD) change patterns and three photosynthetic photon flux density (PPFD) levels. The RSPFD change patterns were as follows: BR/BR, simultaneous blue- and red-light irradiation (BR) for 24 h; R/BR, red-light monochromatic irradiation (R) for 12 h followed by 12 hours of BR; B/BR, blue-light monochromatic irradiation (B) for 12 hours followed by 12 hours of BR; and B/R, 12 hours of B followed by 12 hours of R. Each RSPFD change pattern was conducted at three daily average photosynthetic photon flux densities (PPFDave) of 50, 100, and 200 µmol·m−2·s−1. The RSPFD change patterns that included B (B/BR and B/R) resulted in elongated leaves. A low ratio of active phytochrome to total phytochrome under B was considered the reason for leaf elongation. Shoot dry weight was significantly greater under the RSPFD change patterns that included B when the PPFDave was 200 µmol·m−2·s−1. The leaf elongation caused by B would have increased the amount of light received and thereby promoted growth. However, excessive leaf elongation caused the plants to fall, and growth was not promoted under the RSPFD change patterns that included B when the PPFDave was 50 µmol·m−2·s−1. Thus, 12-hour B promoted growth under conditions in which leaf elongation leads to increases in the amount of light received.

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Nihal C. Rajapakse, Roy E. Young, Margaret J. McMahon, and Ryu Oi

The interest in using nonchemical alternatives for growth control of horticultural crops has recently increased due to public concerns for food safety and environmental pollution. Several research teams around the world are investigating alternative growth control measures, such as genetic manipulation, temperature, water and nutrient management, mechanical conditioning, and light quality manipulation. This review discusses the recent developments in light quality manipulation as a nonchemical alternative for greenhouse plant height control.

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Neil C. Yorio, Gregory D. Goins, Hollie R. Kagie, Raymond M. Wheeler, and John C. Sager

Radish (Raphanus sativus L. cv. Cherriette), lettuce (Lactuca sativa L. cv. Waldmann's Green), and spinach (Spinacea oleracea L. cv. Nordic IV) plants were grown under 660-nm red light-emitting diodes (LEDs) and were compared at equal photosynthetic photon flux (PPF) with either plants grown under cool-white fluorescent lamps (CWF) or red LEDs supplemented with 10% (30 μmol·m-2·s-1) blue light (400-500 nm) from blue fluorescent (BF) lamps. At 21 days after planting (DAP), leaf photosynthetic rates and stomatal conductance were greater for plants grown under CWF light than for those grown under red LEDs, with or without supplemental blue light. At harvest (21 DAP), total dry-weight accumulation was significantly lower for all species tested when grown under red LEDs alone than when grown under CWF light or red LEDs + 10% BF light. Moreover, total dry weight for radish and spinach was significantly lower under red LEDs + 10% BF than under CWF light, suggesting that addition of blue light to the red LEDs was still insufficient for achieving maximal growth for these crops.

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Jill A. Montgomery, Ray A. Bressan, and Cary A. Mitchell

Obtaining uniform mechano-dwarfing of Arabidopsis thaliana (L.) Heynh. seedlings within dense plantings is problematic. Alternative forms of mechano-stimulation were applied to seedlings in effort to obtain uniform growth reduction compared with undisturbed controls in both greenhouse and controlled growth environments. Arabidopsis grown under low photosynthetic photon flux (PPF) artificial light grew upright with limited leaf expansion, which enhanced mechano-responsiveness compared to that of rosette-growing plants under filtered sunlight or high PPF artificial light. Hypocotyls of seedlings grown at PPFs >60 μmol·m-2·s-1 elongated less and had 6% less sensitivity to mechanical stress than seedlings grown at PPFs <60 μmol·m-2·s-1. Fluorescent lamps alone (F) or fluorescent plus incandescent (F+I) lamps were compared for seedling responses to mechanical stress. Under F lighting, hypocotyl elongation was reduced 25% to 40% by twice-daily brush or plate treatments, and brushed seedlings exhibited more growth reduction than did plate treatments. Seedlings grown under F+I lamps exhibited similar stress-induced growth reduction compared to seedlings grown under F only, but stressed F+I seedlings lodged to a greater extent due to excessive hypocotyl elongation. Temperature-response studies using standardized F-only lighting indicated increased hypocotyl elongation but decreased leaf expansion, and decreased mechano-responsivity to brushing over the temperature range from 20 to 28 °C. Daylength studies indicated similar degrees of mechano-inhibition of hypocotyl elongation over the daylength range of 12, 16, 20, and 24 hours, whereas fresh weight of stressed seedling shoots declined compared to controls. A combination of environmental growth parameters that give repeatable, visual mechanical dwarfing of Arabidopsis include low-PPF fluorescent lighting from 55 to 60 μmol·m-2·s-1, ambient temperatures from 22 to 25 °C, and twice-daily brush treatments.

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Nihal C. Rajapakse, Robert K. Pollock, Margaret J. McMahon, John W. Kelly, and Roy E. Young

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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Frank M. Maas and Edwin J. Bakx

Growth and flowering of shoots of `Mercedes' rose was investigated as a function of the level and spectral quality of photosynthetic photon flux (PPF). Experiments were performed with single-shoot plants decapitated above the two most basal leaves with five leaflets. The development of the two lateral shoots emerging from the axillary buds of these leaves was studied for 4 to 6 weeks. To discriminate between the effects of irradiance and light quality, plants were grown in growth chambers in which PPF and its spectral composition could be controlled. At a photoperiod of 12 hours, the length, weight, and flowering of the shoots strongly increased with irradiance. The growth and number of flowering shoots were always higher for the uppermost than for the second shoot. At the highest PPF (270 μmol·m-2·s-1), flowering occurred in 89% and 33% of the uppermost and second shoots, respectively. At an irradiance level of 90 μmol·m-2·s-1, these percentages were 6% and 0%. Although length and dry weight of both types of shoots were significantly increased by reducing the amount of blue light at constant PPF, flower development was not affected. In a second experiment, plants grown in white light (12 h/day) received a short treatment with low-intensity red or far-red `light at the end of each photoperiod. An end-of-day treatment with red light resulted in significantly more flowering shoots than far-red. The red far-red reversibility of this flowering response indicates the involvement of the photoreceptor phytochrome.

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Carlos L. Ballaré and Ana L. Scopel

Our understanding of how plants use light signals to detect and respond to the proximity of neighbors has increased dramatically over the last few years. At the same time, the explosion of biotechnological techniques has opened a variety of opportunities to manipulate the photosensory systems of cultivated plants. Therefore, the idea is beginning to emerge that plastic responses of cultivated plants to population density could he deliberately changed by engineering genotypes with altered photomorphogenic behavior. This talk will provide a review of recent developments in the area of seedling photomorphogenesis, which will be used as a platform to evaluate the realism of current models of plant competition and the agricultural implications of interfering with plant photophysiology.

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Sandra L. Barbour, Margaret J. McMahon, John J. Frett, and Dennis R. Decoteau

Similarities exist between the effects of phytochrome and cytokinins on plant growth and development (e.g., chloroplast development, amaranthin synthesis. seed germination, photomorphogenesis). It is unclear, however, if and how these two systems interact.

As a beginning step to determine cytokinin-phytochrome interactions, we developed a strategy utilizing ipt -transgenic tobacco in phytochrome/light treatment investigations. The sour-cc of the ipt gene was Agrobacterium tumefaciens Ti plasmid 15955. This gene encodes for isopentenyl transferase which is an enzyme active in cytokinin biosynthesis.

Ipt -transgenic tobacco cultures (grown on MS medium supplemented with kanamycin but no plant growth regulators) were treated with end-of-day red or far-red light for 15 minutes. After 30 days of treatment, the plant tissue was harvested and either homogenized for SDS-PAGE or fixed for transmission electron microscopic analysis.

Results from immuno-gold labelling using polyclonal antibodies specific to iptase will he used to Indicate the influence of phytochrome on cytokinin activity. Also, structural changes at the ultra-cellular level will be determined.

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Sandra L. Barbour and Dennis R. Decoteau

Similarities exist between the effects of phytochrome and cytokinins on plant growth and development (e.g., chloroplast development. amaranthin synthesis, seed germination, photomorphogenesis). It is unclear. however, if and how these two systems interact.

To determine the effects of phytochrome activity on cytokinin synthesis and ultracellular plant development, we utilized tobacco transformed with the Agrobacterium tumefaciens isopentenyl transferase (ipt) gene. This gene encodes for isopentenyl transferase (iptase) which is an enzyme active in cytokinin biosysthesis.

Ipt-transgenic tobacco cultures were treated with end-of-day red or far-red light for 15 minutes. After 15-30 days of treatment, the plant tissue was harvested and ipt expression was verified by SDS-PAGE and western blot analysis. Polyclonal antibodies specific to iptase were used as a primary antibody. Colloidal gold conjugated to goat. anti-rabbit antiserum served as an electron dense, secondary antibody and a probe to light-influenced iptase synthesis and distribution within the cell.

A Hitachi 600AB transmission electron microscope was used to determine the influence of phytochrome/light treatments on the ultrastructure of ipr-transgenic cells.

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Gary W. Stutte

photosynthesis ( McCree, 1972 ) to allow the photosynthetic efficiency of a given wavelength to be evaluated as well. These well-defined parameters allow the spectra to be optimized for both photosynthesis and photomorphogenesis. Table 2. Spectral files of