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B. Castillo, M.A.L. Smith, D.L. Madhavi, and U.L. Yadava

Interactions between irradiance levels (5–40 μmol·m-2·s-1) and iron chelate sources (FeEDTA and FeEDDHA) were observed for Carica papaya shoot tip cultures during both the establishment and proliferation stages of microculture. Reduced levels of irradiance (5 μmol·m-2·s-1) favored shoot tip establishment regardless of the source or level of iron. However, the highest percentage of successful explant establishment (100%), and significantly greater leaf length (1.16 cm; over double the size attained in any other treatment), resulted when a low concentration of FeEDTA alone was used at low irradiance. During the subsequent shoot proliferation stage, however, higher irradiance levels (30 and 40 μmol·m-2·s-1) were required, and FeEDTA failed to support culture growth when used as the sole iron source. The highest multiplication rates (3.6 shoots per explant) and leaf chlorophyll concentrations (0.22 mg/g fresh mass), and significantly improved shoot quality were achieved at 30 μmol·m-2·s-1 irradiance when both iron chelate formulations were combined (each at a 100 μM concentration) in the proliferation medium. Chemical names used: benzylamino purine (BA); ferric disodium ethylenediamine tetraacetate or FeNa2EDTA (FeEDTA); ferric monosodium ethylenediamine di(o-hydroxyphenylacetate), (FeNaEDDHA) or Sequestrene 138Fe (FeEDDHA); indoleacetic acid (IAA); 1-naphthaleneacetic acid (NAA).

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Roger Kjelgren

Shade acclimation response of Emerald Queen Norway maple street trees to variable urban irradiance levels was investigated. Specific leaf area, trunk growth, and crown density were measured from trees in 13 sites ranging from urban canyons in the business core to open exposures in residential areas of Seattle, Wash. Percentage of potential seasonal input of global shortwave radiation for each site was modeled based on the azimuth and elevation angles of the surrounding horizon topography. Building height in the business core reduced estimated irradiance to a range of 27% to 90% of that for an unobstructed horizon topography, while those outside the business core had 90% to 95% irradiance. As estimated potential irradiance decreased, growth of these maple street trees exhibited responses characteristic of shade acclimation in a dose-response pattern. Specific leaf area increased and trunk growth and crown density decreased to acclimated levels at -70% of potential irradiance. These acclimation responses did not degrade the function of the trees in their urban-canyon locations. Their foliage was healthy, and reduced crown density was not apparent since there were no full-sun-grown trees for comparison.

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Meriam G. Karlsson and Jan T. Hanscom

Seeds of cyclamen `Laser Scarlet' were germinated at 20C in darkness. Four weeks after germination, the seedlings were moved to a greenhouse at 20C and 16 hours daylength and 4 weeks later transplanted into 10 cm (520 cm3) pots. Plants were grown under 8 or 16 hours daylength in combination with 3.0, 7.5 or 12.0 mol·day-1m-2 for 9 weeks after transplant. The instantaneous irradiance was adjusted based on photoperiod to provide the desired total daily irradiance levels. After the 9 weeks photoperiod and irradiance treatment, plants were allowed to develop and flower at 16 hours daylength, 7.5 mol·day-1m-2 (130 μmol·s-1m-2) and 15C. There was a trend for an increased number of leaves for cyclamen grown at higher total daily irradiance levels at either 8 or 16 hours photoperiod. The largest number of leaves (14 ± 2.2 leaves) after the 9 weeks was observed for cyclamens grown at 16 hours photoperiod and 12 mol·day-1m-2 (210 μmol·s-1m-2). The plants grown at the longer day length and highest irradiance level also, accumulated most dry weight (92 ± 18.7 mg) during the 9 weeks of photoperiod and it-radiance treatment.

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Michael R. Mason and William B. Miller

Interactions of ethephon and irradiance reduction were investigated in terms of flower bud blasting in Easter lily (Lilium longiflorum Thunb. `Nellie White'). Silver thiosulfate (STS) was investigated as an inhibitor of ethylene-induced bud abortion. Fourteen days of 92% irradiance reduction significantly increased bud abortion when plants were exposed to 2.1 mm ethephon. Bud abortion was 39% and 60% for plants grown in ambient and reduced irradiance, respectively. Silver thiosulfate was applied to plants 2 or 3 weeks after the date of the first visible bud, followed by ethephon treatment 2 days later. Bud abortion was significantly reduced by 1 or 2 mm STS, without phytotoxicity. Pretreatment with 1 or 2 mm STS as early as 4 weeks before ethephon exposure significantly reduced ethephon-induced bud abortion. Silver thiosulfate application could inexpensively reduce flower bud abortion during latter stages of greenhouse forcing of Easter lilies.

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John E. Erwin

The interaction among temperature, photoperiod, and irradiance on survival of Chamaecereus silvestrii (yellow sport) flat-grafted onto Hylocereus trigonus Haw. rootstock was studied in an effort to understand the basis for elevated scion necrosis during winter. Plants were placed in glasshouses maintained at 12, 16, 20, or 24 °C under either daylight (moles per day), 66% daylight or daylight + 100 μmol·s−1·m−2 irradiance levels. Plants were grown with an 8-hour (short day) or 8-hour + 4-hour night interruption (long day) photoperiod. Cactus scion necrosis increased under short days and a growing temperature of 12 °C and was nearly eliminated by long-day conditions and a growing temperature of 16 °C. Irradiance did not affect scion necrosis. Plant quality rating was highest when plants were grown under long-day conditions at 16 °C.

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Joseph P. Albano and William B. Miller

Irradiation of FeDTPA-containing nutrient solutions by a fluorescent plus incandescent light source resulted in the loss of both Fe-chelate and soluble Fe, the formation of a precipitate that was composed mostly of Fe, and a rise in pH. The rate of Fe-chelate photodegradation in solution increased with irradiance intensity and with solution temperature under irradiation, but irradiance had the greater effect. Fe-chelates absorb in the blue and UV regions of the spectrum. Removal of these wavelengths with a spectral filter eliminated photodegradation. Chemical name used: ferric diethylenetriaminepentaacetic acid (FeDTPA).

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Amanda M. Miller, James M. Garner, and Allan M. Armitage

Five cultivars of the Angel Mist series of Angelonia angustifolia L. were evaluated in the Univ. of Georgia New Crop Program to determine the influence of temperature, irradiance, and photoperiod on crop growth and flowering. When the temperature was increased from 15 to 30 °C, days to visible bud and days to flower significantly decreased while height of flowers, vegetative height, and total height significantly increased. As irradiance increased, plant growth increased but little influence on flowering time was observed. Angelonia angustifolia appears to be a day-neutral plant with respect to flowering. The influence on growth regulators will also be discussed.

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William J. Carpenter, Eric R. Ostmark, and John A. Cornell

Begonia ×semperflorens-cultorum Hort. `Prelude Scarlet' seeds varied within irradiance treatments in the irradiance level and duration that they required to reach the light saturation value and germinate. At high photosynthetically active radiation (PAR), seeds required light for only part of the germination period to terminate photodormancy. Germination >90% was achieved after 4 and 1 day of 24 hours/day exposure to PAR at 15 and 150 μmol·m–2·s–1, respectively, but 82% germination occurred after 4 days of irradiance at 1.5 μmol·m–2·s–1 at 27C. Fewer days to 50% of final germination (T50) and between 10% and 90% germination (T90 – T10) were required when light saturation was achieved after 1 day at high PAR rather than after 4 days at a low PAR level. The total PAR that seeds received during 6, 12, or 24 hours of light daily determined the total percentage of the seeds that germinated. Seeds receiving 150 μmol·m–2·s–1 continuously for ≥24 hours achieved 90% germination, but 6 or 12 hours daily at this irradiance level required 4 days and 3 days, respectively. Trends in total germination percentages (G), T50, or T90 – T10 with increased PAR levels, hours of light daily, or days of light were found by fitted regression equations and Tukey's hsd procedure. Begonia seed germination was promoted by PAR levels of 1.5 to 150 μmol·m–2·s–1 for periods ≤4 days, with darkness thereafter until cotyledon emergence.

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M.P. Kaczperski and A.M. Armitage

The effects of storage conditions before transplanting were examined for Petunia × hybrida Vilm. `Supercascade Lilac', viola × wittrockiana Gams `Universal Beaconsfield', and Salvia splendens F. Sellow ex Roem. & Schult `Red Hot Sally'. Plug grown seedlings were stored for 0, 7, 14, or 21 days at 5 or 10C and with continuous irradiance levels from incandescent bulbs at 0, 2, or 12 μmol·m-2·s-1. A second group was stored at 18C with irradiance from fluorescent bulbs at 105 μmol·m-2·s-1 and a 16-hour photoperiod for the same durations. Temperature was more important than irradiance in maintaining a commercially acceptable plant during the storage period. Petunia and pansy could be stored successfully for 21 days at 5 or 10C with no appreciable loss of quality; salvia could be stored for a minimum of 14 days. Seedlings of all species elongated excessively when stored >7 days at 18C and 105 μmol·m-2·s-1 irradiance. After 14 days of storage, petunias stored at 18C flowered sooner than those stored at 5 or 10C but time in a production environment (days to flower - days in storage) was similar for petunias stored at 5 or 18C.

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Martin P.N. Gent

Solution electrical conductivity (EC) and the supply of nitrate in proportion to other elements (nitrate supply ratio) should effect tissue composition of lettuce (Lactuca sativa L.) grown in hydroponic solution. These parameters were varied in several series of successive plantings in greenhouses in the northeast United States. In 1996, when the treatments differed only in EC, 0.65 and 0.9 dS·m-1, but not in nitrate supply ratio, leaf tissue had more nitrate and total reduced-N and lettuce grew faster in the solution with higher EC. Over four series of plantings in 1997 and 1998, the nitrate supply ratio of a low-N treatment was only 60% of that for a high-N treatment, and EC was varied from 1.2 to 2.0 dS·m-1. In 1997 and 1998, tissue nitrate was lower in the low-N treatment only when EC was less than in the high-N treatment. However, under irradiance greater than 10 MJ m-2 per day, the lower EC also slowed growth. Stepwise regression over data from all experiments showed leaf nitrate was primarily a function of EC, and a term that described the interaction between irradiance and EC. Due to selective uptake by the plants, the ratio of elements in the recirculating solution differed from the ratio in which they were supplied. Under irradiance less than 10 MJ m-2 per day and solution EC greater than 1.5 dS·m-1, nitrate accumulated in solution to a concentration greater than expected from simple dilution of the concentrates. Tissue nitrate was also related to solution nitrate, increasing by 0.08-0.09 mg·g-1 dry weight per 1 mg·L-1 increase in solution nitrate. To prevent a rise in tissue and solution nitrate under low irradiance, both solution EC and nitrate supply ratio had to be reduced by about one-third, compared to the conditions required for rapid growth under high irradiance.