The National Renewable Energy Laboratory developed the National Solar Radiation Database to provide accessible solar radiation data to the research community for various uses. Previously, we created a series of monthly daily light integral (DLI) maps to provide a tool for horticulturists to estimate the potential growth and flowering responses for various plants throughout the year. The original DLI maps were based on solar radiation data from 239 sites recorded from 1961 to 1990. The DLI maps presented in this article were created from an updated database that included data from 1998 to 2009. This database provides higher resolution data modeled from satellite images of cloud cover. The data are presented in pixels with each pixel representing 100 km ² of land across the lower 48 United States and Hawaii, whereas the Alaska data are 1600 km ² pixels. The database provided global horizontal irradiance data that were converted to DLI (mol·m ⁻² ·d ⁻¹ ) using the conversion factor of 0.007265 mol (400–700 nm)·Wh ⁻¹ (400–2700 nm), which assumes that 45% of the solar radiation is in the photosynthetically active radiation ( PAR , 400–700 nm) region and 4.48 μmol·J ⁻¹ is the conversion from radiometric to quantum units. The updated DLI maps provide more geographically precise data reflecting recent weather patterns. We present a comprehensive review of recent research exploring the growth and flowering responses of horticultural crops to DLI.

Mass balance principles are a cornerstone of efficient fertilizer use and can be utilized to optimize plant nutrition without discarding or leaching solution. Here, we describe the maintenance of closed hydroponic and soilless substrate systems based on mass balance. Water removed by transpiration is restored with solution that replaces the nutrients that were taken up with the water. The concentration of nutrients in this refill/irrigation solution is determined by multiplying the optimal concentration of each nutrient in plant tissue by the water-use efficiency (WUE; ratio of dry mass to water transpired). Optimal leaf nutrient concentrations are well established, but WUE in controlled environments varies widely and is less well characterized. Elevated CO₂ increases photosynthesis and demand for nutrients, but partially closes stomata and reduces transpiration; so high CO₂ dramatically increases WUE. The concentration of the refill/irrigation solution must be adjusted to account for a two-fold range of WUE, from 3 g L⁻¹ in ambient CO₂ in lower humidity, to 6 g L⁻¹ in elevated CO₂ in higher humidity. WUE and nutrient requirements vary during the vegetative and reproductive stages of growth, and adjustment of the solution over the lifecycle can be beneficial. Measurement of solution electrical conductivity (EC) is helpful, but if the solution is appropriate, low EC usually means healthy plants and active nutrient uptake. The ammonium to nitrate ratio is critical to pH management. We have applied these principles across multiple species and environments to achieve long-term, steady-state nutrient concentrations with no discharge or leaching of solution.

Botrytis blight on petunia flowers causes significant losses in the postharvest environment. Infection occurs during greenhouse production, and symptoms are expressed during transport. This phenomenon is termed petunia flower meltdown because of the rapid collapse of flower petal tissue as the plants are transported from the production greenhouse to the retail store. The objective of this study was to determine the effect of calcium (Ca) spray applications on botrytis blight severity in petunia flowers. For the first experiment, petunia ‘Pretty Grand Red’ plants were sprayed twice per week for 2 weeks with calcium chloride (CaCl ₂ ) at rates of 0, 400, 800, and 1200 mg·L ⁻¹ Ca. A fungicide (cyprodinil, 37.5%; fludioxonil, 25%) was used as an additional control treatment. Twenty-four hours after the last treatment, freshly opened flowers were harvested, placed into a humidity chamber with 99% relative humidity, and inoculated with a Botrytis cinerea spore suspension (1 × 10 ⁴ conidia/mL). Disease progression was recorded every 12 hours for 72 hours. The results showed a 96% reduction in botrytis blight severity as Ca concentration increased from 0 to 1200 mg·L ⁻¹ Ca. The Ca treatments provided better disease control than the fungicide treatment because of the fungicide resistance of the isolate used in the study. A second experiment was performed to determine whether the beneficial response to CaCl ₂ application was influenced by chlorine (Cl) or the electrical conductivity (EC) of the spray solutions, and no significant responses were observed. These studies prove Ca is the sole source of the reduction in botrytis blight severity following treatment with CaCl ₂ sprays, and demonstrate the benefit of using Ca as a tool for the management of botrytis blight on petunia flowers.

The amount of fertilizer applied during the commercial production of bedding plants has decreased in recent years because of increasing concerns about environmental impacts and the need to minimize production costs. However, reduced fertilization affects plant growth and flowering during production and in the postproduction environment. Plants grown with lower nutrient levels may perform satisfactorily during greenhouse production, but they may possess insufficient nutrients to sustain further growth in the postproduction environment, where fertilizer application is frequently lacking. This study examined conventional and alternative fertilizer delivery strategies that produce high-quality petunia ( Petunia × hybrida ) during greenhouse production and continue to support plant growth and flowering in the postproduction environment. The fertilizer treatments during production consisted of four constant liquid fertilization (CLF) treatments of 0, 50, 100, or 200 ppm nitrogen (N) and three controlled-release fertilization (CRF) treatments (0, 4, or 8 lb/yard ³ ). Three pulse fertilization (PF) treatments (0, 300, or 600 ppm N) were applied immediately before moving the plants to the postproduction environment. During production, petunia growth and development increased as CLF increased from 0 to 200 ppm N, but the addition of CRF resulted in the increase occurring at a declining rate. During postproduction, the interactive effects of CLF and CRF continued in a similar pattern as that seen in the production environment. The additional PF treatments resulted in further increases in plant growth. Across all CLF and CRF treatments, the leaf area increased from 466 to 540 cm ² as PF increased from 0 to 300 ppm N, and the leaf area increased further to 631 cm ² as PF increased from 300 to 600 ppm N. Based on our findings, two alternative strategies are possible. First, 0 to 50 ppm N CLF can be combined with 4 lb/yard ³ CRF. The second strategy maintains the standard commercial practice of applying 100 ppm N CLF treatment and then applying a 300- to 600-ppm N PF treatment. These results suggest that a relatively low CLF rate can be used to achieve the desired production characteristics while reducing the cost of plant growth regulation, and that additional plant nutrition can be provided with CRF and/or PF to enhance the postproduction performance.

Blue and supplemental far-red light were observed to affect in vitro shoot growth with Cannabis sativa (‘BaOx’ and ‘Cherry 1’) in RV750 vessels. A modified “hedging” and fed-batch system for multiple harvests using Oasis^® foam and 120 mL DKW medium was used. Fifteen nodal and/or apical tips were planted and placed into PAR light treatments providing various red to blue ratios (polychromatic white 9:1 RB and dichromatic 2–15: 1 RB, with and without 5% far-red light). Treatments had similar light intensities (190–240 µmol · m⁻² · s⁻¹ PPFD) for a 16 h photoperiod. Shoot tips were harvested in vitro on five successive two-week cycles, with 15 mL of DKW media supplemented to each vessel following harvest. Shoot numbers, length, and fresh and dry mass were recorded at each cycle harvest. Five randomly selected shoot tips per vessel were rooted ex vitro on greenhouse mist bench for 16 days. Over multiple cycles, 5% far-red increased shoot numbers and length in both genotypes tested, regardless of polychromatic or dichromatic source. Shoots harvested per vessel increased from 15 to 28 in three cycles (6 weeks), but increased from 15 to 18 without far-red treatment. Shoot length in far-red-treated plants increased from 19 to 25 mm during cycles 1–3. Plants without far-red treatment were approximately 15 mm during the first three cycles. By cycle 5, both far-red- and non-far-red-treated plants decreased to 10 mm. Dry mass was greatest in cycle 1 for both genotypes (‘Cherry 1’ was 6 mg and ‘BaOx’ was 7 mg) under the highest amount of blue light, but 2 mg under the lowest amount of blue light. Dry mass decreased by 50% in cycle 3, to 4 mg, where it remained for the duration of the experiment. Sixty eight percent of shoots rooted ex vitro on the mist bench, regardless of any prior in vitro treatment.

This study investigated the preharvest carbohydrate status and postharvest ethylene action of unrooted shoot-tip cuttings of lantana ‘Dallas Red’ harvested at three times during the day (0800, 1200, and 1600 hr ) in relation to subsequent leaf abscission, shoot apices blackening, and adventitious root formation. The cuttings harvested at various times during the day were stored in darkness at 20 ± 1 °C for 4 days in sealed polyethylene bags. The cuttings harvested at 0800 hr had lowest total nonstructural carbohydrate concentrations; however, the amount of ethylene production during postharvest storage was similar among harvest times and increased during the storage period. After 4 days of storage, 69% of the leaves of cuttings harvested at 0800 hr abscised, but only 22% and 8% of the leaves abscised in cuttings harvested at 1200 and 1600 hr , respectively. Application of 1-methylcyclopropene (1-MCP) increased ethylene production and suppressed leaf abscission regardless of the harvest time, but cuttings harvested at 0800 hr developed blackened shoot apices. Leaf abscission was negatively correlated with total nonstructural carbohydrate concentration in the leaves, but no relationship was found with ethylene production. These results indicate that a high endogenous carbohydrate status decreases the postharvest ethylene sensitivity in unrooted shoot-tip cuttings of lantana. Time of harvest influenced subsequent rooting response; however, 1-MCP application did not inhibit rooting. Among various storage treatments, the best rooting response was observed in cuttings harvested at 1600 hr and treated with 1-MCP. Therefore, significant improvement of postharvest storage quality in vegetative lantana cuttings could be achieved by harvesting cuttings late in the day and treating with 1-MCP.