Environmental control computers allow regulation of greenhouse environments based on some model driven factor or factors other than fixed heating and cooling setpoints. A quantitative understanding of how environmental factors influence rate of plant development, flower initiation, and plant morphology is necessary to develop models for environmental control. The major limitation to the use of models for greenhouse climate and crop control is the lack of quantitative models. Examples of model development for environmental control will be discussed.
Royal D. Heins and Paul Fisher
Height control is a major challenge in the production of high quality poinsettia crops. Graphical tracking is a technique where growers make height control decisions by comparing actual measured plant height with a desired height. A computer decision support tool, the Poinsettia Care System, is being developed to combine graphical display of plant height with an expert system to provide height control advice. A simulation model is used to predict future growth of the crop based on greenhouse temperature, growth retardant applications, plant spacing, plant maturity, and light quality. Growth retardant and temperature recommendations are made based on a crop's deviation from the target height, expected future growth rate, and crop maturity. The program was beta tested by 8 Michigan growers over the 1991 poinsettia season. The test growers reacted positively to the program in a follow-up survey. Perceived benefits included improved height control, consistent crop recording, and a `second opinion' when making height control decisions. Improvements were suggested to combine the advice of different crops within the same greenhouse zone, to improve the predictive growth model, and to streamline data entry and output.
Royal D. Heins and James Faust
A decision-support tool for the Oriental lily `Stargazer' was developed from developmental data. Flower buds were measured twice a week on plants growing in greenhouses maintained at 15, 18, 21, 24, or 27°C. For each temperature, days to flower (DTF) was modeled as a linear function of the natural logarithm of bud length, DTF = b0 + b1 * In (bud length). Both parameters (b0 and b1) of the linear function were a quadratic function of average daily temperature (ADT). Both parameters of the linear function were then modeled so DTF = (397.6 - 24.5 * ADT + 0.469 * ADT2) + (-83.5 + 5.13 * ADT - 0.098 * ADT2) * In (budlength in mm). A decision-support tool, shown below, was developed from the model to assist with crop timing.
Royal D. Heins and James Faust
Photoperiod studies in a greenhouse usually require that the natural photoperiod be modified to increase or decrease the daylength. Modification involves using lights to extend the daylength or using some opaque material (e.g., black sateen cloth or black plastic) to shorten the photoperiod by excluding light. Air temperatures under the material can deviate from those of the surrounding air. It is common knowledge that when plants are covered by the cloth prior to sunset, solar radiation will increase the temperature under the it. It is not as widely known that temperature under the cloth will be lower than surrounding air temperature during the night. Radiant cooling of the material occurs when the greenhouse glazing material is cooler than the air temperature, resulting in cooling of the air and plants contained under the material. We have observed radiant cooling exceeding 150 W·m-2 when glazing is cold (-7°C), resulting in a temperature reduction under the material of up to 4°C. The difference in temperature between short-day and normal- or long-day treatments can lead to incorrect conclusions about the effect of photoperiod on plant development rate. Data will be presented with a sample control system to correct the problem.
Yaping Si and Royal D. Heins
Sweet pepper (Capsicum annuum `Resistant Giant #4') seedlings were grown in 128-cell plug trays under 16 day/night temperature (DT/NT) regimes from 14 to 26C. In this temperature range, plant stem height, leaf unfolding rate, plant volume, internode length, stem diameter, leaf area, and shoot dry weight were primarily functions of average daily temperature (ADT). Internode length increased as ADT or the difference between day and night temperature (DIF) increased. The root-to-shoot ratio decreased linearly as DT increased and was not significantly affected by NT. Leaves were darker green under positive DIF than negative DIF temperature regimes. Increasing NT from 14 to 26C reduced the node at which the first flower appeared by an average of 1.2 nodes. Percent abortion of the first flower increased as DT increased. Plant quality, as defined by seedling index [(dry weight × stem diameter)/internode length], increased as DIF became more negative.
Yaping Si and Royal D. Heins
Sweet pepper (Capsicum annuum `Resistant Giant no. 4') seedlings were grown for 6 weeks in 128-cell plug trays under 16 day/night temperature (DT/NT) regimes from 14 to 26 °C. Seedling stem length, internode length, stem diameter, leaf area, internode and leaf count, plant volume, shoot dry weight (DW), seedling index, and leaf unfolding rate (LUR) were primarily functions of average daily temperature (ADT); i.e., DT and NT had similar effects on each growth or development parameter. Compared to ADT, the difference (DIF, where DIF = DT - NT) between DT and NT had a smaller but still statistically significant effect on stem and internode length, leaf area, plant volume, stem diameter, and seedling index. DIF had no effect on internode and leaf count, shoot DW, and LUR. The root: shoot ratio and leaf reflectance were affected by DT and DIF. Positive DIF (DT higher than NT) caused darker-green leaf color than negative DIF. The node at which the first flower initiated was related to NT. The number of nodes to the first flower on pepper plugs grown at 26 C NT was 1.2 fewer than those of plants grown at 14 °C NT.
Bin Liu and Royal D. Heins
Photothermal ratio (PTR) is defined as the ratio of radiant energy (light) to thermal energy (temperature). The objective of this study was to quantify the effect of PTR during the vegetative (PTRv) and reproductive phase (PTRr) on finished plant quality of `Freedom' poinsettia. In Expt. I, plants were grown under 27 combinations of three temperatures, three daily light integrals (DLI), and three plant spacings from pinch to the onset of short-day flower induction and then moved to a common PTR until anthesis. In Expt. II, plants were grown under a common PTR during the vegetative stage and then assigned to nine combinations of one temperature, three DLIs, and three plant spacings after the onset of short-day flower induction. Both PTRr and PTRv affected final plant dry weight. All components of dry weight (total, stem, green leaf, and bract) responded in a linear way to PTRr and in a quadratic way to PTRv. Stem strength was more dependent on PTRv than PTRr. When PTRv increased from 0.02 to 0.06 mol/degree-day per plant, stem diameter increased about 24% while stem strength increased 75%. The size of bracts and cyathia was linearly correlated to PTRr, but not affected by PTRv. When PTRr increased from 0.02 to 0.06 mol/degree-day per plant, bract area, inflorescence diameter, and cyathia diameter increased 45%, 23%, and 44%, respectively.
Hiroshi Shimizu and Royal D. Heins
The effects of photoperiod and the difference between day temperature (DT) and night temperature (NT) (DIF) on stem elongation in Verbena bonariensis L. (tall verbena) were investigated. Plants were exposed to nine treatment combinations of -10, 0, or 10 °C DIF and 8-, 12-, or 16-hour photoperiods. Stem elongation was measured and analyzed by a noncontact computer-vision-based system. Total daily elongation increased as DIF increased; it also increased as photoperiod increased under positive DIF (DT > NT) and zero DIF (DT = NT), but not under negative DIF. Under positive DIF, daily elongation was 90% greater under the 16-hour photoperiod than under the 8-hour photoperiod. DIF affected elongation rate during the daily light span but not during the daily dark span. Total light-span elongation increased as DIF or photoperiod increased. Total dark-span elongation was not influenced by DIF or photoperiod. Elongation rates per hour in the light and dark were not significantly affected by photoperiod but increased in the light as DIF increased. Therefore, for a particular DIF, total elongation during 16-hour photoperiods (long days) was greater than that under 8-hour photoperiods (short days) because there were more hours of light under long days.
Bin Liu and Royal D. Heins
Plant growth and development are driven by two forms of energy: radiant and thermal. This study was undertaken to determine the effect of the ratio of radiant energy to thermal energy on plant quality of Euphorbia pulcherrima `Freedom'. Plants were grown under 27 combinations of temperature (thermal energy), light (radiant energy), and spacing, i.e., factorial combinations of three levels of constant temperature (19, 23, or 27°C:), three levels of daily light integral (5, 10, or 20 mol·m–2·d–1), and three levels of plant spacing (15 × 15, 22 × 22, or 30 × 30 cm), from pinch to the onset of short-day flower induction. Plants were treated for 450 degree-days (base temperature = 5°C) in Expt. 1 or 5 weeks in Expt. 2. The results showed that increasing radiant energy or decreasing average daily temperature during accumulation of 450 degree-days increased plant dry weight. When radiant and thermal energy were calculated into the ratio, plant dry weight increased linearly as the ratio increased Plants exposed to low light: levels and high temperatures, i.e., those at a low ratio, developed thin, weak stems. Higher radiant-to-thermal energy ratios produced thicker stems.
Bin Liu and Royal D. Heins
Shoot elongation of `Stargazer' lily is rapid during the first 15 to 20 days after planting (1 to 2 cm·day–1 is common). Lower stem leaves are small, separated by long internodes. We determined if dipping `Stargazer' bulbs in uniconazole (5-, 10-, 20-, or 40-ppm solutions for 1 min) before planting would slow initial stem elongation, decrease final height, and improve appearance. Emergence, visible bud, anthesis dates, and flower bud count were recorded. Plant height was measured three times per week until anthesis. Uniconazole did not affect time to emergence, visible bud, anthesis, or flower bud count. Compared to the final height of 48 cm (untreated plants), height was reduced 7, 17, 22, and 30 cm (5%, 35%, 46%, and 62%) at anthesis for plants in the 5-, 10-, 20-, and 40-ppm treatments, respectively. The uniconazole bulb dips did not affect stem elongation rate for the first 10 days after treatment or from 45 days after treatment through anthesis (day 65). Relative to untreated plants, stem elongation rate of treated plants decreased linearly from 10 to about 35 days after treatment, with a maximum reduction of 55%, 75%, 85%, and 100% for plants in the 5-, 10-, 20-, and 40-ppm treatments, respectively.