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 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.
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.
Hiroshi Shimizu and Royal D. Heins
A computer vision system for noncontact growth analysis was developed. Front and side images of a plant were captured simultaneously using a mirror system and CCD camera and were magnetically stored on a magneto-optical disk. Images acquired at night were obtained by irradiating plants with incandescent light filtered to wavelengths of 850 nm and greater. Images were automatically captured and saved every 12 minutes. After images were collected, outlines of plant shape were extracted from stored images, a three-dimensional center line of the plant was extracted from the outline, and the elongation rate was computed. The outline extraction algorithm was modified to improve spatial resolution of images, and the thinning algorithm created a representative line of the plant by calculating a center line of the stem so the three-dimensional length could be calculated. Results of growth analysis on Verbena bonariensis L. plants grown under three photoperiods (8, 12, and 16 hours) and three day/night air-temperature combinations (15/25, 20/20, and 25/15) will be presented.
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.
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.
Bin Liu and Royal D. Heins
The objectives of this study were to quantify the effects of the radiant-to-thermal energy ratio (RRT) on poinsettia plant growth and development during the vegetative stage and develop a simple, mechanistic model for poinsettia quality control. Based on greenhouse experiments conducted with 27 treatment combinations; 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/m2 per day), and three plant spacings (15 × 15, 22 × 22, or 30 × 30 cm), from pinch to the onset of short-day flower induction, the relationship between plant growth/development and light/temperature has been established. A model for poinsettia quality control was constructed using the computer software program STELLA II. The t-test shows that there were no significant differences between model predictions and actual observations for all considered plant characteristics; i.e., total, leaf and stem dry weight, leaf unfolding number, leaf area index, and leaf area. The simulation results confirm that RRT is an important parameter to describe potential plant quality in floral crop production.
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.
Bin Liu and Royal D. Heins
Light (radiant energy) and temperature (thermal energy) affect quality of greenhouse crops. Radiant energy drives photosynthesis and, consequently, plant biomass accumulation. Thermal energy is the primary environmental factor driving developmental rate. The concept of a photothermal ratio (PTR), the ratio of radiant energy [moles of photosynthetic (400 to 700 nm) photons/m2] to thermal energy (degree-day), was proposed to describe the balance between plant growth and plant development in greenhouse crops. The objective of this study was to quantify the effect of PTR during vegetative (PTRv) or reproductive (PTRr) phases on finished plant quality of `Freedom' poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch). In Expt. 1, plants were grown under 27 combinations of three constant temperatures (19, 23, or 27 °C), three daily light integrals (DLIs) as measured by the number of photosynthetic (400 to 700 nm) photons (5, 10, or 20 mol·m-2·d-1), and three plant spacings (15 × 15, 22 × 22, or 30 × 30 cm) from pinch to the start of short-day flower induction, and then moved to a common PTR until anthesis. In Expt. 2, plants were grown under a common PTR during the vegetative stage and then moved to combinations of three DLIs (5, 10, or 15 mol·m-2·d-1) and three plant spacings (25 × 25, 30 × 30, or 35 × 35 cm) at a constant 20 °C from the start of short days until anthesis. Both PTRr and PTRv affected final plant dry weight (DW). All components of DW (total, stem, leaf, and bract) increased linearly as PTRr increased, and responded quadratically to PTRv, reaching a maximum when PTRv was 0.04 mol/degree-day per plant. Stem strength depended more on PTRv than PTRr. When PTRv increased from 0.02 to 0.06 mol/degree-day per plant, stem diameter increased ≈24%, while stem strength increased 75%. The size of bracts and cyathia increased linearly as PTRr increased, but was unaffected 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.