New electronic biosensors that directly monitor plant physiological and morphological processes are now being developed for use in research and commercial applications. Although methods for measuring sap flow by applying heat to stems have been used for more than 20 years, they have usually been intrusive, required empirical calibrations and conversions, and been too fragile for rugged commercial environments. A more-promising method for monitoring sap flow is balancing the thermal fluxes into and out of a stem segment using heat sources wrapped around a stem. Constant heat-balance sap-flow gauges have been used for the direct, accurate, non-invasive, and continuous measurement of sap flow rate in many herbaceous and woody plants, including forest and fruit trees, vines, landscaping shrubs, and numerous agronomic plants. The performance of sap-flow gauges has steadily improved as they have been used in a wider range of basic and applied research. Research is now being conducted to use these gauges as on-line sensors to schedule irrigations, monitor plant stress, and even control the greenhouse environment.
Mary Ann Rose and Mark A. Rose
A closed-loop photosynthesis system and a heat-balance sap-flow gauge independently confirmed oscillatory transpiration in a greenhouse-grown Rosa hybrids L. Repetitive sampling revealed 60-minute synchronized oscillations in CO2-exchange rate, stomatal conductance, and whole-plant sap-flow rate. To avoid confusing cyclical plant responses with random noise in measurement, we suggest that gas-exchange protocols begin with frequent, repetitive measurements to determine whether transpiration is stable or oscillating. Single measurements of individual plants would be justified only when transpiration is steady state.
Mary Ann Rose, John W. White and Mark A. Rose
`Celebrate 2' poinsettias (Euphorbia pulcherrima Willd.) received either a constant application rate of 200 mg N/liter or a variable rate that was linked to the N accumulation pattern of the crop. At final harvest, shoot N content, N concentration, dry weight, leaf area, and quality were similar for the treatments. However, N recovery efficiency of the variable treatment was greater (58% vs. 38%), and 41% less total N was applied compared to the constant-rate treatment. Growth analysis revealed that N accumulation rates for both treatments increased rapidly as side branches developed, reaching a maximum 50 to 60 days after potting, and decreased throughout bract development. The decrease in N accumulation rates after day 60 reflected a shift in N allocation from leaves to bracts, a tissue with a lower N concentration.
Mark A. Rose and John W. White
Temperature affects all major plant physiological processes. Traditional methods of controlling greenhouse temperatures use aerial sensors that do not monitor temperatures within each component of the soil-plant-atmosphere continuum.
Bench, pot, plant canopy, and aerial temperatures were monitored using thermocouples and thermistors processed by environmental computers during a wide range of greenhouse conditions. These include diurnal cycles of high and low solar radiation, night periods with and without artificial lighting, and various ventilation and heating conditions. Spatial temperature gradients of 10-22 °C were discovered during both day and night conditions. These spatial variations cause significant differences in average temperatures between and within benches over diurnal and even seasonal cycles.
Preliminary surveys of microclimatic variations that occur within the greenhouse experimental area are essential for choosing the proper experimental design. Continuous environmental monitoring during the experiment is necessary for interpreting experimental results.
E. Jay Holcomb, Loren D. Tukey and Mark A. Rose
Mark A. Rose, John W. White and Joel L. Cuello
Recently developed stem flow gauges that allow for direct, accurate, non-invasive, and continuous measurement of plant sap flow rates have not been used to monitor transpiration of floricultural plants grown in greenhouses.
A Dynamax SGA10 heat-balance sap-flow sensor was mounted on a potted rose plant's main stem containing a total leaf area of 0.52 m in order to monitor transpiration. The sensor was connected to a CR21X Micrologger for data calculation and temporary storage. The results showed average midday sap-flow rates range from 20-30 g·hr-1 to 50-70 g·hr-1 at low and high levels of PPF, respectively. Nighttime levels of 4-7 g·hr-1 persisted throughout early winter trials. Monitoring transpiration of the same rose stem using a lysimeter revealed a significant linear correlation (r2 = 0.999) between the lysimeter and the stem flow gauge values.
In the future, research will be conducted with the gauge to investigate relationships between microclimatic variables, photosynthesis, and transpiration.
Niels Ehler, Mark A. Rose and Jesper Mazanti Hansen
Currently, greenhouse environmental computers are programmed to monitor and control the macroclimate instead of directly controlling plant growth and development, which are features of more interest to growers. Our objective was to develop a generic system to represent and control the dynamic plant processes that regulate plant growth in the greenhouse. Before plant growth can be directly controlled, the dynamic interactions between the microclimate around plants and plant physiological processes must be further understood. Future computerized control systems must be able to display an intuitive, interactive software program that helps the user understand and make use of the dynamic relationships between climate controls, climate processes, and plant processes. A conceptual framework was designed for a user interface with a biological orientation. This software consists of five different elements: the information provider, the information monitor, the information browser, the growth system controller, and the system visualizer. A demonstrator application illustrating this concept was developed and connected in real time to a standard greenhouse environmental computer. Crop tissue temperature is calculated and used instead of conventional irradiance limits to control shading screens to optimize the amount of radiation absorbed by the crop. The application is based on a set of generic automatically created paradox databases. A graphical user interface on the screen displays virtual plants that are used for visualizing, understanding, and controlling the different processes governing the crop tissue temperature.
Mark A. Rose, David J. Beattie and John W. White
Two distinct patterns of whole-plant transpiration (WPT) were observed in `Moonlight' rose (Rosa hybrida L.) using an automated system that integrated a greenhouse climate computer, a heat-balance sap-flow gauge, an electronic lysimeter, and an infrared leaf temperature sensor. One pattern consisted of a steady rate of transpiration in a stable greenhouse environment. The second pattern consisted of large oscillations in transpiration unrelated to any monitored microclimate rhythms. These oscillations had a sine-wave pattern with periods of 50 to 90 minutes and ranged from 2 to 69 g·h-1 in natural light and 3 to 40 g·h-1 under high-pressure sodium lamps at night. Leaf-air temperature difference (T1 - Ta) also oscillated and was inversely related to transpiration rate. Oscillatory transpiration has not been reported in roses. Plant scientists need to recognize the complex and dynamic nature of plant responses such as the oscillatory pattern of WPT monitored in Rosa hybrida when selecting monitoring and control strategies.