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Marc W. van Iersel*

Literature reports on the Q10 for respiration vary widely, both within and among species. Plant size and metabolic activity may be responsible for some of this variation. To test this, respiration of whole lettuce plants was measured at temperatures ranging from 6 to 31 °C during a 24-h period. Subsequently, plant growth rate (in moles of carbon per day) was determined by measuring the CO2 exchange rate of the same plants during a 24-h period. Environmental conditions during this 24-h period resembled those that the plants were exposed to in the greenhouse. The measured growth rate was then used to estimate the relative growth rate (RGR) of the plants. The respiratory Q10 ranged from 1.4 for small plants to 1.75 for large plants. The increase in Q10 with increasing plant size was highly significant, as was the decrease in Q10 with increasing RGR. However, growth rate had little or no effect on the respiratory Q10. One possible explanation for these findings is that the Q10 depends on the ratio of growth to maintenance respiration (which is directly related to RGR). The growth respiration coefficient generally is considered to be temperature-insensitive, while the maintenance respiration coefficient normally increases with increasing temperature. Based on this concept, the Q10 for the maintenance respiration coefficient can be estimated as the estimated Q10 at a RGR of zero (i.e. no growth and thus no growth respiration), which was 1.65 in this experiment. Although the concept of dividing respiration into growth and maintenance fractions remains controversial, it is useful for explaining changes in respiratory Q10 during plant development.

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Marc W. van Iersel

Do you accurately measure and report the growing conditions of your controlled environment experiments? Conditions in controlled environment plant growth rooms and chambers should be reported in detail. This is important to allow replication of experiments on plants, to compare results among facilities, and to avoid artefacts due to uncontrolled variables. The International Committee for Controlled Environment Guidelines, with representatives from the U.K. Controlled Environment Users' Group, the North American Committee on Controlled Environment Technology and Use (NCR-101), and Australasian Controlled Environment Working Group (ACEWG), has developed guidlines to report environmental conditions in controlled environment experiments. These guidelines include measurements of light, temperature, humidity, CO2, air speed, and fertility. A brochure with these guidelines and a sample paragraph on how to include this information in a manuscript will be available.

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Marc W. van Iersel

Bedding plants are exposed to a wide range of environmental conditions, both during production and in the landscape. This research compared the effect of short-term temperature changes on the CO2 exchange rates of four popular bedding plants species. Net photosynthesis (Pnet) and dark respiration (Rdark) of geranium (Pelargonium ×hortorum L.H. Bail.), marigold (Tagetes patula L.), pansy (Viola ×wittrockiana Gams.), and petunia (Petunia ×hybrida Hort. Vilm.-Andr.) were measured at temperatures ranging from 8 to 38 °C (for Pnet) and 6 to 36 °C (for Rdark). Net photosynthesis of all species was maximal at 14 to 15 °C, while Rdark of all four species increased exponentially with increasing temperature. Gross photosynthesis (Pgross) was estimated as the sum of Pnet and Rdark, and was greater for petunia than for the other three species. Gross photosynthesis was less sensitive to temperature than either Pnet or Rdark, suggesting that temperature effects on Pnet were caused mainly by increased respiration at higher temperatures. Gas exchange-temperature response curves were not useful in determining the heat tolerance of these species. There were significant differences among species in the estimated Rdark at 0 °C and the Q10 for Rdark. Differences in the Q10 for Rdark were related to growth rate and plant size. Large plants had a greater Q10 for Rdark, apparently because these plants had a higher ratio of maintenance to growth respiration than small plants. The Q10 of the maintenance respiration coefficient was estimated from the correlation between the Q10 and relative growth rate, and was found to be 2.5 to 2.6.

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Krishna S. Nemali and Marc W. van Iersel

Environmental conditions and incorporation of nutrients into the growing medium can affect the fertilizer needs of bedding plants. To evaluate the effects of photosynthetic photon flux (PPF) and starter fertilizer on the fertilizer requirements of subirrigated plants, we grew wax begonias (Begonia semperflorens-cultorum Hort.) under three PPF levels (averaging 4.4, 6.2, and 9.9 mol·m-2·d-1) and four fertilizer concentrations [electrical conductivity (EC) of 0.15, 0.33, 0.86, and 1.4 dS·m-1] in a normal (with starter fertilizer, EC = 2.1 dS·m-1) and heavily leached (with little starter fertilizer, EC = 0.9 dS·m-1) growing medium. Except for shoot dry mass, we did not find any significant interactions between PPF and fertilizer concentration on any of the growth parameters. There was an interactive effect of fertilizer concentration and starter fertilizer on all growth parameters (shoot dry mass, leaf area, plant height, and number of flowers). When the growing medium contained a starter fertilizer, fertilizer concentration had little effect on growth. When the growing medium was leached before transplanting, growth was best with a fertilizer EC of 0.86 or 1.4 dS·m-1. Water-use efficiency (WUE) was calculated from 24-hour carbon exchange and evapotranspiration measurements, and used to estimate the required [N] in the fertilizer solution to achieve a target tissue N concentration of 45 mg·g-1. Increasing PPF increased WUE and the required [N] (from 157 to 203 mg·L-1 at PPF levels of 4.4 and 9.9 mol·m-2·d-1, respectively). The PPF effect on the required [N] appeared to be too small to be of practical significance, since dry mass data did not confirm that plants grown at high light needed higher fertilizer concentrations. Thus, fertilizer concentrations need not be adjusted based on PPF.

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Krishna S. Nemali and Marc W. van Iersel

To evaluate the effects of increasing photosynthetic photon flux (PPF) on optimal fertilizer concentrations, we grew wax begonia (Begonia semperflorens-cultorum Hort.) and petunia (Petunia ×hybrida Hort. Vilm-Andr.) seedlings in a soilless growing medium without starter fertilizer under three PPF treatments (high, medium, and low corresponding to an average daily PPF of 23.2, 15.6, and 9.8 mol·m-2.d-1, respectively) and subirrigated with six fertilizer concentrations [electrical conductivity (EC) of 0.12, 0.65, 1.18, 1.71, 2.24, and 2.77 dS·m-1]. Compared to low PPF, shoot dry mass of wax begonia and petunia seedlings increased 2- and 3-fold, respectively, at high PPF. Fertilizer EC resulting in maximum shoot dry mass was the same (1.28 and 1.87 dS·m-1 for wax begonia and petunia, respectively) in the three PPF treatments. Shoot dry mass and leaf area of petunias decreased little at higher than optimal fertilizer EC in the three PPF treatments, while growth of begonia was inhibited at high fertilizer EC. The optimal fertilizer range, calculated as the lower and upper limits of fertilizer EC within which plant growth was not reduced by >10% as compared to the optimum EC was 0.65 to 1.71 dS·m-1 in wax begonia and 1.18 to >2.77 dS·m-1 for petunia. Compared to those grown at 1.18 dS·m-1, wax begonias grown at 1.71 dS·m-1 had similar dry mass, but were shorter in all three PPF treatments (average height reduction of 6.5%). In general, EC of the top layer of the growing medium was higher than that of the bottom layer of the growing medium, and this difference increased with increasing EC.

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Marc W. van Iersel and Krishna S. Nemali

We examined the effectiveness of an elevated capillary mat system to maintain constant and different moisture levels in the growing medium and verify the potential of drought stress conditioning in producing small and compact bedding plants. To differentiate between plant height and compactness, we determined compactness as the leaf area or dry mass per unit stem length. Marigold `Queen Sophia' (Tagetes erecta L.) seedlings were grown in square, 9-cm-wide, 10-cm-high containers filled with a soilless growing medium. A capillary mat was laid on top of a greenhouse bench which was raised by 15 cm on one side compared to the other side to create an elevation effect. Seedlings were subirrigated by immersing the low end of the capillary mat in a reservoir of water. The amount of water moving to the higher end of the mat progressively decreased with elevation. The moisture content in the growing medium averaged from 26 to 294 mL/pot at different elevations. Regression analysis indicated that growth parameters including, shoot dry mass, leaf area, leaf number, and plant height decreased linearly with decreasing soil moisture content in the growing medium. Of all the measured growth parameters, plant height was found to be least sensitive to decreasing moisture content in the growing medium. Plants in high moisture treatments had more dry mass and leaf area per unit length of the stem compared to those in low moisture treatments. Our results indicate that drought stress can produce small, but not truly compact bedding plants.

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Marc W. van Iersel and David Gianino

Supplemental lighting in greenhouses is often needed for year-round production of high-quality crops. However, the electricity needed for supplemental lighting can account for a substantial part of overall production costs. Our objective was to develop more efficient control methods for supplemental lighting, taking advantage of the dimmability of light-emitting diode (LED) grow lights. We compared 14 hours per day of full power supplemental LED lighting to two other treatments: 1) turning the LEDs on, at full power, only when the ambient photosynthetic photon flux (PPF) dropped below a specific threshold, and 2) adjusting the duty cycle of the LEDs so that the LED lights provided only enough supplemental PPF to reach a preset threshold PPF. This threshold PPF was adjusted daily from 50 to 250 μmol·m−2·s−1. Turning the LED lights on at full power and off based on a PPF threshold was not practical since this at times resulted in the lights going on and off frequently. Adjusting the duty cycle of the LED lights based on PPF measurements underneath the light bar provided excellent control of PPF, with 5-minute averages typically being within 0.2 μmol·m−2·s−1 of the threshold PPF. Continuously adjusting the duty cycle of the LED lights reduced electricity use by 20% to 92%, depending on the PPF threshold and daily light integral (DLI) from sunlight. Simulations based on net photosynthesis (An) − PPF response curves indicated that there are large differences among species in how efficiently supplemental PPF stimulates An. When there is little or no sunlight, An of Heuchera americana is expected to increase more than that of Campanula portenschlagiana when a low level of supplemental light is provided. Conversely, when ambient PPF >200 μmol·m−2·s−1, supplemental lighting will have little impact on An of H. americana, but can still results in significant increases in An of C. portenschlagiana (1.7 to 6.1 μmol·m−2·s−1 as supplemental PPF increases from 50 to 250 μmol·m−2·s−1). Adjusting the duty cycle of the LEDs based on PPF levels assures that supplemental light is provided when plants can use that supplemental light most efficiently. Implementing automated duty cycle control of LED grow lights is simple and low cost. This approach can increase the cost effectiveness of supplemental lighting, because of the associated energy savings.

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Krishna S. Nemali and Marc W van Iersel

Efficient use of irrigation water is increasingly important in the production of bedding plants. Two approaches to efficient water use include reducing irrigation water wastage during production by growing plants at the optimal substrate water content (θ) and growing species with high water-use efficiency (WUE). However, there is little information on the effects of different θ levels on leaf physiology of bedding plants and variation in WUE among different species of bedding plants. The objectives of this study were to determine the effects of θ on leaf water relations, gas exchange, chlorophyll fluorescence, and WUE of bedding plants and to identify the physiological basis for differences in WUE between two bedding plant species. We grew salvia ‘Bonfire Red’ (Salvia splendens Sellow ex Roemer & J.A. Schultes), vinca ‘Cooler Peppermint’ [Catharanthus roseus (L.) G. Don.], petunia ‘Lavender White’ (Petunia × hybrida Hort ex. Vilm.), and impatiens ‘Cherry’ (Impatiens walleriana Hook F.) at four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m−3) using an automated irrigation controller. Regardless of species, leaf water potential (Ψw) and leaf photosynthesis (A) of all four species were lower at a θ of 0.09 m3·m−3 and were not different among the other θ levels, but stomatal conductance to H2O (g S) was lower at 0.09 than at 0.15 and 0.22 m3·m−3 and highest at 0.32 m3·m−3. WUE of bedding plants at different θ levels depended on species. The WUE of petunia was unaffected by θ, whereas for the other three species, WUE was higher at a θ of 0.09 m3·m−3 than at 0.32 m3·m−3. Differences in WUE between petunia and salvia were partly from differences in photosynthetic capacity between the two species. Based on the response of A to leaf internal CO2 concentration (Ci), mesophyll conductance to CO2 [gm (a measure of photosynthetic capacity)] was higher in petunia than salvia, whereas gas phase conductance to CO2 (gCO2) was similar for these two species, which resulted in higher WUE in petunia than salvia.

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Julián Miralles-Crespo and Marc W. van Iersel

Irrigation control systems that irrigate container-grown plants based on crop water needs can reduce water and fertilizer use and increase the sustainability of ornamental crop production. The use of soil moisture sensors to determine when to irrigate is a viable option. We tested a commercially available irrigation controller (CS3500; Acclima, Meridian, ID), which uses time domain transmissometry (TDT) sensors to measure soil volumetric water content (θ). The objectives of this study were: 1) to test the accuracy of TDT sensors in soilless substrate; 2) to quantify the ability of the Acclima CS3500 irrigation controller to maintain stable θ readings during the production of container-grown begonia (Begonia semperflorens L.) by turning a drip irrigation system on and off as needed; and 3) to study the growth and photosynthetic physiology of begonia at six θ levels. Calibration of the TDT sensors in pots filled with substrate (but without plants) showed that the θ determined by the TDT sensors had a very close relationship (R 2 = 0.99) with the gravimetrically determined θ, but the TDT sensors underestimated θ by ≈0.08 m3·m−3. Therefore, a custom calibration of the TDT sensors for the soilless substrate was necessary to get accurate θ data. The irrigation controller was programmed to maintain six θ thresholds, ranging from 0.136 to 0.472 m3·m−3 (based on our own sensor calibration), and was able to maintain θ readings within 0.008 m3·m−3 of the threshold. Theta and Sigma probes were used to collect comparative θ and bulk electrical conductivity (EC) data, respectively. The results showed a strong correlation with TDT sensor measurements of θ (R 2 = 0.92) but a moderate relationship for bulk EC (R 2 = 0.53). The begonias had similar dry weight at θ levels of 0.348 m3·m−3 and higher, whereas total evapotranspiration increased linearly with the θ threshold. The lowest θ threshold reduced leaf size, net photosynthesis (Pn), and stomatal conductance (g S). Overall, the TDT sensors can provide accurate measurements of θ in soilless substrate but need substrate-specific calibration. The Acclima CS3500 controller, using TDT sensors, was able to maintain stable θ readings throughout a production cycle. These results suggest that this irrigation controller may be suitable for production of greenhouse crops as well as in drought stress research.

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Geoffrey Weaver and Marc W. van Iersel

Plant light use efficiency decreases as light intensity is increased, and a better understanding of crop-specific light responses can contribute to the development of more energy-efficient supplemental lighting control strategies for greenhouses. In this study, diurnal chlorophyll fluorescence monitoring was used to characterize the photochemical responses of ‘Green Towers’ lettuce (Lactuca sativa L.) to photosynthetic photon flux density (PPFD) and daily light integral (DLI) in a greenhouse during a production cycle. Plants were monitored continuously for 35 days, with chlorophyll fluorescence measurements collected once every 15 minutes. Quantum yield of photosystem II (ΦPSII) decreased exponentially with PPFD, whereas electron transport rate (ETR) increased asymptotically to 121 µmol·m–2·s–1. Daily photochemical integral (DPI) is defined as the integral of ETR over a 24-hour period; DPI increased asymptotically to 3.29 mol·m–2·d–1 with increasing DLI. No effects of plant age or prior day’s DLI and a negligible effect of PPFDs 15 or 30 minutes before measurements within days were observed. Simulations were conducted using the regression equation of ETR as a function of PPFD {ETR = 121[1 – exp(–0.00277PPFD)]} to illustrate methods of increasing photochemical light use efficiency for improved supplemental lighting control strategies. For a given DLI, DPI can be increased by providing light at lower PPFDs for a longer period of time, and can be maximized by providing light with a uniform PPFD throughout the entire photoperiod. Similarly, the DLI required to achieve a given DPI is reduced using these same methods.