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Modern greenhouses are intensive farming systems designed to achieve high efficiency and productivity. Plants are produced year-round in greenhouses by maintaining the environment at or near optimum levels regardless of extreme weather conditions. Many scientific discoveries and technological advancements that happened in the past two centuries paved the way for current state-of-the-art greenhouses. These include, but are not limited to, advancements in climate-specific structural designs and glazing materials, and temperature control, artificial lighting, and hydroponic production systems. Greenhouse structures can be broadly grouped into four distinct designs, including tall Venlo greenhouses of the Netherlands, passive solar greenhouses of China, low-cost Parral greenhouses of the Mediterranean region, and gutter-connected polyethylene houses of India and African countries. These designs were developed to suit local climatic conditions and maximize the return on investment. Although glass and rigid plastic options are available for glazing, the development of low-cost and lightweight plastic glazing materials (e.g., polyethylene) enabled widespread growth of the greenhouse industry in the developing world. For temperate regions, supplemental lighting technology is crucial for year-round production. This heavily relies on advancements in electro-lighting during the 19th and 20th centuries. The development of hydroponic production systems for the controlled delivery of nutrients further enhanced crop productivity. This article addresses important historical events, scientific discoveries, and technological improvements related to advancements in these areas.

High energy-use cost for electric lighting is one of the major issues challenging sustainability of the indoor lettuce-farming industry. Thus, maximizing electrical energy-use efficiency (EUE, g·KWh⁻¹), defined as the ratio of dry matter production (g) to electrical energy consumption (EEC, KWh⁻¹), is crucial during indoor production. Light-emitting diodes (LEDs) are energy efficient and highly suitable for indoor farms. Research on optimal spectral quality of LEDs for lettuce growth is extensive; however, there is limited research examining LED spectral quality effects on EEC and EUE. Photon efficiency, defined as the ratio of light output to electrical energy input (PE, µmol·J⁻¹), generally is used for selection of LED fixtures. Because PE does not account for differences in emitted light spectrum, it is not clear whether light-fixture selection based on PE can maximize EUE in lettuce production. This study comprised two experiments. In Expt. 1, we used four “phosphor-converted” commercial LEDs with different light intensities and spectra to model the effect of light spectral quality on lettuce shoot dry weight (SDW), EEC, and EUE. We also evaluated relations between EUE vs. PE and EUE vs. PE_R (PE based on red light) for indoor lettuce production. Results indicated that light spectral quality affected SDW, EEC, and EUE in lettuce production. Fitted models indicated that EEC increased linearly with increasing percentage of red-light output and was unaffected by other spectral colors or ratios. However, EUE increased in a curvilinear fashion with an increasing ratio of red to blue (R:B) light and reached a maximum at a ratio of 4.47. Similar to EUE, SDW also responded in a curvilinear fashion to R:B. Results also indicated that EUE correlated poorly with PE but linearly to PE_R. In Expt. 2, we grew three lettuce varieties under two commercial LED fixtures. They had similar levels of PE but different percentages of red, R:B, and PE_R values. Regardless of the variety, fixtures with greater percentages of red, R:B, and PE_R significantly increased EUE. We conclude that red-light quality is an important determinant of EUE and growers should select fixtures based on R:B and high PE_R in indoor lettuce farming.

Operational cost of producing lettuce (Lactuca sativa) during the winter in greenhouses is high in the northern regions of the United States due to the addition of supplemental lighting (SL) and heating. Crop productivity in greenhouses should increase to offset high operational costs and maintain profits. Factors including SL composition, heating efficiency, suitability of production systems (PS), and cultivar performance can affect crop productivity. Research-based information on optimizing the above environmental- and production-related factors is limited. This information is critical for growers to make informed decisions and increase profits during winter hydroponic production. We evaluated the interactive effects of SL composition, solution temperature, PS, and cultivar treatments on lettuce shoot dry weight (SDW, g·m⁻²) and shoot water content (SWC, %) in a greenhouse maintained at suboptimal air temperature (13.7 °C) using a split-plot design. There were three light treatments (sunlight without SL, sunlight + narrow-spectrum SL at nighttime, and sunlight + full-spectrum SL at nighttime), two solution temperature levels [heated (18.8 °C) and unheated (13.2 °C)], two hydroponic PS [constant flood technique (CFT) and nutrient film technique (NFT)], and eight cultivars included in the study. Results indicated that 1) a narrow-spectrum SL at nighttime in combination with heated solution resulted in maximum SDW of lettuce, 2) the SDW and SWC (major determinant of economic yield) increase between the heated and unheated solution temperature treatments was higher in the CFT than in the NFT, and 3) the positive effects of using heated solution were seen mainly in the green-color cultivars. Our research identified the optimal spectral composition of nighttime SL, tested the positive effects of alternate heating methods using heated solution on plant growth under suboptimal air temperature conditions, compared the suitability of two hydroponic PS for lettuce production, and quantified yield potential of several lettuce cultivars in hydroponic production during winter. Growers can use our research findings to make informed decisions about their investment and to maximize hydroponic lettuce productivity and profits during winter.

Bedding plants are at increased risk for exposure to drought stress during production because they are grown in small containers. Physiological mechanisms of bedding plants at leaf and cellular scales that regulate whole-plant photosynthesis under drought conditions are not well understood. This information can be useful for screening bedding plant cultivars with improved drought-tolerance and generate guidelines to mitigate drought stress during production. We subjected drought-sensitive salvia (Salvia splendens ‘Bonfire Red’) and drought-tolerant vinca (Catharanthus roseus ‘Cooler Peppermint’) to gradual drought stress inside whole-plant gas exchange chambers. Substrate water content (Θ), whole-plant net photosynthesis (P_{n_avg} ), whole-plant respiration (R_{d_avg} ), and daily carbon gain (DCG) were measured continuously, whereas stomatal conductance (g _S) to water, leaf water (Ψ_L), osmotic (Ψ_S), and turgor (Ψ_P) potentials were measured at the start and end of the drought phase. In addition, Ψ_S was measured before exposure to stress and after thoroughly rehydrating plants. Dark-adapted quantum efficiency (dark-adapted Φ_PSII) was measured after rehydrating plants. The results indicated that, at whole-plant scale, vinca continued to uptake water at lower Θ levels than the Θ level that resulted in wilting of salvia. There were no differences in R_{d_avg} ; however, P_{n_avg} and DCG of salvia decreased at a higher Θ level than that of vinca. This indicated that salvia experienced drought stress at a higher Θ level than did vinca. At the leaf scale, there were no differences in Ψ_L; however, a more negative Ψ_S (P = 0.06) and significantly higher Ψ_P were observed in vinca (compared to salvia) under drought conditions. In addition, Ψ_S was not different between species before exposure to drought, whereas Ψ_S of rehydrated leaves after exposure to drought in vinca was significantly lower than that in salvia. Moreover, Ψ_S of rehydrated leaves after exposure to drought was significantly lower than that observed before exposure to drought in vinca. This indicated osmotic adjustment (OA) in vinca under drought conditions. Dark-adapted Φ_PSII was lower in salvia than in vinca after exposure to drought, indicating damage to photosynthetic mechanisms. Our results suggested that increased OA likely helped to maintain higher Ψ_P under drought conditions and continuation of water uptake at lower Θ in vinca compared to salvia. In addition, healthier photosynthetic mechanisms of vinca (compared to salvia) under drought conditions likely resulted in its higher P_{n_avg} and DCG at lower Θ. Screening for OA and dark-adapted Φ_PSII may be useful for developing drought-tolerant bedding plant cultivars.

Monitoring moisture status of the growing medium is essential as growth and quality of greenhouse crops largely depend on the amount of available water. Recently, two new types of moisture sensors have been developed (ECH₂ O, Decagon devices, Inc., Pullman, Wash.; Theta probe ML2X, Delta -T devices Ltd., Burwell, Cambridge, U.K.). We studied the performance of these sensors for measuring the volumetric water content (VWC) of a soilless growing medium. We also tested the sensitivity of these sensors to temperature and electrical conductivity (EC) of the growing medium. Our results indicate that these sensors can be calibrated and used effectively for measuring a wide range of moisture contents in the growing medium; however media specific calibration may be required. Regression analysis indicated that the output of ECH₂ O probes was affected by changes in the EC and temperature of the growing medium. Effects of EC were too small to be of practical significance, while the measured VWC increased by 0.003 m³/m³ for each °C increase in temperature. The output from the Theta Probe was not affected by changes in the EC or temperature of the growing medium. In a comparison study, both probes were found to give similar estimates of the VWC of the growing medium within the common range seen under greenhouse production.

Subjecting bedding plants to non-lethal moisture stress is an established irrigation practice for bedding plants; however information on physiological responses of bedding plants to moisture stress is limited. We examined the CO₂ exchange rates (CER) and water relations of salvia (Salvia splendens) and vinca (Catharanthus roseus) during moisture stress. Seedlings of both species were grown from seed in 7-L trays containing a soilless growing medium. After plants completely covered the trays, they were irrigated and shifted into whole-plant gas exchange chambers (27 °C and daily light integral of 7.5 mol/m²) arranged inside a growth chamber. Inside the gas exchange chambers, the growing medium was allowed to dry and plants were re-watered after wilting. Results from this study indicate that the growth rate (moles of CO₂ gained by plants in a day) of salvia was higher than vinca before experiencing moisture stress; however the volumetric moisture content of the growing medium at which plant growth decreased was higher for salvia than for vinca. During moisture stress, the decrease in growth rate of salvia was gradual and that of vinca was rapid. After re-watering the plants, leaf water potential (Ψ_L) and growth rate of vinca revived completely, and Ψ_L of salvia remained low (more negative), whereas its growth rate revived completely. This study shows that bedding plant species respond differently to moisture stress, particularly with respect to the critical substrate moisture level for initiating moisture stress and the rate of development of moisture stress.

Physiological acclimation of plants to light has been studied mostly at the leaf level; however whole-plant responses are more relevant in relation to crop growth. To examine the physiological changes associated with different daily light integrals (DLI) during growth of wax begonia (Begonia semperflorens-cultorum Hort.), we grew plants under DLI of 5.3, 9.5, 14.4, and 19.4 mol·m^-2·d^-1 in a whole-plant gas exchange system. Photosynthesis-light response curves of groups of 12 plants were determined after 25 d of growth. Physiological parameters were estimated per m² ground area and per m² leaf area. On a ground area basis, significant increases in dark respiration (R_d), quantum yield (α), the light compensation point (LCP), and maximum gross photosynthesis (P_g,max) were seen with increasing DLI. Variations in physiological parameters among different treatments were small when corrected for differences in leaf area. On a leaf area basis, α, LCP, and the light saturation point (LSP) did not change significantly, whereas significant increases in R_d and P_g,max were seen with increasing DLI. There was a small decrease in leaf chlorophyll concentration (6.3%, measured in SPAD units) with increasing DLI. This study indicates that wax begonias acclimate to low DLI by increasing their leaf chlorophyll concentration, presumably to more efficiently capture the available light, and to high DLI by increasing P_g,max to efficiently utilize the available light, thereby maximizing carbon gain under both situations.

The effect of increasing daily light integral (DLI; 5.3, 9.5, 14.4, and 19.4 mol·m^-2·d^-1) on photosynthesis and respiration of wax begonia (Begonia semperflorens-cultorum Hort.) was examined by measuring CO₂ exchange rates (CER) for a period of 25 d in a whole-plant gas exchange system. Although plant growth rate (GR, increase in dry weight per day) increased linearly with increasing DLI, plants grown at low DLI (5.3 or 9.5 mol·m^-2·d^-1) respired more carbohydrates than were fixed in photosynthesis during the early growth period (13 and 4 d, respectively), resulting in a negative daily carbon gain (DCG) and GR. Carbon use efficiency [CUE, the ratio of carbon incorporated into the plant to C fixed in gross photosynthesis (P_g)] of plants grown at low DLI was low, since these plants used most of the C fixed in P_g for maintenance respiration (R_m), leaving few, if any, C for growth and growth respiration (R_g). Maintenance respiration accounted for a smaller fraction of the total respiration with increasing DLI. In addition, the importance of R_m in the carbon balance of the plants decreased over time, resulting in an increase in CUE. At harvest, crop dry weight (DW_CROP) increased linearly with increasing DLI, due to the increased photosynthesis and CUE at high PPF.

Optimal substrate volumetric water content (θ) and drought tolerance of impatiens, petunia, salvia, and vinca were investigated by growing plants under four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m³·m^-3). Gas exchange, quantum efficiency (ΦPSII), electron transport rate (ETR), non-photochemical quenching (NPQ), and leaf water potential (ϒ) were measured for all species, and response of photosynthesis (Pn) to internal CO₂ concentration (Ci) was studied in petunia and salvia. Leaf photosynthesis (Pmax) was highest at a θ of 0.22 m³·m^-3 for all species and did not differ between a θ of 0.15 and 0.22 m³·m^-3 for vinca and petunia. The Pn-Ci response curves for petunia were almost identical at a θ of 0.22 and 0.15 m³·m^-3. Regardless of species, ETR and ΦPSII were highest and NPQ was lowest at a θ of 0.22 m³·m^-3. Based on these results, a θ of 0.22 m³·m^-3 for salvia and impatiens and a slightly lower θ of 0.15 m³·m^-3 for vinca and petunia, is optimal. Mean osmotic potential in all treatments was lower in vinca and salvia and resulted in higher turgor potential in these species than other species. Analysis of Pn-Ci response curves indicated that Pn at a θ of 0.09 m³·m^-3 was limited by both gas phase (stomatal and boundary layer) and non-gas phase (mesophyll) resistance to CO₂ transfer in salvia. At the lowest θ level, Pn in petunia was only limited by gas phase resistance, indicating that absence of mesophyll resistance during drought may play a role in the drought tolerance of petunia.

We have developed a completely automated irrigation system that measures and maintains substrate volumetric water content (θ) at a target level for any length of time. Advantages of this system include complete automation of irrigation and simulation of precise levels of drought stress for potted plants. This system uses ECH₂O moisture sensors interfaced with a CR10X datalogger and solenoid valves connected to the datalogger by a SDM CD16 AC/DC controller. The datalogger measures the θ of the substrate hourly. When the θ of the substrate drops below the set point, the datalogger opens the solenoid valves, which results in irrigation. Substrate θ is maintained at a constant level as the datalogger is programmed to increase θ by 2% to 3% during each irrigation. When the system was validated for its accuracy, we determined that the θ measured in the substrate within the range of 0.15 to 0.35 m³·m^-3 was close (2% to 3%) to the θ determined by the conventional volumetric analysis. The daily average θ maintained in the substrate was slightly higher (within 3%) than the target level. Using this system, we were able to maintain four distinct levels of substrate θ for a prolonged period (40 days), regardless of differences in plant size and environmental conditions. Significant increases in number of irrigations, total water-use, and transpiration rate of impatiens, salvia, vinca, and petunia were noticed with increasing target θ of the substrate. For all species, highest and lowest water-use efficiency (WUE) were seen at 0.09 and 0.32 m³·m^-3, respectively, while WUE was not different between 0.15 and 0.22 m³·m^-3.