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  • Author or Editor: Krishna Nemali x
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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.

Open Access

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. PER (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 PER. 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 PER values. Regardless of the variety, fixtures with greater percentages of red, R:B, and PER 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 PER in indoor lettuce farming.

Free access

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 (Pn_avg ), whole-plant respiration (Rd_avg ), and daily carbon gain (DCG) were measured continuously, whereas stomatal conductance (g S) to water, leaf water (ΨL), osmotic (ΨS), and turgor (ΨP) potentials were measured at the start and end of the drought phase. In addition, ΨS was measured before exposure to stress and after thoroughly rehydrating plants. Dark-adapted quantum efficiency (dark-adapted ΦPSII) 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 Rd_avg ; however, Pn_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 ΨL; however, a more negative ΨS (P = 0.06) and significantly higher ΨP were observed in vinca (compared to salvia) under drought conditions. In addition, ΨS was not different between species before exposure to drought, whereas ΨS of rehydrated leaves after exposure to drought in vinca was significantly lower than that in salvia. Moreover, ΨS 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 ΦPSII 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 ΨP 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 Pn_avg and DCG at lower Θ. Screening for OA and dark-adapted ΦPSII may be useful for developing drought-tolerant bedding plant cultivars.

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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 (ΨL) and growth rate of vinca revived completely, and ΨL 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.

Free access

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.

Free access

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.

Open Access

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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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 (Pg)] of plants grown at low DLI was low, since these plants used most of the C fixed in Pg for maintenance respiration (Rm), leaving few, if any, C for growth and growth respiration (Rg). Maintenance respiration accounted for a smaller fraction of the total respiration with increasing DLI. In addition, the importance of Rm in the carbon balance of the plants decreased over time, resulting in an increase in CUE. At harvest, crop dry weight (DWCROP) increased linearly with increasing DLI, due to the increased photosynthesis and CUE at high PPF.

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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 (Rd), quantum yield (α), the light compensation point (LCP), and maximum gross photosynthesis (Pg,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 Rd and Pg,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 Pg,max to efficiently utilize the available light, thereby maximizing carbon gain under both situations.

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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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