Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures

Lighting technologies for plant growth are improving rapidly, providing numerous options for supplemental lighting in greenhouses. Here we report the photosynthetic (400–700 nm) photon efficiency and photon distribution pattern of two double-ended HPS fixtures, five mogul-base HPS fixtures, ten LED fixtures, three ceramic metal halide fixtures, and two fluorescent fixtures. The two most efficient LED and the two most efficient double-ended HPS fixtures had nearly identical efficiencies at 1.66 to 1.70 micromoles per joule. These four fixtures represent a dramatic improvement over the 1.02 micromoles per joule efficiency of the mogul-base HPS fixtures that are in common use. The best ceramic metal halide and fluorescent fixtures had efficiencies of 1.46 and 0.95 micromoles per joule, respectively. We also calculated the initial capital cost of fixtures per photon delivered and determined that LED fixtures cost five to ten times more than HPS fixtures. The five-year electric plus fixture cost per mole of photons is thus 2.3 times higher for LED fixtures, due to high capital costs. Compared to electric costs, our analysis indicates that the long-term maintenance costs are small for both technologies. If widely spaced benches are a necessary part of a production system, the unique ability of LED fixtures to efficiently focus photons on specific areas can be used to improve the photon capture by plant canopies. Our analysis demonstrates, however, that the cost per photon delivered is higher in these systems, regardless of fixture category. The lowest lighting system costs are realized when an efficient fixture is coupled with effective canopy photon capture.

Nitrogen retention and partitioning at the initiation of lipid accumulation in nitrogen-deficient algae

Nitrogen (N) deficiency promotes lipid accumulation in many oleaginous algae, but we have a poor understanding of the associations between the initiation of lipid accumulation and algal N retention and partitioning. Here, we report on total cell N, five bulk pools of N in the cell (protein, free amino acids, DNA, RNA, chl), and lipids from N saturation to growth cessation in three species. While the maximum level of N uptake differed among species, the ratio of minimum retained N to N retained at the initiation of lipid accumulation was consistent among species at 0.5 ± 0.04. This suggests that the cellular initiation of lipid accumulation was associated with a common magnitude of N deficiency among species. Concerning the partitioning of N, the concentration of RNA and the protein to RNA ratio were most similar among species at the initiation of lipid accumulation with averages of 3.2 ± 0.26 g · L(-1) (8.2% variation) and 16 ± 1.5 (9.2% variation), respectively. All other pools and physiologically relevant ratios were considerably more variable. The species commonalities in RNA and protein show a similar reduction in general cellular function due to N deficiency before cellular initiation of lipid accumulation. These results provide insight into the physiological drivers for lipid accumulation in N-deficient algae and data for modeling these associations.

Photobiological interactions of blue light and photosynthetic photon flux: effects of monochromatic and broad-spectrum light sources

Photosynthesis (Pn) and photomorphogenesis (Pm) are affected by light quality, light intensity and photoperiod. Although blue light (BL) is necessary for normal development, it is less efficient in driving Pn than other wavelengths of photosynthetically active radiation. The effects of BL on Pm are highly species dependent. Here we report the interacting effects of BL and photosynthetic photon flux (PPF) on growth and development of lettuce, radish and pepper. We used light-emitting diode (LED) arrays to provide BL fractions from 11% to 28% under broad-spectrum white LEDs, and from 0.3% to 92% under monochromatic LEDs. All treatments were replicated three times at each of two PPFs (200 and 500 μmol m(-2) s(-1)). Other than light quality, environmental conditions were uniformly maintained across chambers. Regardless of PPF, BL was necessary to prevent shade-avoidance responses in radish and lettuce. For lettuce and radish, increasing BL reduced stem length, and for both species, there were significant interactions of BL with PPF for leaf expansion. Increasing BL reduced petiole length in radish and flower number in pepper. BL minimally affected pepper growth and other developmental parameters. Pepper seedlings were more photobiologically sensitive than older plants. Surprisingly, there were few interactions between monochromatic and broad-spectrum light sources.

Volatilization of trichloroethylene from trees and soil: measurement and scaling approaches

Trichloroethylene (TCE) volatilization from leaves, trunk, and soil was measured to assess the significance of these pathways from phytoremediation sites at Travis and Fairchild Air Force Bases. Measurements were scaled temporally and spatially to estimate the annual volatilization of TCE at the Travis (0.82 ± 0.51 kg/yr) and Fairchild sites (0.014 ± 0.008 kg/yr). Volatilization was primarily through the leaf (0.34 ± 0.16 kg/yr at Travis and 0.01 ± 0.06 kg/yr at Fairchild) and soil (0.48 ± 0.36 kg/yr at Travis, 0.003 ± 0.002 kg/yr at Fairchild) pathways. The larger volatilization estimate at Travis was expected because of the site's higher TCE groundwater concentrations. Using groundwater data collected in 2004 and 2009, calculations show that over the 5 year period, 1.7 and 0.015 kg of TCE were removed each year at the Travis and Fairchild sites, respectively. On the basis of the scaled field measurements, volatilization from the leaves and soil may play a significant role in TCE removal at both sites. Daily and seasonal variations were not addressed during the limited daytime sampling events, but the methods described here provide a novel and practical framework for evaluating the potential importance of volatilization of TCE and similar compounds at phytoremediation sites.

Understanding precision nitrogen stress to optimize the growth and lipid content tradeoff in oleaginous green microalgae

Nitrogen deficiency promotes lipid formation in many microalgae, but also limits growth and lipid productivity. In spite of numerous studies, there is poor understanding of the interactions of growth and lipid content, the time course of lipid accumulation and the magnitude of nitrogen deficiency required to stimulate lipid formation. These relationships were investigated in six species of oleaginous green algae, comparing high and low levels of deficiency. Nitrogen stress typically had disproportionate effects on growth and lipid content, with profound differences among species. Optimally balancing the tradeoffs required a wide range in nitrogen supply rate among species. Some species grew first and then accumulated lipids, while other species grew and accumulated lipids concurrently which resulted in increased lipid productivity. Accumulation of high lipid content generally resulted from a response to minimal stress. The data highlight the tremendous biodiversity that may be exploited to optimally produce lipids with precision nitrogen stress.

A model of canopy photosynthesis incorporating protein distribution through the canopy and its acclimation to light, temperature and CO2

BACKGROUND AND AIMS

The distribution of photosynthetic enzymes, or nitrogen, through the canopy affects canopy photosynthesis, as well as plant quality and nitrogen demand. Most canopy photosynthesis models assume an exponential distribution of nitrogen, or protein, through the canopy, although this is rarely consistent with experimental observation. Previous optimization schemes to derive the nitrogen distribution through the canopy generally focus on the distribution of a fixed amount of total nitrogen, which fails to account for the variation in both the actual quantity of nitrogen in response to environmental conditions and the interaction of photosynthesis and respiration at similar levels of complexity.

MODEL

A model of canopy photosynthesis is presented for C(3) and C(4) canopies that considers a balanced approach between photosynthesis and respiration as well as plant carbon partitioning. Protein distribution is related to irradiance in the canopy by a flexible equation for which the exponential distribution is a special case. The model is designed to be simple to parameterize for crop, pasture and ecosystem studies. The amount and distribution of protein that maximizes canopy net photosynthesis is calculated.

KEY RESULTS

The optimum protein distribution is not exponential, but is quite linear near the top of the canopy, which is consistent with experimental observations. The overall concentration within the canopy is dependent on environmental conditions, including the distribution of direct and diffuse components of irradiance.

CONCLUSIONS

The widely used exponential distribution of nitrogen or protein through the canopy is generally inappropriate. The model derives the optimum distribution with characteristics that are consistent with observation, so overcoming limitations of using the exponential distribution. Although canopies may not always operate at an optimum, optimization analysis provides valuable insight into plant acclimation to environmental conditions. Protein distribution has implications for the prediction of carbon assimilation, plant quality and nitrogen demand.

Chemical hydrophobicity and uptake by plant roots

The transpiration stream concentration factor (TSCF), the ratio between a compound's concentration in the xylem to that in the solution adjacent to the roots, is commonly used to describe the relative ability of an organic compound to be passively transported from root to shoot. Widely cited bell-shaped curves relating TSCFto the octanol/water partition coefficient (log Kow) imply that significant root uptake and transfer into shoot tissues occurs only for compounds falling within an intermediate hydrophobicity range. However, recent laboratory and field data for relatively water soluble compounds such as sulfolane, methyl tert-butyl ether (MTBE), and 1,4-dioxane suggest that these relationships are not universally applicable, especiallyfor nonionizable, highly polar, water soluble organics. To re-evaluate the relationship between root uptake and chemical hydrophobicity, TSCFs were measured for 25 organic chemicals ranging in log Kow from -0.8 to 5 using a pressure chamber technique. Using the TSCF values measured in this study, a new empirical relationship between TSCF (0 and 1) and log Kow (-0.8 to 5) is presented that indicates that nonionizable, polar, highly water soluble organic compounds are most likely to be taken up by plant roots and translocated to shoot tissue.

Changes in crested wheatgrass root exudation caused by flood, drought, and nutrient stress

Root exudates can chelate inorganic soil contaminants, change rhizosphere pH, and may increase degradation of organic contaminants by microbial cometabolism. Root-zone stress may increase exudation and enhance phytoremediation. We studied the effects of low K+, high NH4+/NO3- ratio, drought, and flooding on the quantity and composition of exudates. Crested wheatgrass (Agropyron cristatum) was grown in Ottawa sand in sealed, flow-through glass columns under axenic conditions for 70 d. Root exudates were collected and analyzed for total organic carbon (TOC) and organic acid content to compare treatment effects. Plants in the low K+ treatment exuded 60% more TOC per plant per day (p = 0.01) than the unstressed control. Drought stress increased cumulative TOC exuded per gram dry plant by 71% (p = 0.05). The flooded treatment increased TOC exuded per gram dry plant by 45%, although this was not statistically significant based on the two replicate plants in this treatment. Exudation from the high NH4+/NO3- ratio treatment was 10% less than the control. Exudation rates in this study ranged from 8 to 50% of rates in four other published studies. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that malic acid was the predominant organic acid exuded. Fumaric, malonic, succinic, and oxalic acids were also detected in the exudates of all treatments. These results demonstrate that nutrient and water stress have significant effects on the quantity and composition of root exudates. Cultural manipulations to induce stress may change the quantity of root exudates and thus increase the effectiveness of phytoremediation.

Trichloroethylene uptake by apple and peach trees and transfer to fruit

A greenhouse study was conducted to quantify 14C-trichloroethylene (TCE) uptake and transfer into the edible fruit of apple and peach trees. Trees were subsurface irrigated with solutions of 14C [TCE] that bracketed groundwater concentrations (5 and 500 microg/L) found in residential areas surrounding Hill Air Force Base, UT, where trace amounts of TCE had been found in several fruits during a preliminary field survey. Nondosed control trees were grown within the canopy of the dosed trees and in a separate greenhouse. Tissue samples were analyzed for 14C and TCE using combustion/liquid scintillation counting (LSC) and headspace/gas chromatography/mass spectrometry (HS/GC/MS). Tissue was also extracted and analyzed by GC/MS for dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), and trichloroethanol (TCEt), three specific TCE metabolites that have been previously identified in laboratory and field studies. No 14C was detected in the nonexposed control trees. Exposed trees contained levels of 14C that were proportional to the exposure concentration. 14C concentrations were greatest in leaves followed by branches and fruits. At the end of the study, TCE was detected only in roots implying that the 14C in the leaves, branches, and fruit was associated with unidentified nonvolatile TCE transformation products and/or is nonextractable. However, TCAA and DCAA were positively identified only in leaves collected during the first year from an apple tree exposed to the high dose treatment. Additional data for other chemicals and fruittrees are needed to better understand the potential transfer of organic compounds to edible fruit.

An axenic plant culture system for optimal growth in long-term studies

The symbiotic co-evolution of plants and microbes leads to difficulties in understanding which of the two components is responsible for a given environmental response. Plant-microbe studies greatly benefit from the ability to grow plants in axenic (sterile) culture. Several studies have used axenic plant culture systems, but experimental procedures are often poorly documented, the plant growth environment is not optimal, and axenic conditions are not rigorously verified. We developed a unique axenic system using inert components that promotes plant health and can be kept sterile for at least 70 d. Crested wheatgrass (Agropyron cristatum cv. CDII) plants were grown in sand within flow-through glass columns that were positively pressured with filtered air. Plant health was optimized by regulating temperature, light level, CO2 concentration, humidity, and nutrients. The design incorporates several novel aspects, such as pretreatment of the sand with Fe, graduated sand layers to optimize the air-water balance of the root zone, and modification of a laminar flow hood to serve as a plant growth chamber. Adaptations of several sterile techniques were necessary for maintenance of axenic conditions. Axenic conditions were verified by plating and staining leachates as well as a rhizoplane stain. This system was designed to study nutrient and water stress effects on root exudates, but is useful for assessing a broad range of plant-microbe-environment interactions. Based on total organic C analysis, 74% of exudates was recovered in the leachate, 6% was recovered in the bulk sand, and 17% was recovered in the rhizosphere sand. Carbon in the leachate after 70 d reached 255 microg d(-1). Fumaric, malic, malonic, oxalic, and succinic acids were measured as components of the root exudates.

Uptake of nonylphenol and nonylphenol ethoxylates by crested wheatgrass

Nonylphenol (NP) and other hydrophobic biodegradation intermediates of nonylphenol ethoxylate (NPE) surfactants have been identified in wastewater treatment biosolids. These biosolids often are land applied, but little is known regarding the potential uptake of biosolid-derived contaminants by plants. Hydroponic experiments, 11 to 14 weeks in duration, were conducted to examine the uptake and translocation of 14C and unlabeled NP, nonylphenol tetraethoxylate (NPE4), and nonylphenol nonylethoxylate (NPE9) by crested wheatgrass (Agropyron cristatum). Phenol also was evaluated for comparison. Plant tissue was analyzed for 14C and for the parent compounds. Volatilization from the hydroponic system and rhizosphere mineralization also were quantified. At the conclusion of the study, most of the plant-associated 14C was found in the roots (NP = 98%, NPE4 = 92%, and NPE9 = 81%). Concentrations of 14C in the foliar tissue ranged from 0.002 to 0.045 mg-equivalent per kg (dry wt), but no parent compounds were detected, implying that the 14C was unextractable or in the form of metabolites. Transpiration stream concentration factors for NP, NPE4, and NPE9, calculated assuming the 14C was parent compound, were 0.012, 0.032, and 0.066, respectively. Little mineralization was observed for NP, NPE4, and NPE9 in the hydroponic system; however, for phenol, 16 to 30% of the added 14C was mineralized.

Ethylene synthesis and sensitivity in crop plants

Closed and semi-closed plant growth chambers have long been used in studies of plant and crop physiology. These studies include the measurement of photosynthesis and transpiration via photosynthetic gas exchange. Unfortunately, other gaseous products of plant metabolism can accumulate in these chambers and cause artifacts in the measurements. The most important of these gaseous byproducts is the plant hormone ethylene (C2H4). In spite of hundreds of manuscripts on ethylene, we still have a limited understanding of the synthesis rates throughout the plant life cycle. We also have a poor understanding of the sensitivity of intact, rapidly growing plants to ethylene. We know ethylene synthesis and sensitivity are influenced by both biotic and abiotic stresses, but such whole plant responses have not been accurately quantified. Here we present an overview of basic studies on ethylene synthesis and sensitivity.

Night temperature has a minimal effect on respiration and growth in rapidly growing plants

BACKGROUND AND AIMS

Carbon gain depends on efficient photosynthesis and adequate respiration. The effect of temperature on photosynthetic efficiency is well understood. In contrast, the temperature response of respiration is based almost entirely on short-term (hours) measurements in mature organisms to develop Q(10) values for maintenance and whole-plant respiration. These Q(10) values are then used to extrapolate across whole life cycles to predict the influence of temperature on plant growth.

METHODS

In this study, night temperature in young, rapidly growing plant communities was altered from 17 to 34 degrees C for up to 20 d. Day temperature was maintained at 25 degrees C. CO(2) gas-exchange was continuously monitored in ten separate chambers to quantify the effect of night-temperature on respiration, photosynthesis and the efficiency of carbon gain (carbon use efficiency).

KEY RESULTS

Respiration increased only 20-46 % for each 10 degrees C rise in temperature (total respiratory Q(10) of between 1.2 to about 1.5). This change resulted in only a 2-12 % change in carbon use efficiency, and there was no effect on cumulative carbon gain or dry mass. No acclimation of respiration was observed after 20 d of treatment.

CONCLUSIONS

These findings indicate that whole-plant respiration of rapidly growing plants has a small sensitivity to temperature, and that the sensitivity does not change among the species tested, even after 20 d of treatment. Finally, the results support respiration models that separate respiration into growth and maintenance components.

Exploring the limits of crop productivity: beyond the limits of tipburn in lettuce

The productivity of lettuce in a combination of high light, high temperature, and elevated CO2 has not been commonly studied because rapid growth usually causes a calcium deficiency in meristems called tipburn, which greatly reduces quality and marketability. We eliminated tipburn by blowing air directly onto the meristem, which allowed us to increase the photosynthetic photon flux (PPF) to 1000 micromoles m-2 s-1 (57.6 mol m-2 d-1); two to three times higher than normally used for lettuce. Eliminating tipburn doubled edible yield at the highest PPF level. In addition to high PPF, CO2 was elevated to 1200 micromoles m-2 mol-1, which increased the temperature optimum from 25 to 30 degrees C. The higher temperature increased leaf expansion rate, which improved radiation capture and more than doubled yield. Photosynthetic efficiency, measured as canopy quantum yield in a whole-plant gas exchange system, steadily increased up to the highest temperature of 32 degrees C in high CO2. The highest productivity was 19 g m-2 d-1 of dry biomass (380 g d-1 fresh mass) averaged over the 23 days the plants received light. Without the limitation of tipburn, the combination of high PPF, high temperature, and elevated CO2 resulted in a 4-fold increase in growth rate over productivity in conventional environments.

Exploring the limits of crop productivity: beyond the limits of tipburn in lettuce

The productivity of lettuce in a combination of high light, high temperature, and elevated CO2 has not been commonly studied because rapid growth usually causes a calcium deficiency in meristems called tipburn, which greatly reduces quality and marketability. We eliminated tipburn by blowing air directly onto the meristem, which allowed us to increase the photosynthetic photon flux (PPF) to 1000 micromoles m-2 s-1 (57.6 mol m-2 d-1); two to three times higher than normally used for lettuce. Eliminating tipburn doubled edible yield at the highest PPF level. In addition to high PPF, CO2 was elevated to 1200 micromoles m-2 mol-1, which increased the temperature optimum from 25 to 30 degrees C. The higher temperature increased leaf expansion rate, which improved radiation capture and more than doubled yield. Photosynthetic efficiency, measured as canopy quantum yield in a whole-plant gas exchange system, steadily increased up to the highest temperature of 32 degrees C in high CO2. The highest productivity was 19 g m-2 d-1 of dry biomass (380 g d-1 fresh mass) averaged over the 23 days the plants received light. Without the limitation of tipburn, the combination of high PPF, high temperature, and elevated CO2 resulted in a 4-fold increase in growth rate over productivity in conventional environments.

Sensitivity of wheat and rice to low levels of atmospheric ethylene

Ethylene (C2H4) gas is produced throughout the life cycle of plants and can accumulate in closed growth chambers to levels 100 times higher than in outside environments. Elevated atmospheric C2H4 can cause a variety of abnormal responses, but the sensitivity to elevated C2H4 is not well characterized. We evaluated the C2H4 sensitivity of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) in five studies. The first three studies compared the effects of continuous C2H4 levels ranging from 0 to 1000 nmol mol-1 (ppb) in a growth chamber throughout the life cycle of the plants. A short-term 1000 nmol mol-1 treatment was included in which exposure was stopped at anthesis. Yield was reduced by 36% in wheat and 63% in rice at 50 nmol mol-1 and both species were virtually sterile when continuously exposed to 1000 nmol mol-1. However, the yield reductions were much less with exposure that stopped at anthesis, suggesting the detrimental effect of C2H4 on yield was greatest around the time of seed set. Two additional studies evaluated the differential sensitivity of two wheat cultivars (Super Dwarf and USU-Apogee) to 50 nmol mol-1 C2H4 at three CO2 levels [350, 1200, 5000 micromoles mol-1 (ppm)] in a greenhouse. Yield of USU-Apogee was not significantly reduced by C2H4 but the yield of Super Dwarf was reduced by 60%. Elevated CO2 did not influence the sensitivity to C2H4. A difference in the C2H4 sensitivity of USU-Apogee between greenhouse and growth chamber trials suggests that C2H4 sensitivity is dependent on the environment. Collectively, the data suggest that relatively low levels of C2H4 could induce anomalous plant responses by accumulation in greenhouses and growth chambers with inadequate ventilation. The data also suggest that C2H4 sensitivity can be reduced by both genetic and environmental manipulations.

Anaerobic conditions improve germination of a gibberellic acid deficient rice

Dwarf plants are useful in research because multiple plants can be grown in a small area. Rice (Oryza sativa L.) is especially important since its relatively simple genome has recently been sequenced. We are characterizing a gibberellic acid (GA) mutant of rice (japonica cv 'Shiokari,' line N-71) that is extremely dwarf (20 cm tall). Unfortunately, this GA mutation is associated with poor germination (70%) under aerobic conditions. Neither exogenous GA nor a dormancy-breaking heat treatment improved germination. However, 95% germination was achieved by germinating the seeds anaerobically, either in a pure N2 environment or submerged in unstirred tap water. The anaerobic conditions appear to break a mild post-harvest dormancy in this rice cultivar.

Diffuse light and wheat radiation-use efficiency in a controlled environment

Radiation-use efficiency (dry matter produced per unit absorbed radiation) of a spring wheat (Triticum aestivum L., cv. Veery-10) was 40% higher in controlled growth chamber experiments than under optimal field conditions. Simulations with CERES-Wheat, a field model modified to account for growth chamber conditions, suggest that the observed increase in radiation-use efficiency was due to the large fraction of diffuse light in the experimental chamber. Under optimal conditions in the field, the highest crop growth rates occur when the daily photosynthetic photon flux (PPF) is at its highest levels (50-60 mol m-2 d-1). However, these high growth rates do not appear to be associated with the highest radiation-use efficiency. High PPF levels in the field occur on clear days when the fraction of direct radiation is high and the diffuse fraction is low. In controlled environments with reflective walls, high PPF levels with a large fraction of diffuse radiation can be obtained. Diffuse radiation penetrates to the lower leaves of a canopy better than direct radiation, with the result that the upper leaves are less light saturated and the lower leaves receive more light, increasing radiation-use efficiency, and thus growth rates. The data and model simulations presented here suggest that when diffuse light is a high fraction of the total PPF crop productivity can exceed the highest values attainable in the field under optimal conditions.

An approach to crop modeling with the energy cascade

Use of plants in advanced life support requires models of crop growth to analyze data, to evaluate areas for improvement, and, for design and engineering, to predict the gas exchanges of crops. We used data from experiments at Utah State University and the Kennedy Space Center for wheat (Triticum aestivum L.) and examined it for time dependence of the major three components in the energy cascade: photosynthetic photon absorption, canopy quantum yield, and carbon use efficiency. From the Utah State data, we developed a model with a total of five trends: absorption increasing until canopy closure, then constant; quantum yield as constant, then decreasing during senescence; carbon use as constant. This system probably is the lower limit of simplicity to which a model can be reduced and yet provide substantial utility. We demonstrated this utility by using the model to predict photosynthesis and respiration for experiments at Kennedy Space Center. The most uncertainty arose in predicting a start time for the senescent decrease of canopy quantum yield. The model should be generally applicable to other crops grown in controlled environments, as a generic tool for the design of life support systems.

Mass transfer in the biological fast lane: high CO2 and a shallow root zone

Elevated atmospheric CO2, which is common in regenerative systems, increases photosynthesis, plant growth, and root respiration, which increases the O2 demand in the root zone. Closed systems must make efficient use of volume and thus have shallow root zones. The root density and O2 demand in these artificial systems is 10 to 100 times higher than in field environments. Rapid hydroponic flow rates supply O2 to the root zone, but anaerobic microsites occur because of nonuniform flow rates. Our measurements suggest that, probably because of low O2 in such microsites, up to 30% of the nitrogen can volatilize from denitrification. We improved nitrogen recovery to about 85% by increasing the solution flow rate and reducing the nitrate concentration in solution to 100 micromoles.

Phasic temperature control appraised with the Ceres-Wheat model

Phasic control refers to the specification of a series of different environmental conditions during a crop's life cycle, with the goal of optimizing some aspect of productivity. Because of the enormous number of possible scenarios, phasic control is an ideal situation for modeling to provide guidance prior to experiments. Here we use the Ceres-Wheat model, modified for hydroponic growth chambers, to examine temperature effects. We first establish a baseline by running the model at constant temperatures from 10 degrees C to 30 degrees C. Grain yield per day peaks at 15 degrees C at a value that is 25% higher than the yield at the commonly used 23 degrees C. We then show results for phasic control limited to a single shift in temperature and, finally, we examine scenarios that allow each of the five phases of the life cycle to have a different temperature. Results indicate that grain yield might be increased by 15-20% over the best yield at constant temperature, primarily from a boosted harvest index, which has the additional advantage of less waste biomass. Such gains, if achievable, would help optimize food production for life support systems. Experimental work should first verify the relationship between yield and temperature, and then move to selected scenarios of phasic control, based on model predictions.