Adaptation to high CO2 concentration in an optimal environment: radiation capture, canopy quantum yield and carbon use efficiency

The physiology of plants in the context of space exploration

The stress that the space environment can induce on plant physiology is of both abiotic and biotic nature. The abiotic space environment is characterized by ionizing radiation and altered gravity, geomagnetic field (GMF), pressure, and light conditions. Biotic interactions include both pathogenic and beneficial interactions. Here, we provide an overall picture of the effects of abiotic and biotic space-related factors on plant physiology. The knowledge required for the success of future space missions will lead to a better understanding of fundamental aspects of plant physiological responses, thus providing useful tools for plant breeding and agricultural practices on Earth.

Opportunities to produce food from substantially less land

The vast majority of the food we eat comes from land-based agriculture, but recent technological advances in agriculture and food technology offer the prospect of producing food using substantially less or even virtually no land. For example, indoor vertical farming can achieve very high yields of certain crops with a very small area footprint, and some foods can be synthesized from inorganic precursors in industrial facilities. Animal-based foods require substantial land per unit of protein or per calorie and switching to alternatives could reduce demand for some types of agricultural land. Plant-based meat substitutes and those produced through fermentation are widely available and becoming more sophisticated while in the future cellular agricultural may become technically and economical viable at scale. We review the state of play of these potentially disruptive technologies and explore how they may interact with other factors, both endogenous and exogenous to the food system, to affect future demand for land.

The Impact of a Six-Hour Light-Dark Cycle on Wheat Ear Emergence, Grain Yield, and Flour Quality in Future Plant-Growing Systems

Cultivating wheat (Triticum aestivum) in a closed environment offers applications in both indoor farming and in outer-space farming. Tailoring the photoperiod holds potential to shorten the growth cycle, thereby increasing the annual number of cycles. As wheat is a long-day plant, a night shorter than a critical length is required to induce flowering. In growth chambers, experiments were conducted to examine the impact of a 6 h light-dark cycle on the timing of wheat ear emergence, grain yield, and flour quality. Under equal daily light-integral conditions, the 6 h light-dark cycle promoted growth and development, resulting in accelerated ear emergence when compared to a 12 h cycle, additionally indicating that 12 h of darkness was excessive. To further stimulate heading and increase yield, the 6 h cycle was changed at the onset of stem elongation to a 14 h-10 h, mimicking spring conditions, and maintained until maturity. This successful transition was then combined with two levels of light intensity and nutrient solution, which did not significantly impact yield, while tillering and grain ripening did increase under higher light intensities. Moreover, it enabled manipulation of the baking quality, although lower-end falling numbers were observed. In conclusion, combining a 6 h light-dark cycle until stem elongation with a 14 h-10 h cycle presents a promising strategy for increasing future wheat production in closed environments. The observation of low falling numbers underscores the importance of factoring in flour quality when designing the wheat-growing systems of the future.

The Mechanisms Responsible for N Deficiency in Well-Watered Wheat Under Elevated CO2

Elevated CO₂ concentration [e(CO₂)] often promotes plant growth with a decrease in tissue N concentration. In this study, three experiments, two under hydroponic and one in well-watered soil, including various levels or patterns of CO₂, humidity, and N supply were conducted on wheat (Triticum aestivum L.) to explore the mechanisms of e[CO₂]-induced N deficiency (ECIND). Under hydroponic conditions, N uptake remained constant even as transpiration was limited 40% by raising air relative humidity and only was reduced about 20% by supplying N during nighttime rather than daytime with a reduction of 85% in transpiration. Compared to ambient CO₂ concentration, whether under hydroponic or well-watered soil conditions, and whether transpiration was kept stable or decreased to 12%, e[CO₂] consistently led to more N uptake and higher biomass, while lower N concentration was observed in aboveground organs, especially leaves, as long as N supply was insufficient. These results show that, due to compensation caused by active uptake, N uptake can be uncoupled from water uptake under well-watered conditions, and changes in transpiration therefore do not account for ECIND. Similar or lower tissue NO 3 - -N concentration under e[CO₂] indicated that NO 3 - assimilation was not limited and could therefore also be eliminated as a major cause of ECIND under our conditions. Active uptake has the potential to bridge the gap between N taken up passively and plant demand, but is limited by the energy required to drive it. Compared to ambient CO₂ concentration, the increase in N uptake under e[CO₂] failed to match the increase of carbohydrates, leading to N dilution in plant tissues, the apparent dominant mechanism explaining ECIND. Lower N concentration in leaves rather than roots under e[CO₂] validated that ECIND was at least partially also related to changes in resource allocation, apparently to maintain root uptake activity and prevent more serious N deficiency.

Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids

LED technology facilitates a range of spectral quality, which can be used to optimize photosynthesis, plant shape and secondary metabolism. We conducted three studies to investigate the effect of blue photon fraction on yield and quality of medical hemp. Conditions were varied among studies to evaluate potential interactions with environment, but all environmental conditions other than the blue photon fraction were maintained constant among the five-chambers in each study. The photosynthetic photon flux density (PPFD, 400 to 700 nm) was rigorously maintained at the set point among treatments in each study by raising the fixtures. The lowest fraction of blue photons was 4% from HPS, and increased to 9.8, 10.4, 16, and 20% from LEDs. There was a consistent, linear, 12% decrease in yield in each study as the fraction of blue photons increased from 4 to 20%. Dry flower yield ranged from 500 to 750 g m-2. This resulted in a photon conversion efficacy of 0.22 to 0.36 grams dry flower mass yield per mole of photons. Yield was higher at a PPFD of 900 than at 750 μmol m-2 s-1. There was no effect of spectral quality on CBD or THC concentration. CBD and THC were 8% and 0.3% at harvest in trials one and two, and 12% and 0.5% in trial three. The CBD/THC ratio was about 25 to 1 in all treatments and studies. The efficacy of the fixtures ranged from 1.7 (HPS) to 2.5 μmol per joule (white+red LED). Yield under the white+red LED fixture (10.4% blue) was 4.6% lower than the HPS on a per unit area basis, but was 27% higher on a per dollar of electricity basis. These findings suggest that fixture efficacy and initial cost of the fixture are more important for return on investment than spectral distribution at high photon flux.

Impact of Different Daily Light Integrals and Carbon Dioxide Concentrations on the Growth, Morphology, and Production Efficiency of Tomato Seedlings

Indoor growing systems with light-emitting diodes offer advantages for the growth of tomato seedlings through uniform and optimized environmental conditions which increase consistency between plants and growing cycles. CO2 enrichment has been shown to improve the yield of crops. Thus, this research aimed to characterize the effects of varied light intensities and CO2 enrichment on the growth, morphology, and production efficiency of tomato seedlings in indoor growing systems. Four tomato cultivars, "Florida-47 R," "Rebelski," "Maxifort," and "Shin Cheong Gang," were subjected to three different daily light integrals (DLIs) of 6.5, 9.7, and 13 mol m-2 d-1 with a percent photon flux ratio of 40 blue:60 red and an end-of-day far-red treatment of 5 mmol m-2 d-1. The plants were also subjected to three different CO2 concentrations: 448 ± 32 (400-ambient), 1010 ± 45 (1000), and 1568 ± 129 (1600) μmol mol-1. Temperature was maintained at 24.3°C ± 0.48/16.8°C ± 1.1 (day/dark; 22.4°C average) and relative humidity at 52.56 ± 8.2%. Plant density was 1000 plants m-2 until canopy closure. Morphological measurements were conducted daily to observe the growth response over time. In addition, data was collected to quantify the effects of each treatment. The results showed increases in growth rate with increases in the DLI and CO2 concentration. In addition, CO2 enrichment to 1000-1600 μmol mol-1 increased the light use efficiency (gDM mol-1 applied) by 38-44%, and CO2 enrichment to 1600 μmol mol-1 did not result in any additional increase on shoot fresh mass, shoot dry mass, and stem extension. However, the net photosynthetic rate obtained with 1600 μmol mol-1 was 31 and 68% higher than those obtained with 1000 and 400 μmol mol-1, respectively. Furthermore, the comparison of the light and CO2 treatment combinations with the control (13 mol m-2 d-1-400CO2) revealed that the plants subjected to 6.5DLI-1600CO2, 9.7DLI-1000CO2, and 9.7DLI-1600CO2 treatment combinations exhibited the same growth rate as the control plants but with 25-50% less DLI. Furthermore, two treatment combinations (13.0DLI-1000CO2 and 13.0DLI-1600CO2) were associated with the consumption of comparable amount of energy but increased plant growth by 24-33%.

Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO2 Fixation and Increased Photon Capture During Long-Term Studies: Implications for Re-Defining PAR

Far-red photons regulate shade avoidance responses and can have powerful effects on plant morphology and radiation capture. Recent studies have shown that far-red photons (700 to 750 nm) efficiently drive photosynthesis when added to traditionally defined photosynthetic photons (400-700 nm). But the long-term effects of far-red photons on canopy quantum yield have not yet been determined. We grew lettuce in a four-chamber, steady-state canopy gas-exchange system to separately quantify canopy photon capture, quantum yield for CO₂ fixation, and carbon use efficiency. These measurements facilitate a mechanistic understanding of the effect of far-red photons on the components of plant growth. Day-time photosynthesis and night-time respiration of lettuce canopies were continuously monitored from seedling to harvest in five replicate studies. Plants were grown under a background of either red/blue or white light, each background with or without 15% (50 μmol m^-2 s^-1) of far-red photons substituting for photons between 400 and 700 nm. All four treatments contained 31.5% blue photons, and an equal total photon flux from 400 to 750 nm of 350 μmol m^-2 s^-1. Both treatments with far-red photons had higher canopy photon capture, increased daily carbon gain (net photosynthesis minus respiration at night), and 29 to 31% more biomass than control treatments. Canopy quantum yield was similar among treatments (0.057 ± 0.002 mol of CO₂ fixed in gross photosynthesis per mole of absorbed photons integrated over 400 to 750 nm). Carbon use efficiency (daily carbon gain/gross photosynthesis) was also similar for mature plants (0.61 ± 0.02). Photosynthesis increased linearly with increasing photon capture and had a common slope among all four treatments, which demonstrates that the faster growth with far-red photon substitution was caused by enhanced photon capture through increased leaf expansion. The equivalent canopy quantum yield among treatments indicates that the absorbed far-red photons were equally efficient for photosynthesis when acting synergistically with the 400-700 nm photons.

Wheat yield potential in controlled-environment vertical farms

Wheat is the most important food crop worldwide, grown across millions of hectares. Wheat yields in the field are usually low and vary with weather, soil, and crop management practices. We show that yields for wheat grown in indoor vertical farms under optimized growing conditions would be several hundred times higher than yields in the field due to higher yields, several harvests per year, and vertically stacked layers. Wheat grown indoors would use less land than field-grown wheat, be independent of climate, reuse most water, exclude pests and diseases, and have no nutrient losses to the environment. However, given the high energy costs for artificial lighting and capital costs, it is unlikely to be economically competitive with current market prices.

Scaling current cereal production to a growing global population will be a challenge. Wheat supplies approximately one-fifth of the calories and protein for human diets. Vertical farming is a possible promising option for increasing future wheat production. Here we show that wheat grown on a single hectare of land in a 10-layer indoor vertical facility could produce from 700 ± 40 t/ha (measured) to a maximum of 1,940 ± 230 t/ha (estimated) of grain annually under optimized temperature, intensive artificial light, high CO₂ levels, and a maximum attainable harvest index. Such yields would be 220 to 600 times the current world average annual wheat yield of 3.2 t/ha. Independent of climate, season, and region, indoor wheat farming could be environmentally superior, as less land area is needed along with reuse of most water, minimal use of pesticides and herbicides, and no nutrient losses. Although it is unlikely that indoor wheat farming will be economically competitive with current market prices in the near future, it could play an essential role in hedging against future climate or other unexpected disruptions to the food system. Nevertheless, maximum production potential remains to be confirmed experimentally, and further technological innovations are needed to reduce capital and energy costs in such facilities.

Hardware Validation of the Advanced Plant Habitat on ISS: Canopy Photosynthesis in Reduced Gravity

The Advanced Plant Habitat (APH) is the largest research plant growth facility deployed on the International Space Station (ISS). APH is a fully enclosed, closed-loop plant life support system with an environmentally controlled growth chamber designed for conducting both fundamental and applied plant research during experiments extending as long as 135 days. APH was delivered to the ISS in parts aboard two commercial resupply missions: OA-7 in April 2017 and SpaceX-11 in June 2017, and was assembled and installed in the Japanese Experiment Module Kibo in November 2018. We report here on a 7-week-long hardware validation test that utilized a root module planted with both Arabidopsis (cv. Col 0) and wheat (cv. Apogee) plants. The validation test examined the APH's ability to control light intensity, spectral quality, humidity, CO₂ concentration, photoperiod, temperature, and root zone moisture using commanding from ground facilities at the Kennedy Space Center (KSC). The test also demonstrated the execution of programmed experiment profiles that scheduled: (1) changes in environmental combinations (e.g., a daily photoperiod at constant relative humidity), (2) predetermined photographic events using the three APH cameras [overhead, sideview, and sideview near-infrared (NIR)], and (3) execution of experimental sequences during the life cycle of a crop (e.g., measure photosynthetic CO₂ drawdown experiments). Arabidopsis and wheat were grown in microgravity to demonstrate crew procedures, planting protocols and watering schemes within APH. The ability of APH to contain plant debris was assessed during the harvest of mature Arabidopsis plants. Wheat provided a large evaporative load that tested root zone moisture control and the recovery of transpired water by condensation. The wheat canopy was also used to validate the ability of APH to measure gas exchange of plants from non-invasive gas exchange measurements (i.e., canopy photosynthesis and respiration). These features were evaluated by executing experiment profiles that utilized the CO₂ drawdown technique to measure daily rates of canopy photosynthesis and dark-period CO₂ increase for respiration. This hardware validation test confirmed that APH can measure fundamental plant responses to spaceflight conditions.

Trends in seedling growth and carbon-use efficiency vary among broadleaf tree species along a latitudinal transect in eastern North America

Factors constraining the geographic ranges of broadleaf tree species in eastern North America were examined in common gardens along a ~1500 km latitudinal transect travers in grange boundaries of four target species: trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) to the north vs. eastern cottonwood (Populus deltoides) and sweet gum (Liquidambar styraciflua) to the south. In 2006 and 2007, carbon-use efficiency (CUE), the proportion of assimilated carbon retained in biomass, was estimated for seedlings of the four species as the quotient of relative growth rate (RGR) and photosynthesis per unit tree mass (Atree ). In aspen and birch, CUE and RGR declined significantly with increasing growth temperature, which spanned 9 °C across gardens and years. The 37% (relative) CUE decrease from coolest to warmest garden correlated with increases in leaf nighttime respiration (Rleaf ) and the ratio of Rleaf to leaf photosynthesis (R%A ). For cottonwood and sweet gum, however, similar increases in Rleaf and R%A accompanied modest CUE declines, implying that processes other than Rleaf were responsible for species differences in CUE's temperature response. Our findings illustrate marked taxonomic variation, at least among young trees, in the thermal sensitivity of CUE, and point to potentially negative consequences of climate warming for the carbon balance, competitive ability, and persistence of two foundation species in northern temperate and boreal forests.

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.

Variable conductivity and embolism in roots and branches of four contrasting tree species and their impacts on whole-plant hydraulic performance under future atmospheric CO₂ concentration

Anatomical and physiological acclimation to water stress of the tree hydraulic system involves trade-offs between maintenance of stomatal conductance and loss of hydraulic conductivity, with short-term impacts on photosynthesis and long-term consequences to survival and growth. Here, we study the role of variations in root and branch maximum hydraulic specific conductivity (k(s-max)) under high and low soil moisture in determining whole-tree hydraulic conductance (K(tree)) and in mediating stomatal control of gas exchange in four contrasting tree species growing under ambient and elevated CO₂ (CO₂(a) and CO₂(e)). We hypothesized that K(tree) would adjust to CO₂(e) through an increase in root and branch k(s-max) in response to anatomical adjustments. However, physiological changes observed under CO₂(e) were not clearly related to structural change in the xylem of any of the species. The only large effect of CO₂(e) occurred in branches of Liquidambar styraciflua L. and Cornus florida L. where an increase in k(s-max) and a decrease in xylem resistance to embolism (-P₅₀) were measured. Across species, embolism in roots explained the loss of K(tree) and therefore indirectly constituted a hydraulic signal involved in stomatal regulation and in the reduction of G(s-ref), the sap-flux-scaled mean canopy stomatal conductance at a reference vapour pressure deficit of 1 kPa. Across roots and branches, the increase in k(s-max) was associated with a decrease in -P₅₀, a consequence of structural acclimation such as larger conduits, lower pit resistance and lower wood density. Across species, treatment-induced changes in K(tree) translated to similar variation in G(s-ref). However, the relationship between G(s-ref) and K(tree) under CO₂(a) was steeper than under CO₂(e), indicating that CO₂(e) trees have lower G(s-ref) at a given K(tree) than CO₂(a) trees. Under high soil moisture, CO₂(e) greatly reduced G(s-ref). Under low soil moisture, CO₂(e) reduced G(s-ref) of only L. styraciflua and Ulmus alata. In some species, higher xylem dysfunction under CO₂(e) might impact tree performance in a future climate when increased evaporative demand could cause a greater loss of hydraulic function. The results contributed to our knowledge of the physiological and anatomical mechanisms underpinning the responses of tree species to drought and more generally to global change.

On the relative magnitudes of photosynthesis, respiration, growth and carbon storage in vegetation

BACKGROUND AND AIMS

The carbon balance of vegetation is dominated by the two large fluxes of photosynthesis (P) and respiration (R). Mechanistic models have attempted to simulate the two fluxes separately, each with their own set of internal and external controls. This has led to model predictions where environmental change causes R to exceed P, with consequent dieback of vegetation. However, empirical evidence suggests that the R : P ratio is constrained to a narrow range of about 0.4-0.5. Physiological explanations for the narrow range are not conclusive. The aim of this work is to introduce a novel perspective by theoretical study of the quantitative relationship between the four carbon fluxes of P, R, growth and storage (or its inverse, remobilization).

METHODS

Starting from the law of conservation of mass - in this case carbon - equations are derived for the relative magnitudes of all carbon fluxes, which depend on only two parameters: the R : P ratio and the relative rate of storage of carbon in remobilizable reserves. The equations are used to explain observed flux ratios and to analyse incomplete data sets of carbon fluxes.

KEY RESULTS

The storage rate is shown to be a freely varying parameter, whereas R : P is narrowly constrained. This explains the constancy of the ratio reported in the literature. With the information thus gained, a data set of R and P in grassland was analysed, and flux estimates could be derived for the periods after cuts in which plant growth is dominated by remobilization before photosynthesis takes over.

CONCLUSIONS

It is concluded that the relative magnitudes of photosynthesis, respiration, growth and substrate storage are indeed tightly constrained, but because of mass conservation rather than for physiological reasons. This facilitates analysis of incomplete data sets. Mechanistic models, as the embodiment of physiological mechanisms, need to show consistency with the constraints.

Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long-term growth in elevated CO(2) (free-air CO(2) enrichment) and N-fertilization

We investigated how leaf hydraulic conductance (K(leaf)) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO(2) concentrations (CO(2)(a) and CO(2)(e), respectively). We also examined how K(leaf) varies with changes in reference leaf water potential (Psi(leaf-ref)) and stomatal conductance (g(s-ref)) calculated at vapour pressure deficit, D of 1 kPa. We detected significant reductions in K(leaf) caused by N and CO(2)(e), but neither treatment affected pre-dawn or midday Psi(leaf). We also detected a significant CO(2)(e)-induced reduction in g(s-ref) and Psi(leaf-ref). Among treatments, the sensitivity of K(leaf) to Psi(leaf) was directly related to a reference K(leaf) (K(leaf-ref) computed at Psi(leaf-ref)). This liquid-phase response was reflected in a similar gas-phase response, with g(s) sensitivity to D proportional to g(s-ref). Because leaves represented a substantial component of the whole-tree conductance, reduction in K(leaf) under CO(2)(e) affected whole-tree water use by inducing a decline in g(s-ref). The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO(2)(e) controlled morning leaf water status less than CO(2)(a) trees resulting in a higher diurnal loss of K(leaf); (2) the effect of CO(2)(e) on g(s-ref) was manifested only during times of high soil moisture.

Microgravity effects on leaf morphology, cell structure, carbon metabolism and mRNA expression of dwarf wheat

The use of higher plants as the basis for a biological life support system that regenerates the atmosphere, purifies water, and produces food has been proposed for long duration space missions. The objective of these experiments was to determine what effects microgravity (microg) had on chloroplast development, carbohydrate metabolism and gene expression in developing leaves of Triticum aestivum L. cv. USU Apogee. Gravity naive wheat plants were sampled from a series of seven 21-day experiments conducted during Increment IV of the International Space Station. These samples were fixed in either 3% glutaraldehyde or RNAlater or frozen at -25 degrees C for subsequent analysis. In addition, leaf samples were collected from 24- and 14-day-old plants during the mission that were returned to Earth for analysis. Plants grown under identical light, temperature, relative humidity, photoperiod, CO(2), and planting density were used as ground controls. At the morphological level, there was little difference in the development of cells of wheat under microg conditions. Leaves developed in mug have thinner cross-sectional area than the 1g grown plants. Ultrastructurally, the chloroplasts of microg grown plants were more ovoid than those developed at 1g, and the thylakoid membranes had a trend to greater packing density. No differences were observed in the starch, soluble sugar, or lignin content of the leaves grown in microg or 1g conditions. Furthermore, no differences in gene expression were detected leaf samples collected at microg from 24-day-old leaves, suggesting that the spaceflight environment had minimal impact on wheat metabolism.

Climate change impacts on crop yield and quality with CO2 fertilization in China

A regional climate change model (PRECIS) for China, developed by the UK's Hadley Centre, was used to simulate China's climate and to develop climate change scenarios for the country. Results from this project suggest that, depending on the level of future emissions, the average annual temperature increase in China by the end of the twenty-first century may be between 3 and 4 degrees C. Regional crop models were driven by PRECIS output to predict changes in yields of key Chinese food crops: rice, maize and wheat. Modelling suggests that climate change without carbon dioxide (CO2) fertilization could reduce the rice, maize and wheat yields by up to 37% in the next 20-80 years. Interactions of CO2 with limiting factors, especially water and nitrogen, are increasingly well understood and capable of strongly modulating observed growth responses in crops. More complete reporting of free-air carbon enrichment experiments than was possible in the Intergovernmental Panel on Climate Change's Third Assessment Report confirms that CO2 enrichment under field conditions consistently increases biomass and yields in the range of 5-15%, with CO2 concentration elevated to 550 ppm Levels of CO2 that are elevated to more than 450 ppm will probably cause some deleterious effects in grain quality. It seems likely that the extent of the CO2 fertilization effect will depend upon other factors such as optimum breeding, irrigation and nutrient applications.

Microgravity effects on thylakoid, single leaf, and whole canopy photosynthesis of dwarf wheat

The concept of using higher plants to maintain a sustainable life support system for humans during long-duration space missions is dependent upon photosynthesis. The effects of extended exposure to microgravity on the development and functioning of photosynthesis at the leaf and stand levels were examined onboard the International Space Station (ISS). The PESTO (Photosynthesis Experiment Systems Testing and Operations) experiment was the first long-term replicated test to obtain direct measurements of canopy photosynthesis from space under well-controlled conditions. The PESTO experiment consisted of a series of 21-24 day growth cycles of Triticum aestivum L. cv. USU Apogee onboard ISS. Single leaf measurements showed no differences in photosynthetic activity at the moderate (up to 600 micromol m(-2) s(-1)) light levels, but reductions in whole chain electron transport, PSII, and PSI activities were measured under saturating light (>2,000 micromol m(-2) s(-1)) and CO(2) (4000 micromol mol(-1)) conditions in the microgravity-grown plants. Canopy level photosynthetic rates of plants developing in microgravity at approximately 280 micromol m(-2) s(-1) were not different from ground controls. The wheat canopy had apparently adapted to the microgravity environment since the CO(2) compensation (121 vs. 118 micromol mol(-1)) and PPF compensation (85 vs. 81 micromol m(-2) s(-1)) of the flight and ground treatments were similar. The reduction in whole chain electron transport (13%), PSII (13%), and PSI (16%) activities observed under saturating light conditions suggests that microgravity-induced responses at the canopy level may occur at higher PPF intensity.

Microgravity does not alter plant stand gas exchange of wheat at moderate light levels and saturating CO2 concentration

Plant stand gas exchange was measured nondestructively in microgravity during the Photosynthesis Experiment Subsystem Testing and Operations experiment conducted onboard the International Space Station. Rates of evapotranspiration and photosynthesis measured in space were compared with ground controls to determine if microgravity directly affects whole-stand gas exchange of Triticum aestivum. During six 21-day experiment cycles, evapotranspiration was determined continuously from water addition rates to the nutrient delivery system, and photosynthesis was determined from the amount of CO2 added to maintain the chamber CO2 concentration setpoint. Plant stand evapotranspiration, net photosynthesis, and water use efficiency were not altered by microgravity. Although leaf area was significantly reduced in microgravity-grown plants compared to ground control plants, leaf area distribution was not affected enough to cause significant differences in the amounts of light absorbed by the flight and ground control plant stands. Microgravity also did not affect the response of evapotranspiration to changes in chamber vapor pressure difference of 12-day-old wheat plant stands. These results suggest that gravity naïve plants grown at moderate light levels (300 micromol m(-2) s(-1)) behave the same as ground control plants. This implies that future plant-based regenerative life support systems can be sized using 1 g data because water purification and food production rates operate at nearly the same rates as in 1 g at moderate light levels. However, it remains to be verified whether the present results are reproducible in plants grown under stronger light levels.

Seasonal Changes in Canopy Photosynthesis and Respiration, and Partitioning of Photosynthate, in Rice (Oryza sativa L.) Grown Under Free-Air CO2 Enrichment

An increase in atmospheric CO(2) concentration ( [CO(2)]) is generally expected to enhance photosynthesis and biomass. Rice plants (Oryza sativa L.) were grown in ambient CO(2) (AMB) or free-air CO(2)-enrichment (FACE), in which the target [CO(2)] was 200 micromol mol(-1) above AMB. (13)CO(2) was fed to the plants at different stages so we could examine the partitioning of photosynthates. Furthermore, canopy photosynthesis and respiration were measured at those stages. The ratio of (13)C content in the whole plant to the amount of fixed (13)C under FACE was similar to that under AMB at the vegetative stage. However, the ratio under FACE was greater than the ratio under AMB at the grain-filling stage. At the vegetative stage, plants grown under FACE had a larger biomass than those grown under AMB owing to enhancement of canopy photosynthesis by the increased [CO(2)]. On the other hand, at the grain-filling stage, CO(2) enrichment promoted the partitioning of photosynthate to ears, and plants grown under FACE had a greater weight of ears. However, enhancement of ear weight by CO(2) enrichment was not as great as that of biomass at the vegetative stage. Plants grown under FACE did not necessarily show higher canopy photosynthetic rates at the grain-filling stage. Therefore, we concluded that the ear weight did not increase as much as biomass at the vegetative stage owing to a loss of the advantage in CO(2) gain during the grain-filling period.

Using explanatory crop models to develop simple tools for Advanced Life Support system studies

System-level analyses for Advanced Life Support require mathematical models for various processes, such as for biomass production and waste management, which would ideally be integrated into overall system models. Explanatory models (also referred to as mechanistic or process models) would provide the basis for a more robust system model, as these would be based on an understanding of specific processes. However, implementing such models at the system level may not always be practicable because of their complexity. For the area of biomass production, explanatory models were used to generate parameters and multivariable polynomial equations for basic models that are suitable for estimating the direction and magnitude of daily changes in canopy gas-exchange, harvest index, and production scheduling for both nominal and off-nominal growing conditions.

Evaluating and optimizing horticultural regimes in space plant growth facilities

In designing innovative space plant growth facilities (SPGF) for long duration space flight, various limitations must be addressed including onboard resources: volume, energy consumption, heat transfer and crew labor expenditure. The required accuracy in evaluating on board resources by using the equivalent mass methodology and applying it to the design of such facilities is not precise. This is due to the uncertainty of the structure and not completely understanding the properties of all associated hardware, including the technology in these systems. We present a simple criteria of optimization for horticultural regimes in SPGF: Qmax = max [M x (EBI)2/(V x E x T], where M is the crop harvest in terms of total dry biomass in the plant growth system; EBI is the edible biomass index (harvest index), V is volume occupied by the crop; E is the crop light energy supply during growth; T is the crop growth duration. The criterion reflects directly on the consumption of onboard resources for crop production.

Farming in space: environmental and biophysical concerns

The colonization of space will depend on our ability to routinely provide for the metabolic needs (oxygen, water, and food) of a crew with minimal re-supply from Earth. On Earth, these functions are facilitated by the cultivation of plant crops, thus it is important to develop plant-based food production systems to sustain the presence of mankind in space. Farming practices on earth have evolved for thousands of years to meet both the demands of an ever-increasing population and the availability of scarce resources, and now these practices must adapt to accommodate the effects of global warming. Similar challenges are expected when earth-based agricultural practices are adapted for space-based agriculture. A key variable in space is gravity; planets (e.g. Mars, 1/3 g) and moons (e.g. Earth's moon, 1/6 g) differ from spacecraft orbiting the Earth (e.g. Space stations) or orbital transfer vehicles that are subject to microgravity. The movement of heat, water vapor, CO2 and O2 between plant surfaces and their environment is also affected by gravity. In microgravity, these processes may also be affected by reduced mass transport and thicker boundary layers around plant organs caused by the absence of buoyancy dependent convective transport. Future space farmers will have to adapt their practices to accommodate microgravity, high and low extremes in ambient temperatures, reduced atmospheric pressures, atmospheres containing high volatile organic carbon contents, and elevated to super-elevated CO2 concentrations. Farming in space must also be carried out within power-, volume-, and mass-limited life support systems and must share resources with manned crews. Improved lighting and sensor technologies will have to be developed and tested for use in space. These developments should also help make crop production in terrestrial controlled environments (plant growth chambers and greenhouses) more efficient and, therefore, make these alternative agricultural systems more economically feasible food production systems.

Modeling above-canopy CO2 flux and evapotranspiration in wheat

Simulations of above-canopy water vapor and CO2 fluxes were calculated by the USGF linked model of canopy gas exchange and subsurface processes for the 1996-1997 winter wheat season at the AmeriFlux Wheat Site, Oklahoma. Soil surface CO2 flux plus canopy gas exchange and transpiration plus soil evaporation modeled the CO2 and water vapor fluxes, respectively. Parameter values for net photosynthesis, respiration and transpiration were obtained from published sources, generated from Wheat Site data, or estimated by minimizing standard deviation between model and data. The mean measured downward flux of CO2 during rapid growth and maturity of the crop was -0.45 mg m(-2) s(-1) compared to simulated flux of -0.47. Simulated downward CO2 flux exceeded measured values during rapid growth of the crop but underestimated the flux during maturity. For the entire 285-day period, the mean measured upward CO2 flux at night was 0.06 and simulated flux was 0.05.

Effects of atmospheric CO(2) enrichment on plant constituents related to animal and human health

Atmospheric CO(2) enrichment is known to significantly enhance the growth and development of nearly all plants, implying a potential for elevated levels of CO(2) to alter the concentrations of plant constituents related to animal and human health. Our review of this subject indicates that increases in the air's CO(2) content typically lead to reductions in the nitrogen and protein concentrations of animal-sustaining forage and human-sustaining cereal grains when soil nitrogen levels are sub-optimal. When plants are supplied with all the nitrogen they can use, however, no such reductions are observed. CO(2)-enriched plants growing in the natural environment also tend to overcome initial reductions in plant mineral concentrations as time progresses, possibly due to development of larger root systems and consequent enhanced abilities to locate and absorb mineral nutrients. Atmospheric CO(2) enrichment additionally appears to reduce oxidative stresses in plants; and it has been shown to increase the concentration of vitamin C in certain fruits and vegetables. Elevated CO(2) has also been demonstrated to increase the biomass of plants grown for medicinal purposes while simultaneously increasing the concentrations of the disease-fighting substances produced within them. It is likely, therefore, that the ongoing rise in the air's CO(2) content will continue to increase food production around the world, while maintaining the nutritive quality of that food and enhancing the production of certain disease-inhibiting plant compounds.

Wheat response to differences in water and nutritional status between zeoponic and hydroponic growth systems

Hydroponic culture has traditionally been used for controlled environment life support systems (CELSS) because the optimal environment for roots supports high growth rates. Recent developments in zeoponic substrate and microporous tube irrigation (ZPT) also offer high control of the root environment. This study compared the effect of differences in water and nutrient status of ZPT or hydroponic culture on growth and yield of wheat (Triticum aestivum L. cv. USU-Apogee). In a side-by-side test in a controlled environment, wheat was grown in ZPT and recirculating hydroponics to maturity. Water use by plants grown in both culture systems peaked at 15 to 20 L m-2 d-1 up to Day 40, after which it declined more rapidly for plants grown in ZPT culture due to earlier senescence of leaves. No consistent differences in water status were noted between plants grown in the two culture systems. Although yield was similar, harvest index was 28% lower for plants grown in ZPT than in hydroponic culture. Sterile green tillers made up 12 and 0% of the biomass of plants grown in ZPT and hydroponic culture, respectively. Differences in biomass partitioning were attributed primarily to NH4-N nutrition of plants grown in ZPT compared with NO3-N in hydroponic nutrient solution. It is probable that NH4-N-induced Ca deficiency produced excess tillering and lower harvest index for plants grown in ZPT culture. These results suggest that further refinements in zeoponic substrate would make ZPT culture a viable alternative for achieving high productivity in a CELSS.

Canopy photosynthesis and transpiration in microgravity: gas exchange measurements aboard Mir

The SVET Greenhouse on-board the Orbital Station Mir was used to measure canopy photosynthesis and transpiration rates for the first time in space. During the Greenhouse IIB experiment on Mir (June-January 1997), carbon and water vapor fluxes from two wheat (cv. Superdwarf) canopies were measured using the US developed Gas Exchange Measurement System (GEMS). Gas analyzers capable of resolving CO2 concentration differences of 5 micromoles mol-1 against a background of 0.9% CO2, are necessary to measure photosynthetic and respiratory rates on Mir. The ability of the GEMS gas analyzers to measure these CO2 concentration differences was determined during extensive ground calibrations. Similarly, the sensitivity of the analyzers to water vapor was sufficient to accurately measure canopy evapotranspiration. Evapotranspiration, which accounted for over 90% of the water added to the root zone, was estimated using gas exchange and used to estimate substrate moisture content. This paper presents canopy photosynthesis and transpiration data during the peak vegetative phase of development in microgravity.

Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations

We examined the hypothesis that elevated CO2 concentration would increase NO3- absorption and assimilation using intact wheat canopies (Triticum aestivum cv. Veery 10). Nitrate consumption, the sum of plant absorption and nitrogen loss, was continuously monitored for 23 d following germination under two CO2 concentrations (360 and 1000 micromol mol-1 CO2) and two root zone NO3- concentrations (100 and 1000 mmol m3 NO3-). The plants were grown at high density (1780 m-2) in a 28 m3 controlled environment chamber using solution culture techniques. Wheat responded to 1000 micromol mol-1 CO2 by increasing carbon allocation to root biomass production. Elevated CO2 also increased root zone NO3- consumption, but most of this increase did not result in higher biomass nitrogen. Rather, nitrogen loss accounted for the greatest part of the difference in NO3- consumption between the elevated and ambient [CO2] treatments. The total amount of NO3(-)-N absorbed by roots or the amount of NO3(-)-N assimilated per unit area did not significantly differ between elevated and ambient [CO2] treatments. Instead, specific leaf organic nitrogen content declined, and NO3- accumulated in canopies growing under 1000 micromol mol-1 CO2. Our results indicated that 1000 micromol mol-1 CO2 diminished NO3- assimilation. If NO3- assimilation were impaired by high [CO2], then this offers an explanation for why organic nitrogen contents are often observed to decline in elevated [CO2] environments.