O. Monje

The Limits of Crop Productivity

Plant scientists have sought to maximize the yield of food crops since the beginning of agriculture. There are numerous reports of record food and biomass yields (per unit area) in all major crop plants, but many of the record yield reports are in error because they exceed the maximal theoretical rates of the component processes. In this article, we review the component processes that govern yield limits and describe how each process can be individually measured. This procedure has helped us validate theoretical estimates and determine what factors limit yields in optimal environments.

Evaluation and modification of commercial infra-red transducers for leaf temperature measurement

Accurate measurement of the leaf to air temperature gradient is crucial for the determination of stomatal conductance and other plant responses in both single leaves and in plant canopies. This gradient is often less than 1 degree C, which means that leaf temperature must be known to within about +/- 0.1 degree C. This is a challenging task, but new, miniature infra-red transducers from Exergen Corporation (Newton, MA) and Everest Interscience (Tucson, AZ) can be modified and calibrated to achieve this accuracy. The sensors must be modified to add thermal mass and the Exergen sensor requires a measurement of sensor body temperature. Significant error is caused by the discharge of a capacitor in the standard Exergen sensor, but we tested it without the capacitor. The sensors respond rapidly to changes in target temperature, but require 2 to 10 minutes to respond to changes in sensor body temperature, which is often the largest source of error. A new, sensitive method for measuring field of view indicates substantial peripheral vision for both sensors and a wider field of view than specified by the manufacturers. Here we describe sensor output as a function of target and sensor body temperatures, and provide a generic (sensor independent) equation that can be used to achieve +/- 0.2 C accuracy with Exergen sensors. The equation was developed and verified using two black body calibrators.

CO2 Crop Growth Enhancement and Toxicity in Wheat and Rice

The effects of elevated CO2 on plant growth are reviewed and the implications for crop yields in regenerative systems are discussed. There is considerable theoretical and experimental evidence indicating that the beneficial effects of CO2 are saturated at about 0.12% CO2 in air. However, CO2 can easily rise above 1% of the total gas in a closed system, and we have thus studied continuous exposure to CO2 levels as high as 2%. Elevating CO2 from 340 to 1200 micromoles mol-1 can increase the seed yield of wheat and rice by 30 to 40%; unfortunately, further CO2 elevation to 2500 micromoles mol-1 (0.25%) has consistently reduced yield by 25% compared to plants grown at 1200 micromoles mol-1; fortunately, there was only an additional 10% decrease in yield as the CO2 level was further elevated to 2% (20,000 micromoles mol-1). Yield increases in both rice and wheat were primarily the result of increased number of heads per m2, with minor effects on seed number per head and seed size. Yield increases were greatest in the highest photosynthetic photon flux. We used photosynthetic gas exchange to analyze CO2 effects on radiation interception, canopy quantum yield, and canopy carbon use efficiency. We were surprised to find that radiation interception during early growth was not improved by elevated CO2. As expected, CO2 increased quantum yield, but there was also a small increase in carbon use efficiency. Super-optimal CO2 levels did not reduce vegetative growth, but decreased seed set and thus yield. The reduced seed set is not visually apparent until final yield is measured. The physiological mechanism underlying CO2 toxicity is not yet known, but elevated CO2 levels (0.1 to 1% CO2) increase ethylene synthesis in some plants and ethylene is a potent inhibitor of seed set in wheat.

Canopy photosynthesis and transpiration in micro-gravity: 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.

Quantifying energy and mass transfer in crop canopies: Sensors for measurement of temperature and air velocity☆

Here we report on the in situ performance of inexpensive, miniature sensors that have increased our ability to measure mass and energy fluxes from plant canopies in controlled environments: 1. Surface temperature. Canopy temperature measurements indicate changes in stomatal aperture and thus latent and sensible heat fluxes. Infrared transducers from two manufacturers (Exergen Corporation, Newton, MA; and Everest Interscience, Tucson, AZ, USA) have recently become available. Transducer accuracy matched that of a more expensive hand-held infrared thermometer. 2. Air velocity varies above and within plant canopies and is an important component in mass and energy transfer models. We tested commercially-available needle, heat-transfer anemometers (1 x 50 mm cylinder) that consist of a fine-wire thermocouple and a heater inside a hypodermic needle. The needle is heated and wind speed determined from the temperature rise above ambient. These sensors are particularly useful in measuring the low wind speeds found within plant canopies. 3. Accurate measurements of air temperature adjacent to plant leaves facilitates transport phenomena modeling. We quantified the effect of radiation and air velocity on temperature rise in thermocouples from 10 to 500 micrometers. At high radiation loads and low wind speeds, temperature errors were as large as 7 degrees C above air temperature.

Solar Food Production and Life Support in Space Exploration

Long duration space travel and settlement in planetary colonies will ultimately require plant production systems to provide food and bioregenerative life support. Lighting is a critical enabling technology for plant production in human exploration missions. To date, electric lighting has been used for enclosed plant chamber studies for bioregenerative research because it can provide uniform environments among laboratories for comparative crop testing. However, the low efficiency of electric lighting adds thermal control challenges and and increases system mass. This paper discusses the effectiveness of a direct solar plant lighting system based on the fiber optic solarlight transmission. It is shown that the direct solar lighting system will significantly reduce the size and the weight of the plant growth system for food production and life support in human exploration missions.