Net Carbon Dioxide Exchange Rates Research Articles

Double cropping inVitis viniferaL. cv. Pinot Noir: agronomical and physiological validation

Background and Aims Environmental effects of global warming are leading to extended ripening seasons, which may either require or allow new vineyard management techniques. An innovative double-cropping technique is proposed for temperate climate areas. Methods and Results The principle technique was to maintain the primary crop as well as to obtain a second, late ripening crop through release of dormancy of the auxiliary buds during the current season. Potted Pinot Noir vines were subjected to two forcing treatments over 2 years: trimming all the primary shoots at six nodes and removing any developing laterals at the end of the flowering and pea size stages. Vine growth, yield components and grape composition were monitored on both primary and forced shoots. In the second season, seasonal whole-vine gas exchange was evaluated in detail. Forced shoots carried 40–50% of the vine crop compared to primary shoots and fruit quality was greatly enhanced and higher TA was observed. Forcing treatments reached a similar net carbon dioxide exchange rate per vine compared to unforced Control about 2 weeks following auxiliary budburst. For the remainder of the season, forcing treatments maintained much higher net carbon dioxide exchange rate and water-use efficiency than unforced Control vines. Conclusions Detailed agronomical and physiological evaluation for 2 years confirmed the reliability of the double-cropping technique without compromising the pruning point selection for the next cropping year. Significance of the Study Future field application may disclose that this forcing technique is able to warrant two crops potentially suited to different wine styles within a single season.

Net carbon dioxide exchange rate of wild blueberries is influenced by carbon dioxide concentration, light intensity and air temperature

Net carbon dioxide exchange rate of wild blueberries is influenced by carbon dioxide concentration, light intensity and air temperature

Soil pore-water environment and CO2 emission in a Luvisol as influenced by contrasting tillage

Little is known how contrasting tillage (deep ploughing, top- and sub-soil loosening with straight or bent leg cultivator [BLC], direct drilling [DD]) affect important soil physical properties (total porosity [TP], pore size distribution [PSD], water release characteristics [WRC]) and CO2 emissions from a Luvisol. The study was aimed to alleviate compaction on land that had been under reduced tillage for 4 successive years. Undisturbed core samples were collected from 5–10, 15–20 and 25–30 cm depths for soil WRCs, TP and pore-size distribution determination. A closed chamber method was used to quantify the CO2 emissions from the soil. Soil loosening with straight or BLC produced the highest total soil porosity (on average 0.48 m3 m−3) within 5–30 cm soil layer, while conventional tillage (CT) gave 6%, DD up to 25% reduction. Sub-surface loosening with a BLC was the most effective tool to increase the amount of macro- and mesopores in the top- and sub-soil layers. It produced 21% more macro- and mesopores within 25–30 cm soil layer as compared to the soil loosened with a straight leg cultivator. Plant available water content under CT and DD was lower as compared to that under deep loosening with straight or BLC (23% and 18%, respectively). DD produced 12% lower soil surface net carbon dioxide exchange rate than CT and by 25–28% lower than deep soil loosening with straight or BLC. The increase in micropores within 25–30 cm soil layer caused net carbon dioxide exchange rate reduction. The amount of mesopores within the whole 5–30 cm soil layer acted as a direct dominant factor influencing net CO2 exchange rate (NCER) (Pxy = −3.063; r = 0.86).

Influence of Water Regimes and Potassium Chlorate on Floral Induction, Leaf Photosynthesis and Leaf Water Potential in Longan

This study verifies the influence of water regimes and potassium chlorate (KC1O3) on photosynthetic rate, flower emergence and media moisture content of longan trees. The trees were grown in 150 liters lysimeter tanks filled with fine sand. The experimental design was a 2x2 factorial in completely randomized design (CRD) with 2 factors; 1) two levels of water regimes (well-watered and water deficit) and 2) two levels of KClO3 at 10 and 0 g. The results revealed that the well-watered treatment produced faster days of terminal bud break than that of the water deficit treatment. The 10 g KClO3 treatment induced 91 % flower emergence at 35 days after commencing the treatment, while the 0 g KClO3 treatment had 82 % leaf flushing and had no flower emergence. Water deficit or KClO3 treatments reduced the net carbondioxide (CO2) exchange, transpiration and stomatal conductance rates. Moreover, the combination of well-watered and 0 g KClO3 treatments gave the greatest values of the parameters. The well-watered treatment had higher volumetric water content in the growing medium and leaf water potential than the water deficit treatment, while for the 10 g and 0 g KClO3 treatments had similar the media moisture content.

Long‐Term Influence of Tillage and Fertilization on Net Carbon Dioxide Exchange Rate on Two Soils with Different Textures

The importance of agricultural practices to greenhouse gas mitigation is examined worldwide. However, there is no consensus on soil organic carbon (SOC) content and CO emissions as affected by soil management practices and their relationships with soil texture. No-till (NT) agriculture often results in soil C gain, though, not always. Soil net CO exchange rate (NCER) and environmental factors (SOC, soil temperature [T], and water content [W]), as affected by soil type (loam and sandy loam), tillage (conventional, reduced, and NT), and fertilization, were quantified in long-term field experiments in Lithuania. Soil tillage and fertilization affected total CO flux (heterotrophic and autotrophic) through effect on soil SOC sequestration, water, and temperature regime. After 11 yr of different tillage and fertilization management, SOC content was 23% more in loam than in sandy loam. Long-term NT contributed to 7 to 27% more SOC sequestration on loam and to 29 to 33% more on sandy loam compared with reduced tillage (RT) or conventional tillage (CT). Soil water content in loam was 7% more than in sandy loam. Soil gravimetric water content, averaged across measurement dates and fertilization treatments, was significantly less in NT than CT and RT in both soils. Soil organic carbon content and water storage capacity of the loam and sandy loam soils exerted different influences on NCER. The NCER from the sandy loam soil was 13% greater than that from the loam. In addition, NCER was 4 to 9% less with NT than with CT and RT systems on both loam and sandy loam soils. Application of mineral NPK fertilizers promoted significantly greater NCER from loam but suppressed NCER by 15% from sandy loam.

Changes in water relations, photosynthetic activity and proline accumulation in one-year-old olive trees (Olea europaea L. cv. Chemlali) in response to NaCl salinity

The comparative responses of young olive trees (Olea europaea L. cv “Chemlali”) to different NaCl salinity levels were investigated over 11 months. One-year-old own rooted plants were grown in 10-L pots containing sand and perlite mixture (1:3 v/v). Trees were subjected to three irrigation treatments: CP (control plants that were irrigated with fresh water); SS1 (salt stressed plants irrigated with water containing 100 mM NaCl) and SS2 plants (salt stressed plants irrigated with water containing 200 mM NaCl). Shoot elongation rate, relative water content, leaf water potential and net carbon dioxide exchange rates decreased significantly with increased NaCl salinity level. Under stressed conditions, the increase of Na+ and Cl− ions in both leaves and roots was accompanied with that of proline and soluble sugars. The above results show that the accumulation of proline and sugars under stressed conditions could play a role in salt tolerance. The absence of toxicity symptoms under both stress treatments and the superior photosynthetic activity recorded in SS1-treated plants suggest that cv Chemlali is better able to acclimatize to 100 mM NaCl than at 200 mM NaCl. Our findings indicate that saline water containing 100 mM NaCl, the most available water in arid region in Tunisia, can be recommended for the irrigation of cv Chemlali in the arid south of Tunisia.

Carbon dioxide concentration at night affects translocation from soybean leaves.

Studies have indicated that the concentration of carbon dioxide [CO2] during the dark period may influence plant dry matter accumulation. It is often suggested that these effects on growth result from effects of [CO2] on rates of respiration, but responses of respiration to [CO2] remain controversial, and connections between changes in respiration rate and altered growth rate have not always been clear. The present experiments tested whether translocation, a major consumer of energy from respiration in exporting leaves, was sensitive to [CO2]. Nineteen-day-old soybean plants grown initially at a constant [CO2] of 350 micromol mol(-1) were exposed to three consecutive nights with a [CO2] of 220-1400 micromol mol(-1), with a daytime [CO2] of 350 micromol mol(-1). Change in dry mass of the individual second, third and fourth trifoliate leaves over the 3-d period was determined, along with rates of respiration and photosynthesis of second leaves, measured by net CO2 exchange. Translocation was determined from mass balance for second leaves. Additional experiments were conducted where the [CO2] around individual leaves was controlled separately from that of the rest of the plant. Results indicated that low [CO2] at night increased both respiration and translocation and elevated [CO2] decreased both processes, to similar relative extents. The effect of [CO2] during the dark on the change in leaf mass over 3 d was largest in second leaves, where the change in mass was about 50% greater at 1400 micromol mol(-1) CO2 than at 220 micromol mol(-1) CO2. The response of translocation to [CO2] was localized in individual leaves. Results indicated that effects of [CO2] on net carbon dioxide exchange rate in the dark either caused or reflected a change in a physiologically important process which is known to depend on energy supplied by respiration. Thus, it is unlikely that the observed effects of [CO2] on respiration were artefacts of the measurement process in this case.

Net Carbon Dioxide Exchange Rates of Raspberries at Varying Irradiances, Carbon Dioxide Concentrations, and Air and Soil Temperatures

The influence of irradiance, CO2, and temperature on whole-plant net C exchange rate (NCER) of micropropagated raspberries (Rubus idaeus L. cv. `Heritage') was examined in 1994. Irradiances >1000 μmol–m–2–s–1 PAR were required for light saturation, and net photosynthesis (Pn) greatly increased under CO2 enrichment (up to 2000 μl–liter–1) and was optimum at 17C. Temperature effects were separated in another experiment using varying air and soil temperatures (15, 20, 25, 30, and 35C) under saturated light and ambient CO2 levels (350 μl–liter–1). Both air and soil temperature influenced net Pn, with maximum rates occurring at an air/soil temperature of 17/25C and each contributing 71.2% and 26.7%, respectively, to the total variation explained by a polynomial model (R2 = 0.96). Dark respiration and root respiration rates also increased significantly with elevated air and soil temperatures. Therefore, results from this study indicate that maximum net Pn occurred at an air/soil temperature of 17/25C and that irradiance, CO2 levels, and shoot and root temperatures are all important factors in examining NCER in raspberries.

Crop Net Carbon Dioxide Exchange Rate and Radiation Use Efficiency in Soybean

AbstractPlant growth is directly related to the fraction of the C fixed by photosynthesis that is converted into phytomass. We report here the results of a study aiming at the estimation of the net CO2 exchange rates (CNCER) of a soybean crop over the growing season. Hourly values of CNCER were estimated based on eddy correlation measurements of CO2 fluxes above the canopy, on chamber measurements of CO2 fluxes at the soil surface, and on the assumption that root respiration can be computed as a fraction of plant daily photosynthesis. Mean daytime CNCER varied between 0.70 and 0.83 mg m−2s−1 for a leaf area index (LAI) greater than 1, with a maximum hourly value of 1.48 mg m−2s−1. Absolute mean nighttime CNCER values were usually <20% of daytime values. Hourly CNCER varied nonlinearly with intercepted photosynthetically active radiation (IPAR). The accuracy of the CNCER estimates was evaluated by comparing the radiation use efficiency of the soybean crop calculated using CNCER estimates with radiation use efficiency calculated using phytomass accumulation. Agreement between the two approaches was within 10% for LAI>2, but calculated CNCER overestimated radiation use efficiency based on phytomass accumulation for smaller values of LAI.

Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at an elevated concentration of carbon dioxide

ABSTRACTSeedlings of temperate deciduous tree species were grown outdoors at ambient and at an elevated concentration of carbon dioxide to examine how aspects of their gas exchange would be altered by growth at elevated carbon dioxide concentration. Leaf conductances to water vapour and net carbon dioxide exchange rates were determined periodically near midday. Whole‐plant carbon dioxide efflux rates in darkness were also determined. The stomatal conductance of leaves of plants grown and measured at 700 cm3 m−3 carbon dioxide did not differ from that of plants grown and measured at 350 cm3 m−3 in Malus domestica, Quercus prinus and Quercus robur at any measurement time. In Acer saccharinum, lower conductances occurred for plants grown and measured at elevated carbon dioxide concentration only at measurement temperatures above 33°C. Photo‐synthetic adjustment to elevated carbon dioxide concentration was evident only in Q. robur. All species examined had lower rates of dark respiration per unit of mass when grown and measured at elevated carbon dioxide concentration.

Net carbon dioxide exchange in canopies of burned and unburned tallgrass prairie

Net carbon dioxide exchange (NCE) rates were measured in a tallgrass prairie, a grassland with high productivity, to determine photosynthetic rates of the canopy. Canopy measurements were made in large, plexiglass chambers (1.21 m long; 0.91 m wide; 1.40 m tall) placed on burned and unburned areas of the prairie. The NCE rates of the canopy were compared with those of individual leaves ofAndropogon gerardii Vitman (big bluestem). In addition, CO2 flux from the soil was quantified and compared with net photosynthetic flux. The canopy NCE rates were generally lower than those made on individual leaves. In mid-summer (11 July 1987), the maximum canopy NCE rates were 55% and 64% of those measured on individual leaves in burned and unburned treatments, respectively. Canopy NCE rates were lower than individual-leaf NCE rates for two reasons. First, the individualleaf measurements were made on young, unshaded, healthy leaves, while the canopy measurements were made on all types of leaves including senescing, shaded, and damaged leaves. Second, soil CO2 flux into the chambers lowered NCE values. The CO2 flux from the soil ranged from 7.2% to 28.4% of the total NCE. One needs to add soil CO2 flux rates to the measured canopy NCE rates to obtain canopy NCE rates closer to individual-leaf NCE rates. Soil CO2 flux decreased when conditions became dry, reaching a low of 0.06 mg CO2m−2s−1, but increased after rain to 0.16 mg CO2m−2s−1. Also, after rain, when plants were well watered, they were not light saturated at 1 900 µEm−2s−1. The NCE rates on the burned treatment were either higher or similar to those on the unburned treatment. For example, on 11 July 1987, NCE rates were higher on the burned treatment (0.66 mg CO2m−2s−1) compared to the unburned treatment (0.47 mg CO2m−2s−1). During the rest of July and August, the rates of the two treatments were not significantly different. But in September and October, the NCE rates were again higher on the burned treatment compared to the unburned treatment. The results indicated that canopy NCE rates may be more indicative of the productivity of the prairie than individual-leaf measurements made only on young, highly productive leaves.

Carbon dioxide exchange rates in wheat canopies. Part I. Influence of canopy geometry on trends in leaf area index, light interception and instantaneous exchange rates

This field study investigated the influence of different canopy structures on seasonal trends in leaf area index (LAI), light interception, and instantaneous net carbon dioxide (CO 2) exchange rates (NCE) in wheat. Winter wheat ( Triticum aestivum L. cv. Newton) was planted at the Evapotranspiration Research Site (Unit 2) located 14 km south of Manhattan, KS (39°09′N, 96°37′W) in north-south and east-west row orientations with 0.18-, 0.35-, 0.53-, and 0.71-m row spacings during the 1982–1983 and 1983–1984 growing seasons. Seeding rates were 67, 34, 22, and 17 kg ha −1 for the 0.18-, 0.35-, 0.53-, and 0.71-m row spacings, respectively. These treatments modified the wheat canopy structure for a comparative study. For both years, the soil type was a Muir silt loam (fine-silty, Mesic, Pachic Haplustoll). All plots were fertilized with 109 kg N ha −1 incorporated prior to planting. Interplant competition reduced the size, tiller number, and above-ground green phytomass for individual plants planted in narrow-spaced rows. However, the total number of plants, tillers, and green phytomass per ground area were greatest in the narrow-spaced rows. Altering canopy structure significantly influenced the seasonal trends in LAI, with larger values occurring in canopies planted in narrow-spaced rows. NCE rates were consistently higher in wheat canopies grown in the narrow-spaced rows (i.e. greater LAI) and decreased with increasing row width (i.e. smaller LAI). LAI was an adequate indicator of potential NCE rates during the vegetative and reproductive stages of growth. However the maximum NCE rates remained at seasonal high levels during the senescent phase when the LAI dropped below the canopy light saturation threshold (i.e. LAI of 2.5–3.5) suggesting that stem and heads play a vital role in NCE rate during senescence. NCE were consistently greater in the north-south oriented canopies when compared to the east-west ones, particularly at high zenith angles during clear days. However, row orientation did not significantly influence seasonal phytomass production. Narrow-spaced rows obtained canopy closure early in the spring which significantly increases light interception efficiency and NCE rates resulting in higher phytomass production.

Nonstomatal inhibition of photosynthesis by water stress. Reduction in photosynthesis at high transpiration rate without stomatal closure in field-grown tomato

Large underestimates of the limitation to photosynthesis imposed by stomata can occur because of an error in the standard method of calculating average substomatal pressures of carbon dioxide when heterogeneity of those pressures occurs across a leaf surface. Most gas exchange data supposedly indicating nonstomatal inhibition of photosynthesis by water stress could have this error. However, if no stomatal closure occurs, any reduction in photosynthesis must be due to nonstomatal inhibition of photosynthesis. Net carbon dioxide exchange rates and conductances to water vapor were measured under field conditions in upper canopy leaves of tomato plants during two summers in Beltsville, Maryland, USA. Comparisons were made near midday at high irradiance between leaflets in air with the ambient water vapor content and in air with a higher water content. The higher water content, which lowered the leaf to air water vapor pressure difference (VPD), was imposed either one half hour or several hours before measurements of gas exchange. In both seasons, and irrespective of the timing of the imposition of different VPDs, net photosynthesis increased 60% after decreasing the VPD from 3 to 1 kPa. There were no differences in leaf conductance between leaves at different VPDs, thus transpiration rates were threefold higher at 3 than at 1 kPa VPD. It is concluded that nonstomatal inhibition of photosynthesis did occur in these leaves at high transpiration rate.

Endogenous Rhythms in Photosynthesis, Sucrose Phosphate Synthase Activity, and Stomatal Resistance in Leaves of Soybean (Glycine max [L.] Merr.)

Experiments were conducted with soybean (Glycine max [L.] Merr. cv ;Ransom') plants to determine if diurnal rhythms in net carbon dioxide exchange rate (CER), stomatal resistance, and sucrose-phosphate synthase (SPS) activity persisted in constant environmental conditions (constant light, LL; constant dark DD) and to assess the importance of these rhythms to the production of nonstructural carbohydrates (starch, sucrose, and hexose). Rhythms in CER, stomatal resistance, and SPS activity were observed in constant environmental conditions but the rhythms differed in period length, amplitude, and phase. The results indicated that these photosynthetic parameters are not controlled in a coordinated manner. The activity of UDPG pyrophosphorylase, another enzyme involved in sucrose formation, did not fluctuate rhythmically in constant conditions but increased with time in plants in LL. In LL, the rhythm in CER was correlated positively with fluctuations in total chlorophyll (r = 0.810) and chlorophyll a (r = 0.791) concentrations which suggested that changes in pigment concentration were associated with, but not necessarily the underlying mechanism of, the rhythm in photosynthetic rate. Assimilate export rate, net starch accumulation rate, and leaf sucrose concentration also fluctuated in constant light. No single photosynthetic parameter was closely correlated with fluctuations in assimilate export during LL; thus, assimilate export may have been controlled by interactions among the endogenous rhythms in CER, SPS activity, or other metabolic factors which were not measured in the present study.

Effect of Day and Night Temperatures During Floral Induction on Morphology of Soybeans1

AbstractTemperature affects the vegetative and reproductive morphology of soybeans [Glycine max (L.) Merrill]. Descriptive information of the effects of temperature will assist in development of a dynamic simulation model. The purpose of this study was to determine the effects of day and night temperatures on growth and development of the determinate soybean cultivar ‘Ransom’. Seedlings were grown under a long‐day (non‐inductive) photoperiod at 26/22 C day/night temperatures in a temperature controlled greenhouse until the expansion of the first trifoliolate leaf. Plants then were moved into five controlled‐environment chambers programmed for 9‐hour photoperiods with temperature regimes of 14/10, 18/14, 22/18, 26/22, and 30/26 C. At the beginning of each night, groups of plants were moved from the five day temperatures to each of the five night temperatures to obtain a total of 25 day/night temperature combinations. Significant interactions between day and night temperatures for all measured characteristics except branch internode length made general evaluations of simple effects of day and night temperatures difficult. Instead, responses to the specific combinations of day/night temperature were considered. At the conclusion of 50 consecutive short days, area per leaf and vegetative dry weight were greatest at 26/10 C temperatures while pod dry weight was greatest at the higher night temperature combination of 26/22 C. The lowest net carbon dioxide exchange rate was obtained at temperatures of 14/26 C, and the highest rates at 26/14 and 26/18 C. The extent of apical dominance, given as the ratio between main stem height and total branch length, was greatest for plants from either warm day and warm night or cool day and cool night combinations. Warm night temperatures, in conjunction with any of the day temperatures, shortened the time from first exposure to inductive photoperiod to anthesis. At the four coolest day/night temperature combinations (14/10, 14/14, 18/10, and 18/14 C) pods were not formed even though floral initiation had occurred

Soil moisture effects on transpiration and net carbon dioxide exchange of sorghum

This field study was conducted to determine the effect of soil moisture on net carbon dioxide exchange (NCE) and transpiration rates of sorghum ( Sorghum bicolor (L.) Moench) grown under well-watered and water-stressed conditions. Components of the energy balance, soil moisture, leaf and air temperatures, stomatal resistance, and leaf-water potential were measured. A field growth-chamber system was used to determine NCE and transpiration rates. Analysis of the data suggested that transpiration and NCE rates were reduced when available soil moisture was ⩽35% of the maximum. Above that moisture level, transpiration was primarily energy-dependent and the rate was enhanced by air temperatures higher than 33°C. Photosynthetic rates declined when leaf temperatures exceeded 33°C. At and below the critical soil-moisture level, leaf-water potential decreased and the resistance to vapor transport increased; that reduced transpiration and NCE rates.

Effects of temperature on the photosynthesis–irradiance response curves of newly matured leaves of alfalfa

Various carbon dioxide exchange characteristics are described for two alfalfa (Medicago sativa L.) genotypes (AT 171 and CC 120) grown at 20:15 °C and 30:25 °C day:night temperatures and 53 nE cm−2 s−1 irradiance (400–700 nm). Growth at 30:25 °C as compared with 20:15 °C resulted in lower net carbon dioxide exchange rates (NCE) for both genotypes when analyzed at 20 °C, but did not cause any sizeable change for CC 120 at 30 °C. Oxygen inhibition of photosynthesis increased with irradiance to 48 nE cm−2 s−1 but either declined or remained constant with further increase in irradiance. Oxygen inhibition was higher at 30 °C than at 20 °C and was not consistently influenced by growth temperature. However, the ratio of oxygen inhibition to carbon dioxide exchange rate in air containing 1% oxygen and the mesophyll resistance were greater with AT 171 grown at 30:25 °C than at 20:15 °C, particularly at high irradiances. NCE measured at 20 °C instead of 30 °C for plants grown at 30:25 °C was reduced to a much more marked extent with CC 120 than with AT 171; this difference was paralleled by a more marked increase in stomatal resistance length (rSL) for CC 120.rSL decreased with an increase in irradiance, was generally higher at 20 °C than at 30 °C, and did not differ between growth temperatures when measured at an irradiance of 116 nE cm−2 s−1 and a temperature equal to the day temperature of the growth regime. The results are discussed in relation to factors responsible for adaptability to different thermal environments.

EFFECTS OF SOYBEAN LEAFLET TYPE ON NET CARBON DIOXIDE EXCHANGE, WATER USE, AND WATER-USE EFFICIENCY

Two near-isogenic soybean (Glycine max (L.) Merrill) lines with normal and narrow leaflets were compared in the field for differences in net carbon dioxide exchange rates, water use, and water-use efficiency with various combinations of population and row spacing. Net carbon dioxide exchange was determined with an open field chamber system and infrared gas analyzer. Water use was estimated from neutron attenuation readings and precipitation. Leaflet type did not significantly affect net carbon dioxide exchange, water use, or yield. Water-use efficiency was only affected at 53 plants/m2 in 38-cm rows, with greater efficiency for the narrow-leaflet cultivar.

Soil Reflectance Effects on Net Carbon Dioxide Exchange Rates of Sorghum1

Net carbon dioxide exchange (NCE) rates of sorghum (Sorghum bicolor (L.) Moench) canopies were measured in the field using large plastic chambers and an infrared gas analyzer to assess the effect of soil reflectance on photosynthesis. Increasing soil reflectance, by applying white reflective clay, kaolinite, to the soil surface enclosed by the chambers, increased NCE by as much as 16% when crop cover was sparse. The effect decreased as crop coverage increased.Spectral‐reflectance data taken over kaolinite‐treated and nontreated soil showed that reflected light over the treated plots decreased as crop cover increased so by 2 August treated and nontreated plots differed little in the amount of reflected light. Increases in NCE as calculated from spectral‐reflectance data approximated measured values.

Net carbon dioxide exchange of wheat, sorghum, and soybean

Canopy net carbon dioxide exchange (NCE) rates are needed to understand the effect of light on crop growth and development. NCE rates of sorghum, soybean, and wheat canopies were determined using field chambers and an infrared gas analyzer throughout a growing season. Whereas sorghum attained peak NCE rates early in season, soybean and wheat did not reach maximum rates until the late reproductive growth stage. Frequently during the season, NCE rates of sorghum were nearly four times those of soybeans and wheat. Solar radiation had a much greater affect on the NCE of sorghum than on that of soybeans. Apparent light saturation was observed in both soybeans and wheat. An expression for dry matter production of sorghum was derived using chamber data. The expression simulated the total dry matter production for sorghum in 1973, 1972, and 1970 within 20%.