Crop management and agronomic context of the Farm Scale Evaluations of genetically modified herbicide-tolerant crops

The Farm Scale Evaluations of genetically modified herbicide-tolerant crops (GMHT) were conducted in the UK from 2000 to 2002 on beet (sugar and fodder), spring oilseed rape and forage maize. The management of the crops studied is described and compared with current conventional commercial practice. The distribution of field sites adequately represented the areas currently growing these crops, and the sample contained sites operated at a range of management intensities, including low intensity. Herbicide inputs were audited, and the active ingredients used and the rates and the timings of applications compared well with current practice for both GMHT and conventional crops. Inputs on sugar beet were lower than, and inputs on spring oilseed rape and forage maize were consistent with, national averages. Regression analysis of herbicide-application strategies and weed emergence showed that inputs applied by farmers increased with weed densities in beet and forage maize. GMHT crops generally received only one herbicide active ingredient per crop, later and fewer herbicide sprays and less active ingredient (for beet and maize) than the conventional treatments. The audit of inputs found no evidence of bias.     

Effects of increasing weed-beet density on sugar-beet yield and quality

Weed beets are an increasing problem in many sugar-beet crops in many countries. At present about one sugar-beet field in four in England is infested with weed-beet seed. Control in other crops can be achieved using selective herbicides but in sugar beet the weed beets, many of which are of annual habit, are not easily controlled and often compete with the crop. Experiments were done to quantify the yield loss caused by weed beet in sugar-beet crops. Transects were laid out across three fields in 1985 and 1986 and plots located thereon to include the range of weed-beet densities found in the field. Weed beet did not affect the concentration of sugar (sucrose), potassium, sodium, α amino nitrogen or invert sugar in the crop beets. Root and sugar yields were progressively reduced by increasing densities of weed beet. A rectangular hyperbola described the data slightly better than an asymptotic model. There was no indication of a threshold density of weed beet below which there was no yield loss, which averaged 11.7% for each weed beet plant/m². This corresponds to an average 0.6% sugar yield loss for each 1% of bolted weed beet in the root crop up to 100%, which is similar to the reported losses resulting from bolters in the root crop.     

Populations of Rhizoctonia solani in soil under crops in rotation with sugar beet

Using a soil debris isolation method, populations of Rhizoctonia solani were monitored over a 4 -yr period in four fields which were initially cropped to sugar beet and in which four areas of Rhizoctonia crown rot diseased beets (DA) and four areas of apparently healthy beets (AH) had been selected and precisely located. Soil from these areas was assayed during the subsequent crops, which included sugar beet, tomato, cucumber, maize and soybean. No significant differences in colony counts were found between the soils in DA and AH on any of 30 sampling dates. R. solani population counts were, in general, quite low, except under sugar beet and following tomato harvest. Areas of diseased beet and high R. solani soil populations that developed in subsequent sugar beet crops did not necessarily coincide with the previously selected diseased areas. High R. solani populations developed from parasitic activity on sugar beet or saprophytically on tomato crop residues. Of the other crops, both maize and soybean may have slightly increased the low R. solani residual populations in soil. The monitoring of R. solani populations in the season prior to, and during the early season of sugar beet cropping did not provide a basis for forecasting disease in fields or sites within fields. The initiation of disease patches in these sugar beet fields was therefore governed by factors other than inoculum density.     

Effect of soil compaction on growth, yield and light interception of selected crops

The response of spring barley (Hordeum vulgare, cvs Carnival and Atem), faba beans (Vicia faba, cv. Maris Bead), sugar beet (Beta vulgaris, cv. Monoire), forage maize (Zea mays, cv. Leader), forage peas (Pisum sativum, cv. Poneka) and white turnip (Brassica campestris, cv. Barkant) to topsoil compaction was investigated in a three year trial. Soil compaction was induced by tractor wheeling after crop sowing. Compaction reduced leaf area and dry matter accumulation in all crops in every season. Yield of barley was reduced by 29%, 27% and 40% in 1984, 1986 and 1987 respectively. Yield of maize, peas and turnip decreased by 33%, 14% and 13% in 1986 and 25%, 16% and 19% in 1987. Yields of beans and sugar beet were decreased by 34% and 35% respectively in 1984. Light interception was decreased in all crops in all three years of study but, with the exception of maize in 1987, the efficiency of conversion of radiant energy to dry matter was not significantly affected by soil compaction. It is concluded that reduced dry matter production and yield due to soil compaction was more a consequence of reduced light interception because of restricted leaf area development rather than as a result of an impaired ability of crops to utilise intercepted radiant energy.     

Invertebrate responses to the management of genetically modified herbicide-tolerant and conventional spring crops. I. Soil-surface-active invertebrates

The effects of herbicide management of genetically modified herbicide-tolerant (GMHT) beet, maize and spring oilseed rape on the abundance and diversity of soil-surface-active invertebrates were assessed. Most effects did not differ between years, environmental zones or initial seedbanks or between sugar and fodder beet. This suggests that the results may be treated as generally applicable to agricultural situations throughout the UK for these crops. The direction of the effects was evenly balanced between increases and decreases in counts in the GMHT compared with the conventional treatment. Most effects involving a greater capture in the GMHT treatments occurred in maize, whereas most effects involving a smaller capture were in beet and spring oilseed rape. Differences between GMHT and conventional crop herbicide management had a significant effect on the capture of most surface-active invertebrate species and higher taxa tested in at least one crop, and these differences reflected the phenology and ecology of the invertebrates. Counts of carabids that feed on weed seeds were smaller in GMHT beet and spring oilseed rape but larger in GMHT maize. In contrast, collembolan detritivore counts were significantly larger under GMHT crop management.     

Weed seed resources for birds in fields with contrasting conventional and genetically modified herbicide-tolerant crops

The UK Farm Scale Evaluations (FSEs) have shown that the use of broad spectrum herbicides on genetically modified herbicide-tolerant (GMHT) crops can have dramatic effects on weed seed production compared to management of conventional varieties. Here, we use FSE data and information on bird diets to determine how GMHT cropping might change the food resources available to farmland birds. More than 60 fields of each of four crops, spring- and winter-sown oilseed rape, beet and maize, were split, one half being sown with a conventional variety, the other with a GMHT variety. Seed rain from weeds known to be important in the diets of 17 granivorous farmland bird species was measured under the two treatments. In beet and spring oilseed rape, rain of weed seeds important in the diets of 16 bird species was significantly reduced in GMHT compared to conventional halves; for no species did it increase. In winter oilseed rape, rain of weed seeds important in the diets of 10 species was significantly reduced in GMHT halves; for only one species did it increase significantly. By contrast, in maize, rain of weed seeds important in the diets of seven species was significantly greater in GMHT halves; for no species was it reduced. Treatment effects for the total weed seed energy available to each bird species were very similar to those for seed rain alone. Measuring the effects on individual bird species was outside the scope of this study. Despite this, these results suggest that should beet, spring and winter rape crops in the UK be largely replaced by GMHT varieties and managed as in the FSEs, this would markedly reduce important food resources for farmland birds, many of which declined during the last quarter of the twentieth century. By contrast, GMHT maize would be beneficial to farmland birds.     

Economic consequences for UK farmers of growing GM herbicide tolerant sugar beet

Weed control is important and one of the more expensive inputs to sugar beet production. The introduction of genetically modified herbicide tolerant (GMHT) sugar beet would result in a major saving in weed control costs in the crop for growers, including control of problem weeds such as perennial weeds and weed beet. However, there would be other economic consequences of growing GMHT beet, some of which would manifest themselves in other parts of the rotation, such as the previous crop, the cereal stubbles that proceed most beet crops, soil tillage and spray application. The average national saving for UK sugar beet growers if they could use the technology would be in excess of £150 ha^?1 yr^?1 or £23 million yr^?1, which includes reductions in agrochemical use of c. £80 ha,^?1 yr^?1 or £12 million yr^?1. However, for some growers, the gains would be much larger and for a few, less than these figures. The possible cost savings are sufficiently large that they could ensure that sugar beet production, with its regionally important environmental benefits as a spring crop, remains economically viable in the UK post reform of the EU sugar regime.     

Manganese dynamics in the rhizosphere and Mn uptake by different crops evaluated by a mechanistic model

Understanding of the mechanisms of Mn supply from the soil and uptake by the plants can be improved by using simulation models that are based on basic principles. For this, a pot culture experiment was conducted with a sandy clay loam soil to measure Mn uptake by summer wheat (Triticum aestivum L. cv. Planet), maize (Zea mays L. cv. Pirat) and sugar beet (Beta vulgaris L. cv. Orbis) and to simulate Mn dynamics in the rhizosphere by means of a mechanistic model. Seeds of three crops were sown in pots containing 2.9 kg soil in a controlled growth chamber. Root and shoot weight, Mn content of plants, root length and root radius were determined 8 (13 days in case of sugar beet) and 20 days after germination. Soil and plant parameters were determined to run nutrient uptake model calculations. Manganese content of the shoot varied from 25 mg kg^-1 for sugar beet to 34 mg kg^-1 for maize. Sugar beet had the lowest root length/shoot weight ratio but the highest relative shoot growth rate, resulting in the highest shoot demand on the root. This is reflected by the Mn influx which was 0.9 × 10^-7, 1.7 × 10^-7 and 2.5 × 10^-7 nmol cm^-1 s^-1 for wheat, maize and sugar beet, respectively. Nutrient uptake model calculations predicted similar influx values. Initial Mn concentration of 0.2 μM in the soil solution decreased to only 0.16 μM for wheat, 0.13 μM for maize and 0.11 μM for sugar beet at the root surface. This shows that manganese transport to the root was not a limiting step. This was confirmed by the fact that an assumed 20 times increase in maximum influx (I_max) increased the calculated Mn influx by 3.7 times. Sensitivity analysis demonstrated that for controlling Mn uptake the initial soil solution concentration (C _Li), the root radius (r₀), I_max and the Michaelis constant (K _m) were the most sensitive factors in the listed order. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of weed density on the dispersal of Orius majusculus Reuter (Heteroptera Anthocoridae) within maize

Orius majusculus Reuter (Heteroptera: Anthocoridae) is the most common and abundant generalist predator in Spanish maize crops and is sensitive to drastic changes in weed density. We carried out a 2‐year study in the NE Iberian Peninsula to examine the dispersal of O. majusculus in maize plots with moderately high and low weed density. Insects were collected using yellow sticky traps and/or a bug‐vac aspirator. Dispersal was assessed using rubidium as a marker. Dispersal rate of O. majusculus differed between sampling periods, with a maximum between the 16 unfolded leaves (V16) to grain milky (R3) maize growth stages. However, we detected no differences in the distance moved by male and females of O. majusculus from rubidium‐marked areas in plots with moderately high or low weed density suggesting that changes in weed density do not affect the dispersal of O. majusculus within a maize field.     

Effects of genetically modified herbicide-tolerant cropping systems on weed seedbanks in two years of following crops

The Farm Scale Evaluations (FSEs) showed that genetically modified herbicide-tolerant (GMHT) cropping systems could influence farmland biodiversity because of their effects on weed biomass and seed production. Recently published results for winter oilseed rape showed that a switch to GMHT crops significantly affected weed seedbanks for at least 2 years after the crops were sown, potentially causing longer-term effects on other taxa. Here, we seek evidence for similar medium-term effects on weed seedbanks following spring-sown GMHT crops, using newly available data from the FSEs. Weed seedbanks following GMHT maize were significantly higher than following conventional varieties for both the first and second years, while by contrast, seedbanks following GMHT spring oilseed rape were significantly lower over this period. Seedbanks following GMHT beet were smaller than following conventional crops in the first year after the crops had been sown, but this difference was much reduced by the second year for reasons that are not clear. These new data provide important empirical evidence for longer-term effects of GMHT cropping on farmland biodiversity.     

Diet selection by hares (Lepus europaeus) in arable land and its implications for habitat management

Populations of European hares (Lepus europaeus) have experienced a dramatic decline throughout Europe in recent decades. European hares are assumed to prefer weeds over arable crops, and weed abundance was reduced by the intensification of agriculture. Therefore, modern agriculture has been blamed as a major factor affecting European hare populations. However, it is questionable whether European hares select weeds at all, as previous studies had major methodological limitations. By comparing availability and use of plants with Chesson’s Electivity Index, we investigated whether the European hare actually feeds selectively on different plants in arable land. Food availability and use were dominated by cultivated crops (e.g. winter wheat, spring barley and sugar beet). Diet selection analysis revealed that in autumn and winter, European hares predominantly preferred cultivated crops (winter wheat) and food items provided by hunters (tubers of sugar beet and carrot). In spring and summer, apart from soy, only weeds (e.g. clover and corn poppy) were positively selected, especially after cereal crops were harvested. We suggest that the decline in European hare populations throughout Europe was facilitated by the decrease in weed abundance. Wildlife-friendly set-asides in arable land have the potential to reconcile the European Union’s Common Agricultural Policy with wildlife conservation.     

Curly Top of Cultivated Plants and Weeds and Report of a Unique Curtovirus from Iran

The incidence of curly top disease on cultivated plants and weeds was investigated in Kerman Province (southeastern Iran) from October 2003 to November 2004. A total of 1186 samples were collected in fields of sugar beet and other crops as well as within commercial plastic houses. Curtovirus infection of four field crops, three vegetables and 11 weeds was verified by indirect enzyme‐linked immunosorbent assay (ELISA) using a polyclonal antibody. An undescribed curtovirus, tentatively designated Iranian beet curly top virus (IBCTV), was isolated from three symptomatic beet samples collected randomly in widely separated regions of south‐eastern, southern and central Iran and used for molecular studies. A 672 bp segment of the coat protein (CP) gene of each isolate was amplified by PCR and sequenced. The results showed that the three isolates shared 98.5–98.7% nucleotide homology with each other but only 72.1–76.5% with other members of the genus Curtovirus. IBCTV was also detected by PCR using specific primers in other samples of sugar beet, tomato, spinach, turnip and several weed species collected in different parts of Iran. These results indicated that IBCTV is the dominant curtovirus in Iran.     

Fate of Clavibacter michiganensis ssp. sepedonicus, the Causal Organism of Bacterial Ring Rot of Potato, in Weeds and Field Crops

Crops and weeds were tested for their ability to host Clavibacter michiganensis ssp. sepedonicus (Cms), the causal agent of bacterial ring rot in potato. Ten crops grown in rotation with potato in Europe, namely maize, wheat, barley, oat, bush bean, broad bean, rape, pea and onion and five cultivars of sugar beet were tested by stem and root inoculation. About 6–8 weeks after inoculation, Cms could be detected in most crops except onion and sugar beet, in larger numbers in stems (10⁵–10⁶ cells/g of tissue) than in roots (≤10³ cells/g of tissue) in immunofluorescence cell‐staining (IF). Cms was successfully re‐isolated only from IF‐positive stem samples of maize, bush bean, broad bean, rape and pea, but not from roots. Twelve solanaceous weeds and 13 other weeds, most commonly found in potato fields in Europe, were tested in IF as hosts of Cms by stem and root inoculations. Only in Solanum rostratum, a weed present in northern America, Cms persisted in high numbers (10⁸ cells/g tissue) in stems and leaves, where it caused symptoms. In the other solanaceous weeds, Cms persisted at low numbers (approximately 10⁵ cells/g of tissue) in stems but less so in roots. The bacteria could be frequently re‐isolated from stem but not from root tissues. In 2 consecutive years, plants from 14 different weed species were collected from Cms‐contaminated potato field plots and tested for the presence of Cms by dilution plating or immunofluorescence colony‐staining (IFC), and by AmpliDet RNA, a nucleic acid‐based amplification method. Cms was detected in roots but not in stems of Elymus repens plants growing through rotten potato tubers, and in some Viola arvensis and Stellaria media plants, where they were detected both in stems and roots, but more frequently by AmpliDet RNA than by IFC.     

Weeds in fields with contrasting conventional and genetically modified herbicide-tolerant crops. I. Effects on abundance and diversity

We compared the seedbanks, seed rains, plant densities and biomasses of weeds under two contrasting systems of management in beet, maize and spring oilseed rape. Weed seedbank and plant density were measured at the same locations in two subsequent seasons. About 60 fields were sown with each crop. Each field was split, one half being sown with a conventional variety managed according to the farmer's normal practice, the other half being sown with a genetically modified herbicide-tolerant (GMHT) variety, with weeds controlled by a broad-spectrum herbicide. In beet and rape, plant densities shortly after sowing were higher in the GMHT treatment. Following weed control in conventional beet, plant densities were approximately one-fifth of those in GMHT beet. In both beet and rape, this effect was reversed after the first application of broad-spectrum herbicide, so that late-season plant densities were lower in the GMHT treatments. Biomass and seed rain in GMHT crops were between one-third and one-sixth of those in conventional treatments. The effects of differing weed-seed returns in these two crops persisted in the seedbank: densities following the GMHT treatment were about 20% lower than those following the conventional treatment. The effect of growing maize was quite different. Weed density was higher throughout the season in the GMHT treatment. Late-season biomass was 82% higher and seed rain was 87% higher than in the conventional treatment. The difference was not subsequently detectable in the seedbank because the total seed return was low after both treatments. In all three crops, weed diversity was little affected by the treatment, except for transient effects immediately following herbicide application.     

Weeds in fields with contrasting conventional and genetically modified herbicide-tolerant crops. II. Effects on individual species

We compared the effects of the management of genetically modified herbicide-tolerant (GMHT) and conventional beet, maize and spring oilseed rape on 12 weed species. We sampled the seedbank before and after cropping. During the season we counted plants and measured seed rain and biomass. Ratios of densities were used to calculate emergence, survival, reproduction and seedbank change. Treatments significantly affected the biomass of six species in beet, eight in maize and five in spring oilseed rape. The effects were generally consistent, with biomass lower in GMHT beet and spring oilseed rape and higher in GMHT maize. With few exceptions, emergence was higher in GMHT crops. Subsequent survival was significantly lowered for eight species in beet and six in spring oilseed rape in the GMHT treatments. It was increased for five species in maize and one in spring oilseed rape. Significant effects on seedbank change were found for four species. However, for many species in beet and spring oilseed rape (19 out of 24 cases), seed densities were lower in the seedbank after GMHT cropping. These differences compounded over time would result in large decreases in population densities of arable weeds. In maize, populations may increase.     

The implication of irrigation in climate change impact assessment: a European‐wide study

This study evaluates the impacts of projected climate change on irrigation requirements and yields of six crops (winter wheat, winter barley, rapeseed, grain maize, potato, and sugar beet) in Europe. Furthermore, the uncertainty deriving from consideration of irrigation, CO₂ effects on crop growth and transpiration, and different climate change scenarios in climate change impact assessments is quantified. Net irrigation requirement (NIR) and yields of the six crops were simulated for a baseline (1982–2006) and three SRES scenarios (B1, B2 and A1B, 2040–2064) under rainfed and irrigated conditions, using a process‐based crop model, SIMPLACE . We found that projected climate change decreased NIR of the three winter crops in northern Europe (up to 81 mm), but increased NIR of all the six crops in the Mediterranean regions (up to 182 mm yr^?1). Climate change increased yields of the three winter crops and sugar beet in middle and northern regions (up to 36%), but decreased their yields in Mediterranean countries (up to 81%). Consideration of CO₂ effects can alter the direction of change in NIR for irrigated crops in the south and of yields for C3 crops in central and northern Europe. Constraining the model to rainfed conditions for spring crops led to a negative bias in simulating climate change impacts on yields (up to 44%), which was proportional to the irrigation ratio of the simulation unit. Impacts on NIR and yields were generally consistent across the three SRES scenarios for the majority of regions in Europe. We conclude that due to the magnitude of irrigation and CO₂ effects, they should both be considered in the simulation of climate change impacts on crop production and water availability, particularly for crops and regions with a high proportion of irrigated crop area.     

Soil microbial biomass and enzyme kinetics for the assessment of temporal diversification in agroecosystems

In agroecosystems, temporal diversification creates a sequence of short-lived habitats through time. Crop species as well as the diversity of crops grown in sequence might affect soil biodiversity and nutrient cycling processes. In the present study, we focused on a long-term crop rotation established in 2006 in Lower Saxony, Germany on a Luvisol. Winter wheat (WW) and silage maize (SM) were grown in continuous cultivation as well as in rotations. WW rotations span up to six years (including silage maize, sugar beet, winter rape and/or grain pea). Over two years, microbial biomass carbon (MBC) as well as kinetics (Michaelis-Menten V_max and K_m) of extracellular hydrolytic enzymes (β-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG) and acid phosphomonoesterase (AP)) were measured in topsoil (0–10 cm depth) three times during the growing season. Continuous wheat increased soil microbial parameters compared to continuous maize as indicated by the higher microbial biomass to soil organic carbon ratio and higher potential enzymes activities involved in the C- and N-cycles (V_max of BG and NAG). The efficiency of these enzymes was lowest in continuous maize (highest K_m of BG and NAG). Maize and sugar beet as preceding crop of WW significantly decreased MBC in the 1^st year but not in the 2^nd year WW. Sugar beet decreased BG activity as well as its substrate affinity (increased K_m). The effect of sugar beet on MBC and enzyme kinetics depended on the preceding crop and lessened with grain pea as the preceding crop. Soil microorganisms in the wheat phase benefited from winter rape as the preceding crop, shown by an increased biomass and efficiency to turn over chitin and peptidoglycan (decreased K_m of NAG). Differences between cultivated crops, cropping history and fluctuations within the year in soil microbial biomass and enzyme kinetics are shown.     

Reproduction of Heterodera schachtii Schmidt on Resistant Mustard,Radish, and Sugar Beet Cultivars

The reproduction of a Wyoming population of Heterodera schachtii was determined for resistant trap crop radish (Raphanus sativus) and mustard (Sinapis alba) cultivars, and resistant and susceptible sugar beet (Beta vulgaris) cultivars in a greenhouse (21 °C/16 °C) and a growth chamber study (25 °C). Oil radish cultivars also were field tested in 2000 and 2001. In the greenhouse study, reproduction was suppressed similarly by the resistant sugar beet cultivar Nematop and all trap crop cultivars (P ≤ 0.05). In the growth chamber study, the radish cultivars were superior to most of the mustard cultivars in reducing nematode populations. All trap crops showed less reproduction than Nematop (P ≤ 0.05). In both studies, Nematop and all trap crops had lower Pf than susceptible sugar beet cultivars HH50 and HM9155 (P ≤ 0.05). In field studies, Rf values of radish cultivars decreased with increasing Pi of H. schachtii (r² = 0.59 in 2000 and r² = 0.26 in 2001). In 2000, trap crop radish cv. Colonel (Rf = 0.89) reduced nematode populations more than cv. Adagio (Rf = 4.67) and cv. Rimbo (Rf = 13.23) (P ≤ 0.05) when Pi was lower than 2.5 H. schachtii eggs and J2/cm³ soil. There were no differences in reproductive factors for radish cultivars in 2001 (P ≤ 0.05); Rf ranged from 0.23 for Adagio to 1.31 for Commodore for all Pi.     

Tracing back seed and pollen flow within the crop-wild Beta vulgaris complex: genetic distinctiveness vs. hot spots of hybridization over a regional scale

Hybrids between transgenic crops and wild relatives have been documented successfully in a wide range of cultivated species, having implications on conservation and biosafety management. Nonetheless, the magnitude and frequency of hybridization in the wild is still an open question, in particular when considering several populations at the landscape level. The Beta vulgaris complex provides an excellent biological model to tackle this issue. Weed beets contaminating sugar beet fields are expected to act as a relay between wild populations and crops and from crops-to-crops. In one major European sugar beet production area, nine wild populations and 12 weed populations were genetically characterized using cytoplasmic markers specific to the cultivated lines and nuclear microsatellite loci. A tremendous overall genetic differentiation between neighbouring wild and weed populations was depicted. However, genetic admixture analyses at the individual level revealed clear evidence for gene flow between wild and weed populations. In particular, one wild population displayed a high magnitude of nuclear genetic admixture, reinforced by direct seed flow as evidenced by cytoplasmic markers. Altogether, weed beets were shown to act as relay for gene flow between crops to wild populations and crops to crops by pollen and seeds at a landscape level.     

Cellular Localization of CO(2) Fixation and Translocation of Metabolites

Leaves of maize (Zea mays L.) and sugar beet (Beta vulgaris L.) were enclosed in an illuminated chamber in air for 30 min after which time ¹⁴CO₂ was released into the chamber. Two min after the ¹⁴CO₂ was released, the leaves were removed from the chamber, and small sections were cut from them. The sections were put in small wire baskets and frozen in isopentane cooled by liquid nitrogen. Approximately 1.5 min elapsed from the removal of the leaf from the illuminated chamber until the tissue was frozen. The tissue was freeze-dried, embedded in paraffin and the cellular location of the isotopic activity was determined by radiography of leaf cross sections. Isotopic activity in maize leaves was localized in bundle sheath parenchyma. In contrast, the label in sugar beet leaves was generally distributed in the mesophyll cells. The bundle sheath cells in maize contain specialized chloroplasts which appear to have a unique capacity to incorporate CO₂. Translocation from leaves of maize was 3-fold as rapid as from sugar beet leaves in the same environment. Low light intensity did not alter the distribution pattern of fixed CO₂.