Effects of awns on the photosynthesis and yield of wheat, Triticum aestivum

Two pairs of awned and awnless near-isogenic lines of winter wheat were used in a field study in which canopy enclosure apparatus and carbon-14 dosing were employed to assess the contribution of the awns to photosynthesis and grain yield. Awns contributed an average of 12-2% to canopy gross photosynthesis but did not increase the net photosynthesis of the complete canopy. The presence of awns decreased photosynthesis in the remaining ear structures and in the flag and penultimate leaf laminae. Seven days after dosing during the phase of rapid grain filling, 80% of the carbon recovered was located in the grains. The awns intercepted 9% of the incident visible radiation when fully green, and senesced at similar rates as the ears and flag leaves. In a second experiment the effect of awns on grain yield and its components was investigated in crosses segregating for height and presence of awns. Awns did not increase grain yields in either experiment. It appears that for British conditions in the absence of severe drought there is little advantage to be gained at present by breeding awned varieties of wheat.     

Structures in the cell wall that enable hygroscopic movement of wheat awns

The dispersal unit of wild wheat bears two prominent filaments called awns. The awns bend as they dry and straighten in a damp environment. This hygroscopic movement is explained by the orientation of the cellulose fibrils that build the cell wall, as follows. The stiff fibrils are embedded in a soft hygroscopic matrix. When the cell wall dries, the matrix shrinks but the fibrils do not. Therefore, the cell wall contracts in a direction perpendicular to the fibril orientation. Using X-ray scattering we identified a region at the base of the awn that contains fibrils aligned in all directions. This is the active part, which contracts as it dries and pulls the awn to a bent position. Cryo-scanning electron microscopy revealed sequential laminas which are rotated to form a nano-scale plywood construction, implying planar local order within the global isotropy. Water molecules absorbed into the matrix probably cause large microscopic distortions by expanding neighboring layers in perpendicular directions. This is thought to cause opening of tiny gaps between fiber layers, to facilitate the exchange and the transport of water through the cell wall, and thereby to increase the sensitivity of the actuating unit to moderate changes in humidity.     

Use of carbon-13 and carbon-14 natural abundances for stream food web studies

We review the use of stable carbon isotope ratios (δ¹³C) and radiocarbon natural abundances (Δ¹⁴C) for stream food web studies. The δ¹³C value of primary producers (e.g., periphytic algae, hereafter periphyton) in streams is controlled by isotopic fractionation during photosynthesis and variable δ¹³C of dissolved CO₂. When periphyton δ¹³C differs from that of terrestrial primary producers, the relative contribution of autochthony and allochthony to stream food webs can be calculated. Moreover, the variation in periphyton δ¹³C can reveal how much stream consumers rely on local resources because each stream habitat (e.g., riffle vs. pool, open vs. shaded) usually has a distinctive δ¹³C. However, periphyton δ¹³C often overlaps with that of terrestrial organic matter. On the other hand, periphyton Δ¹⁴C is less variable than δ¹³C among habitats, and reflects the Δ¹⁴C of dissolved CO₂, which could be a mixture of “aged” (Δ¹⁴C < 0 ‰) and “modern” (Δ¹⁴C > 0 ‰) carbon. This is because the Δ¹⁴C is corrected by its δ¹³C value for the isotopic fractionation during photosynthesis. Recent studies and our data indicate that many stream food webs are supported by “aged” carbon derived from the watershed via autochthonous production. The combined use of δ¹³C and Δ¹⁴C allows robust estimation of the carbon transfer pathway in a stream food web at multiple spatial scales ranging from the stream habitat level (e.g., riffle and pool) to watershed level (autochthony and allochthony). Furthermore, the Δ¹⁴C of stream food webs will expand our understanding about the time frame of carbon cycles in the watersheds.     

Nuclear energy: Health impact of carbon-14

Summary Estimates are presented on the carbon-14 generation rates in several reactor types and in peaceful nuclear explosions. If the carbon-14 generated in light water reactors is released, the population radiationdose rate it causes initially will be comparable to that resulting from the krypton-85 and tritium generated in these reactors. Because of the long half-life, the radiationdose commitment and thedose rate resulting from the environmental build-up of carbon-14 are considerably larger than those of the two other radionuclides.This work received support from the Program on Science, Technology, and Society at Cornell University, with funds given them by the Sloan Foundation, Inc.     

The production of ryegrass labelled with carbon-14

Summary The construction and operation of a growth chamber for producing plant material labelled with carbon-14 to a specified degree of uniformity is described.The specific activity of the plant material is measured by a method based on scintillation counting.     

Effect of awns and drought on the supply of photosynthate and its distribution within wheat ears

The presence of awns doubled the net photosynthetic rate of wheat ears and also increased the proportion of ¹⁴CO₂ assimilated by the ear that moved to the grain. The effect of water supply on photosynthesis and movement of assimilates was greater for leaves than ears, so that drought increased the proportion of assimilate contributed by ear photosynthesis to grain filling from 13% to 24% in the awnless ears, and from 34% to 43% in the awned ears. ¹⁴C assimilated by the ears was most important to the economy of the upper spikelets and to the distal florets in each spikelet, whereas flag leaf assimilate went mainly to the spikelets in the lower half of the ear, and to the proximal florets. Awns increased grain yield in the dry but not in the irrigated treatment, despite the large contribution of awned ears to grain filling. Either the supply of assimilate did not limit grain yield when water supply was not limiting, or there were compensating disadvantages to awns. However, they did not seem to have any adverse effect on the development of the upper florets, nor did they reduce grain number per ear.     

A comparative study of translocation of assimilated14C from leaves of different species

Summary Translocation of assimilated¹⁴C from the leaves of different species varied both in the rate of export and in the total percentage moved out. Those species which are known to have high photosynthetic rates, such as the tropical grasses sorghum and millet, exported 70% or more of the assimilated¹⁴C during the first 6 h after assimilation, compared to values of 45 to 50% for tomato, castor bean,Nicotiana affinis and soybean.The compounds in which the¹⁴C was retained in the leaves varied from species to species. Except for castor bean only small amounts were retained in sucrose, with generally much higher amounts in fructose, glucose and malic acid. Most of the¹⁴C was retained in the ethanol-insoluble fraction.This work was supported by a grant from the National Research Council (A 1720).Deceased June, 1968.     

Distribution patterns of assimilated 14C in vegetative and reproductive shoots of Lolium perenne and L. temulentum

The movement of ¹⁴C-labelled assimilate to the terminal meristem, stem, mature leaves, tillers and roots was measured in Loliurn perenn and Lolium temulentum after exposure to ¹⁴C0₂ of the youngest fully-expanded leaf and, on fewer occasions, the oldest healthy leaf on the main shoot. During early vegetative growth, the terminal meristem, tillers and roots received most of the ¹⁴C exported from the youngest leaf. As the shoot aged, more ¹⁴C was exported to the terminal meristem and tillers and less to roots. When the stem became a sizeable sink for ¹⁴C at the six-leaf (L. temulentum) or eleven-leaf (L. perenne) stage, less ¹⁴C moved to tillers and much less to roots. The terminal meristem continued to receive ¹⁴ at a steady rate throughout late vegetative growth. The transition from vegetative to reproductive growth in both species was marked by an abrupt increase in the export of ¹⁴C to stem from the upper leaf, but there was little change in the proportion of ¹⁴C which moved to the developing leaves and incipient inflorescence at the terminal meristem. At the same time, less ¹⁴C moved to tillers and much less to roots. Immediately before ear emergence, the export of ¹⁴C from the upper leaf (flag leaf) to the stem declined and the proportion moving to the ear increased, reaching a maximum of 55–75% as the ear emerged. The relative patterns of export of upper and lower leaves showed that while some ¹⁴ moved from each leaf to all meristems, the proximity of actively growing meristems appeared to be the main factor which determined the destination of most exported ¹⁴C. The distribution of ¹⁴C from upper and lower leaves was most alike in young vegetative plants of L. perenne. At later stages of development of both species, the terminal meristem and stem received most 14¹⁴C from the upper leaf, while roots and tillers received mos 14¹⁴C from the oldest leaf at the base of the shoot.     

Stomatal control of gas exchange in barley awns

The gas exchange of barley ears and awns was measured in the field using a gas analysis system and a diffusion porometer. Awn stomatal resistance decreased with increasing irradiance but to a smaller extent than leaf stomatal resistance. Measurements on ears immediately before and after successively removing awns showed that awn transpiration and photosynthesis were proportional to awn area and that awns accounted for 73% of transpiration by the ear. Although the maximum rates of photosynthesis of which awns were capable declined with age, awns accounted for 80–115% of the net CO₂ uptake of complete ears because the ears-less-awns could respire more CO₂ than they absorbed. Ear photosynthesis accounted for 52% of the weekly increment in ear dry weight after ear emergence, but 5 weeks later photosynthesis by the ear balanced respiration. Overall photosynthesis by the ear accounted for 35 % of its final weight. Differences in the light response curves of leaves and ears can be fully accounted for by the different relationships between stomatal resistance and irradiance of the two organs.     

Labelling patterns of carbon-14 in net plankton during a winter-spring bloom

Weekly surface samples were collected in lower Narragansett Bay, Rhode Island, during the 1975 winter-spring bloom and fractionated by nets to nannoplankton (<20 μm) and total (< 158 μm) size fractions. Each size fraction was assayed for paniculate carbon, nitrogen, carbohydrate, protein, chlorophyll a, and cell counts. The <20 μm values were subtracted from the <158 μm values to estimate the composition of the 20 μm to 158 μm fraction (termed net plankton). As nutrients (primarily nitrogen) decreased to undetectable levels with the culmination of the diatom bloom, the ratios of protein/carbohydrate, carbohydrate/carbon, and carbon/chlorophyll a in the net plankton indicated the diatom population was increasingly nutrient-limited. Each size fraction was also incubated at a saturating light intensity with carbon-14; following filtration, the cells were extracted with solvents to obtain labelled polysaccharide and protein. The daily rates of polysaccharide and protein synthesis in the net plankton declined as the bloom entered the stationary phase. When the diatom population was at its maximum density the majority of the carbon-14 was found in the ethanol-soluble fraction; this may be due to high light intensities or nutrient effects.     

Bicarbonate as tracer for assimilated C and homogeneity of 14C and 15N distribution in plants by alternative labeling approaches

Aims

Application of carbon (C) and nitrogen (N) isotopes is an essential tool to study C and N flows in plant-soil-microorganisms systems. When targeting single plants in a community the tracers need to be added via e.g., leaf-labeling or stem-feeding approaches. In this study we: (i) investigated if bicarbonate can be used to introduce ¹⁴C (or ¹³C) into white clover and ryegrass, and (ii) compared the patterns of ¹⁴C and ¹⁵N allocation in white clover and ryegrass to evaluate the homogeneity of tracer distribution after two alternative labeling approaches.

Methods

Perennial ryegrass and white clover were pulse labeled with ¹⁵N urea via leaf-labeling and ¹⁴C either via a ¹⁴CO₂ atm or with ¹⁴C bicarbonate through leaf-labeling. Plants were sampled 4 days after labeling and prepared for bulk isotope analysis and for ¹⁴C imaging to identify plant parts with high and low ¹⁴C activity. Subsequently, plant parts with high and low ¹⁴C activity were separated and analyzed for ¹⁵N enrichment.

Results

Bicarbonate applied by leaf-labeling efficiently introduced ¹⁴C into both white clover and ryegrass, although the ¹⁴C activity in particular for white clover was found predominantly in the labeled leaf. Using ¹⁴C imaging for identification of areas with high (hotspots) and low ¹⁴C activity showed that ¹⁴C was incorporated very heterogeneously both when using bicarbonate and CO₂ as expected when using pulse labeling. Subsequent analysis of ¹⁵N enrichment in plant parts with high and low ¹⁴C activity showed that ¹⁵N also had a heterogeneous distribution (up to two orders of magnitude).

Conclusion

Bicarbonate can efficiently be used to introduce ¹⁴C or ¹³C into plant via the leaf-labeling method. Both ¹⁴C and ¹⁵N showed heterogeneous distribution in the plant, although the distribution of ¹⁵N was more even than that of ¹⁴C.