Is stimulation of leaf photosynthesis by elevated carbon dioxide concentration maintained in the long term? A test with Lolium perenne grown for 10 years at two nitrogen fertilization levels under Free Air CO2 Enrichment (FACE)

Photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration ([CO2]), a physiological response that could significantly alter the future carbon cycle if it persists in the long term.. Yet an acclimation of photosynthetic capacity suggested by theoretical models and short-term experiments could completely remove this effect of CO2. Perennial ryegrass (Lolium perenne L. cv. Bastion) was grown under an elevated [CO2] of 600 mumol mol(-1) for 10 years using Free Air CO2 Enrichment (FACE), with two contrasting nitrogen levels and abrupt changes in the source: sink ratio following periodic harvests. More than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated [CO2] across each growing season for the duration of the experiment. Over the 10 years as a whole, growth at elevated [CO2] resulted in a 43% higher rate of light-saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake. Photosynthetic stimulation was maintained despite a 30% decrease in stomatal conductance and significant decreases in both the apparent, maximum carboxylation velocity (V-c,V-max) and the maximum rate of electron transport (J(max)). Immediately prior to the periodic (every 4-8 weeks) cuts of the L. perenne stands, V-c,V-max and J(max), were significantly lower in elevated than in ambient [CO2] in the low-nitrogen treatment. This difference was smaller after the cut, suggesting a dependence upon the balance between the sources and sinks for carbon. In contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no significant change in photosynthetic stimulation across a 10-year period, nor greater acclimation in V-c,V-max and J(max) in the later years in either nitrogen treatment

A CO2 Concentrating System in Leaves of Higher C3-Plants Predicted by a Model Based on RuBP Carboxylase/Oxygenase Kinetics and 14CO2/12CO2 Exchange

Mächler, F., Lehnherr, B., Schnyder, H. and Nösberger, J. 1985. A CO2 concentrating system in leaves of higher C3-plants predicted by a model based on RuBP carboxylase/oxygenase kinetics and 14CO2/12CO2 exchange.−J. exp. Bot. 36: 1542-1550. A model is presented which compares the ratio of the two activities of the enzyme nbulose bisphosphate carboxylase/oxygenase as determined in vitro with the ratio of photosynthesis to photorespiration in leaves as determined from differential 14CO2/12CO2 uptake or from CO2 compensation concentration. Discrepancies between measurements made in vitro and in vivo are attributed to the effect of a CO2 concentrating system in the leaf cells. Interference from dark respiration is discussed. A CO2 concentrating system is postulated which is efficient mainly at low temperature and low CO2 concentratio

Food for thought: Lower than expected crop yield stimulation with rising carbon dioxide concentrations.

published or submitted for publicatio

The effect of removing florets or shading the ear of barley on production and distribution of dry matter

RESP-533

The influence of removing tubers on dry-matter production and net assimilation rate of potato plants

RESP-535

Variation in growth and N2 fixation within and among Centrosema spp. in response to phosphorus supply

Four inoculated ecotypes of each of Centrosema acutifolium, C. brasilianum and C. macrocarpum were grown in undisturbed soil (oxisol) cores in a glasshouse to study their response to 7 phosporus levels (a total of 11, 22, 45, 90, 180, 360 and 720 mg/core P, surface applied as Ca(H2PO4)2.H2O). Seven weeks after planting, dry matter production, leaf area, N yield, nodulation, nodule-P concentration and acetylene reduction activity increased strongly with P supply. Enhanced leaf area was due mainly to increases in individual leaf size, especially in the large-leaved C. macrocarpum. Shoot-N concentration of all species increased with P supply, which was attributed to increased nodule mass and N2 fixation, but not to a greater proportion of leaf material. The response to P supply in leaf number, leaf size and leaf-to-stem ratio as well as nodule number and weight differed among species. However, there was no significant (P > 0.05) species × P rate interactions for dry matter and N yield, shoot-N %, or nodule-P %, but strong intra-specific variation was found. All 3 Centrosema species contain important genetic variability in the growth and N2 fixation responses to P supply during establishment. Therefore both parameters should be taken into account in selecting ecotypes

INTERNAL ROTATIONS IN DINITROMETHANE

Author Institution: Laboratory for Physical Chemistry, ETH, Universit\""{a}tstrasse 22""A theory for describing the rotation-internal rotation spectra of molecules with two equivalent internal rotors of $C_{2{v}}$ symmetry is developed. Examples are dinitromethane, benzophenone, diphenylsulfone, etc. Starting from a semirigid molecular model the quantum mechanical Hamiltonian and its symmetry group are derived. The lowest eigenvalues are obtained through diagonalization of the truncated irreducible blocks of the energy matrix. Group theoretical arguments are used in the derivation of selection, rules for electric dipole transitions and spin-statistical weights. The characteristics of the microwave and far infrared spectra of such molecules will be shown