Modelling the exchange and transformation of matter and energy in ecosystems requires the development of hierarchical structured models of the considered ecosystem compartments. In this context, a model describing the coupled CO2 and H2O gas exchange of a winter wheat canopy was developed and calibrated. The formidation of the model was related to the problems of linking processes at different systém levels. For model calibration, ecophysiological gas exchange characteristics and micro-meteorological data were obtained on both leaf and canopy levels and completed by results of structural and Chemical plant analysis. The gas exchange was measured by a computer-controlled multi-channel systém. On the basis of this data pool, the canopy gas fluxes were calculated by the model as the integrál of the corresponding local fluxes over the area elements of the canopy. The model describes correctly physiological interactions and gas exchange characteristics at both the leaf and canopy levels.
Membrane-bound bicarbonate is believed by some to act as an essential activator of photosystem 2 (PS2) electron transport. Formáte and other inhibitory monovalent anions act by removing bound-bicarbonate. This belief relies to a great extent on the observation that formáte (100 mM) pretreated thylakoids exhibit a non- proportionality between Hill activity (HAR) and chlorophyll (Chl) concentration when preirradiated with bright radiation in reaction mixture that contains only 5 mM formáte. The non-linearity was attributed to a supposed loosening of residual bicarbonate still present after formáte treatment and which would be more abundant at higher Chl concentrations. In repeating this experiment, we observed an increase in HAR at higher Chl concentrations in preirradiated, but also in non-preirradiated samples, the latter were simply left in the dark for 3 min before measurements were made. Therefore, preirradiation is not needed to restore some HAR in formáte pretreated samples; a 3 min wait in the electrode chamber at low formáte concentration is sufficient to partially relieve the formáte inhibition of PS2 activity. Moreover, HAR in samples preirradiated by weak radiation, or not preirradiated at all, was directly proportional to Chl concentration. We can attribute the increase in activity to a dissociation of bound formáte, not necessarily to the effect of residual bicarbonate. Non-linearity in HAR with Chl concentration was found only in high- irradiance pretreated samples. We can attribute this to a greater amount of photoinhibition occurring in the dilute samples, where the effective irradiance was greater. There is no need to postuláte the existence of residual bound bicarbonate to explain these results.
The effect of enhanced air CO2 concentrations (C520 and 0^50 = 520 and 650 cm^ m"^) on the growth of Lamium galeobdolon and Stellaria holostea and on the competition between the two species was examined. After five months growth imder CO2 enrichment the dry masses of both species increased when the plants were grown in monoculture, but the increase in biomass was much more pronounced in Stellaria. When the plants were grown together in competition, the measured shoot masses of Stellaria were again higher under C520 and 0^50 than at ambient CO2 concentration (C390 = 390 cm^ m'^), while the shoot masses of Lamium strongly decreased at Cgso- The effect of CO2 enrichment on the two plant species in monoculture differed significantly from that observed in mixed cultures. In terms of plant relative yield, Stellaria benefitted slightly but insignificantly from competition, while Lamium was significantly suppressed imder c^sq. Total community production of the mixed culture was optimum at C520, while that of the monocultures was highest at c^sq. At C390 and C520, growth of Stellaria depended strongly on irradiance in all types of culture. At C650 no such dependence could be demonstrated.