Growth and yield of winter wheat (Triticum aestivum L.) and corn (Zea mays L.) near a high voltage transmission line.

The objective of this study was to determine the effects of an electromagnetic field from a high voltage transmission line on the yield of agricultural crops cultivated underneath and near the transmission line. For 5 years, experiments with winter wheat and corn were carried out near the 380 kV transmission line Dürnrohr (Austria)-Slavetice (Czech Republic). Different field strengths were tested by planting the crops at different distances from the transmission line. The plants were grown in experimental plots (1.77 m2), aligned to equal electric field strengths, and were cultivated according to standard agricultural practice. The soil for all plots was homogenized layer-specifically to a depth of 0.5 m to guarantee uniform soil conditions in the plant root environment. The soil was sampled annually for determinations of carbon content and the behavior of microbial biomass. During development of the vegetation, samples were collected at regular intervals for growth rate analyses. At physiological maturity, the plots (n = 8) were harvested for grain and straw yield determinations. The average electric and magnetic field strengths at four distances from the transmission line (nominal distances: 40, 14, 8, and 2 m) were between 0.2 and 4.0 kV/m and between 0.4 and 4.5 micro T, respectively. No effect of the field exposures on soil microbial biomass could be detected. The wheat grain yields were 7% higher (average of 5 years) in the plots with the lowest field exposure than in the plots nearer to the transmission line (P <.10). The responses of the plants were more pronounced in years with drought episodes during grain filling than in humid years. No significant yield differences were found for corn yields. The extent of the yield variations attributed to the distance from the transmission line was small compared to the observed annual variations in climatic or soil specific site characteristics.

Does ozone exposure protect from photoinhibition?

Tropospheric ozone and high light intensities are two stress factors that often occur simultaneously under natural conditions. Ozone is well known to form oxygen radicals in the apoplastic water and long lasting photoinhibition can cause photooxidative damage also by formation of several species of oxygen radicals. We were interested whether moderate levels of ozone would be able to modulate the response of leaves to photoinhibitory conditions naturally occurring around noon on a bright day. Cuttings of Populus sp. were cultivated in two separate greenhouse-compartments adapted as fumigation chambers. In the two compartments plants were grown in ambient air containing about 20 nmol mol(-1) ozone and in elevated ozone concentrations supplied for 8 h per day. During the midday of bright days Fv/Fm decreased by the same amount in all leaves, indicating photoinhibition. At the same time Fo increased in control leaves more than in ozone-exposed leaves indicating a higher amount of heat-deactivating PSII centres in the latter. This was confirmed by a higher epoxidation state in ozone-exposed leaves during midday of a bright day. The contents of chlorophyll a and chlorophyll b were significantly decreased in ozone-exposed leaves. In older leaves the ratios chlorophyll a : b, and xanthophylls: chlorophyll b were increased indicating an adaptation to higher light stress. From this we conclude that by increasing the amount of heat-deactivating centres ozone seems to protect PSII from photoinhibition.

Analysis of light-induced reduction of the photochemical capacity in field-grown plants. Evidence for photoinhibition?

It was tested whether field-grown plants (Phaseolus vulgaris, Zea mays and Helianthus annuus) reflect photoinhibitory effects under natural conditions. Attached leaves were used for determination of the photochemical capacity of Photosystem II (Fv/Fm) by means of a portable fluorimeter (PSM, BioMonitor, S.). For a more qualitative description of Fv/Fm, the modifications of the absolute values F0, Fm as well as of the half-rise time of Fm (T/2) were also considered. By comparing artificially shaded and 'sun exposed' plants, the direct influence of light on the photochemical capacity was investigated. Under low natural light conditions the differences of the photochemical capacity between shaded and 'sun exposed' leaves were negligible in all three species. On a day with full sunlight a decline of Fv/Fm was observable at noon-time in the 'sun exposed' leaves of all three species, although the absolute values differed between the species compared. Additionally, the extent of the recovery of Fv/Fm was varying. Both phenomena could be due to differences in the photosynthetic apparatus (e.g., C3-C4, ontogenetic stage, sun-shade type), to self-shading phenomena (comparing leaf layers of Zea and Helianthus) or to differences in the activity of repair mechanisms possibly caused by other environmental factors (vapour pressure deficit = VPD, drought and temperature phenomena).Nevertheless, the results of the shading experiments and the comparison of species lead to the conclusion that primarily light-induced reduction of the photochemical capacity appears at noon in leaves exposed to full sunlight, a partial restoration of Fv/Fm takes place till the evening. Artifically shaded plants show only a slight alteration of the photochemical capacity.

Separating the contribution of the upper and lower mesophyll to photosynthesis in Zea mays L. leaves.

The appearance of transverse sections of maize leaves indicates the existence of two airspace systems serving the mesophyll, one connected to the stomata of the upper epidermis and the other to the stomata of the lower surface, with few or no connections between the two. This study tests the hypothesis that the air-space systems of the upper and lower mesophyll are separated by a defined barrier of measurable conductance. A mathematical procedure, based on this hypothesis, is developed for the quantitative separation of the contributions made by the upper and lower halves of the mesophyll to carbon assimilation using gasexchange data. Serial paradermal sections and three-dimensional scanning-electron-microscope images confirmed the hypothesis that there were few connections between the two air-systems. Simultaneous measurements of nitrous-oxide diffusion across the leaf and of transpiration from the two surfaces showed that the internal conductance was about 15% of the maximum observed stomatal conductance. This demonstrates that the poor air-space connections, indicated by microscopy, represent a substantial barrier to gas diffusion. By measuring the CO2 and water-vapour fluxes from each surface independently, the intercellular CO2 concentration (c i) of each internal air-space system was determined and the flux between them calculated. This allowed correction of the apparent CO2 uptake at each surface to derive the true CO2 uptake by the mesophyll cells of the upper and lower halves of the leaf. This approach was used to analyse the contribution of the upper and lower mesophyll to CO2 uptake by the leaf as a whole in response to varying light levels incident on the upper leaf surface. This showed that the upper mesophyll was light-saturated by a photon flux of approx. 1000 µmol·m(-2)·s(-1) (i.e. about one-half of full sunlight). The lower mesophyll was not fully saturated by photon fluxes of nearly double full sunlight. At low photon fluxes the c i of the upper mesophyll was significantly less than that of the lower mesophyll, generating a significant upward flux of CO2. At light levels equivalent to full sunlight, and above, c i did not differ significantly between the two air space systems. The physiological importance of the separation of the air-space systems of the upper and lower mesophyll to gas exchange is discussed.

Temperature and light dependent modifications of chlorophyll fluorescence kinetics in spruce needles during winter.

Prompt chlorophyll a fluorescence kinetics at room temperature were measured from intact spruce needles. The fluorescence signal was recorded after varying light pretreatments. During the winter, induction curves showed characteristic changes in both the initial peak of fluorescence FV/FP (FP-FO/FP) and the steady state level Fdr (FP-FT/FP). Winter stress induced decreases in both values which showed close correlation to the light and temperature pre-history of the plants. In February changes in fluorescence induction indicative of a restoration of photosynthesis were detected and these corresponded to a rise of temperature above zero in combination with low light levels. In March increasing light intensity combined with chilling temperatures induced again decreases of both values of chlorophyll fluorescence induction suggesting the occurrence of photoinhibition.

Respiration and Energy Turnover During the Seedling Development of Triticum aestivum L., Zea mays L., Helianthus annuus L., and Phaseolus vulgaris L.

Seedling length, CO(2) output, dry weight and energy content of the seeds and seedling organs of Triticum, Zea, Helianthus and Phaseolus were determined in order to study the efficiency of storage substance utilization and energy turnover during seedling development up to 108 h. The respiration rate is lower in the large seeds of Zea and Phaseolus (2.2 and 2.0 mgCO(2)·gDW(-1)·h(-1)) than in the small seeds of Triticum and Helianthus (3.1 and 4.2 mgCO(2)·gDW(-1)·h(-1)). CO(2) output during the synthesis of seedling organs reaches a maximum value in Triticum, Zea and Helianthus after 36 h (1.0, 1.7 and 1.3 gCO(2)·gDW(n)(-1), resp.) and in Phaseolus after 60 h (3.0 gCO(2)·gDW(n)(-1)). This high CO(2) evolution is caused by the lag phase of growth at the beginning of germination, as indicated in the RGR course. The loss of mobilized storage substances caused by CO(2) evolution amounts to 46% in Phaseolus and is less (av. 30%) in the other seeds. The energy loss in relation to the energy of the ungerminated seed is relatively high in the small and energy-rich seeds of Triticum (19%) and Helianthus (21 %) compared with the large seeds of Zea (8 %) and Phaseolus (13 %). The efficiency of energy utilization corresponding to the energy stored in the newly formed seedling organs is low in Helianthus (40%) and Phaseolus (43%) and relatively high in Triticum (55%) and Zea (6 %). The calorific equivalent of the CO(2) amount evolved differs widely (11 to 24 kJ·gCO(2)(-1)). With the exception of Zea these values are much higher than the calorific equivalent of the dry weight loss. The latter may be explained by energy losses without CO(2) evolution, as well as by CO(2) refixation by PEPC and by energy dissipation in the maintenance metabolism.