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Notes: The rate coefficient for the ion–molecule reaction NH+3 +H2→NH+4+H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50–100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH+3+H2+He→NH+4+H+He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.

Notes: Abstract. With the aid of specifically designed potometer experiments, it is shown that, after ozone fumigation, twigs transpiring in gas exchange chambers show poor water balance in decreasing humidity. The quotient of water uptake to water loss never falls below 0.9 in healthy material because of the control capacity of the stomata. In twigs from a tree fumigated with ozone irregular and delayed stomatal closure results in values of 〈 0.5 or even lower, depending on the degree of damage. As a result, in dry air, the transpiration rates of fumigated twigs often fall far below those of the control material, even if they were higher than the latter in humid air. In analogous experiments, the difference in behaviour between twigs of densely (‘healthy’) and sparsely needled (‘damaged’) trees from the natural stand is comparable to the difference between controls and ozone-fumigated trees in most respects. In soil that is more or less dried out and after the best possible saturation of the twigs during the night, the transpiration rates of fumigated trees increase fairly strongly in the humid chamber air at dawn, but finally decrease more or less suddenly to lower values than in the controls. The results are placed in the context of the basic research on plant water relations and compared with histological changes in the stomatal apparatus after a period of fumigation as described earlier. Therefore, long-term effects of pollution can be explained as a specific distrubance of hydroregulation.

Notes: Seasonal changes of some water relations parameters of Norway spruce shoots (Picea abies [L.] Karst.) were studied during two experiments using the pressure-volume analysis. For each experiment only shoots of a single tree were used.During the first study, the course of the turgor loss point (as bulk osmotic pressure when turgor first reaches zero, πp) of shoots developed in late 1986 vegetation period, were measured in 1987. The turgor loss point decreased temporarily from –2.5 MPa at the beginning of the year to –3.3 MPa at the end of March, but then increased to the original level for the rest of the year.During the second study, water relations parameters were measured in late summer 1987 and in late winter 1988. Winter shoots at full water saturation contained up to 20% less water than in late summer. Accordingly, the bulk osmotic pressure at full water saturation (πp) decreased from –1.7 MPa in late summer to –1.9 MPa in winter, πp decreased also from –2.2 MPa to –2.8 MPa. However, the amount of osmotically active substances (mOsmol, N) remained unchanged. The relative amount of apoplastic water in the total shoot water content appeared to drop insignificantly from 17% to 15%.The results show that the decrease in πo and πp in late winter is not due to an accumulation of osmotically active substances in the vacuoles but is due to a decrease in tissue water content. The temporary reduction of the symplastic volume by deposition of osmotically inert substances seems to be the most probable cause of this phenomenon.

Notes: Using the pressure volume analysis (PV analysis) on the shoots of Norway spruce (Picea abies [L.] Karst.) and the here presented capillary microcryoscopy of the needle press sap of the same shoots, it was possible to determine the amount of apoplastic water in the needles (Wan) as well as in the defoliated shoots (Was). Additionally, the bulk osmotic pressure at full water saturation in the symplast of the needles and defoliated shoots (πon and πos) was determined. The dependence of the bulk-averaged turgor pressure (Pt) on the water content and the relationship between the bulk modulus of elasticity of the needles (ɛn) and the bulk-averaged needle turgor pressure (Ptn) was shown with help of the PV analysis on the whole shoots and defoliated shoots. The study was conducted at the end of the vegetation period in 1987 and during winter 1988.The proportion of Wan in the total needle water content (Wtn) was 14% in September 1987 and 12.5% in March 1988. The respective percentage of Was in Wts were 27% and 25%. The amount of apoplastic water depended on the ratio of the dry weight of defoliated shoots to the dry weight of the whole shoots. A standard mean value for the amount of Wan in the total water content of the shoots (Wt) was therefore not possible. The bulk osmotic pressure at full water saturation in the needle symplasts was –1.9 MPa in September 1987 and –2.2 MPa in winter 1988. The respective values of the bulk osmotic pressures in the symplast of the defoliated shoots (πos) were –1.5 MPa and –1.7 MPa. Thus πon was 0.1 MPa lower and πos 0.3–0.4 MPa higher than the average osmotic pressure during full water saturation in the symplast of the whole shoots (πo).The relation between bulk-averaged turgor pressure and water content showed that during water loss Ptn dropped more rapidly than the turgor pressure of defoliated shoots (Pts). Consequently the needles were less elastic than the defoliated shoots. The turgor values of whole shoots followed an intemediate course between Ptn and Pts. The flat course of Pts seems to be the main reason for the often observed “plateau” of ψ-isotherms of whole shoots near full turgor.

Notes: Summary The influence of climatic factors on net photosynthesis, dark respiration and transpiration was investigated in the Negev Desert at the end of the dry summer period when plant water stress was at a maximum. Species studied included: dominant species of the natural vegetation (Artemisia herba-alba, Hammada scoparia, Noaea mucronata, Reaumuria negevensis, Salsola inermis, Zygophyllum dumosum), cultivated plants receiving rainfall and run-off water during the winter season in the run-off farm Avdat (Prunus armeniaca, Vitis vinifera), and irrigated cultivated plants receiving additional water during the summer season (Citrullus colocynthis, Datura metel). 1. Light saturation of net photosynthesis was reached at 60–90 klx conforming to the high solar radiation intensities of the desert. 2. Maximum rates of CO2 uptake per unit of dry weight for the irrigated mesomorphic plants was ten times that of the wild plants. However, in comparison to the other species, maximal rates of CO2 uptake for wild plants were higher when calculated on a leaf area basis than when represented on a dry weight basis. Maximum rates of net photosynthesis per unit chlorophyll content for some of the wild plants (Salsola and Noaea) were comparable to those of the cultivated Vitis and irrigated Citrullus and Datura, Hammada exhibited even higher rates than Prunus. This demonstrates the great photosynthetic capacity of the wild plants even at the end of the dry season. 3. The upper temperature compensation point for net photosynthesis of the wild plants was unusually high as an adaptation to the temperatures of the habitat. Compensation points higher than 49°C exceed the maxima known so far for other flowering species. Maximum rates of net photosynthesis of Hammada were measured when the temperature of the photosynthetic organs was 37°C; at 49°C photosynthesis was only reduced by 50%. 4. Leaf temperature affects plant gas exchange by influencing stomatal aperture. Diffusion resistance of leaves to water vapour was reduced at low temperatures and increased at high temperatures. Reduction of net photosynthesis and transpiration of desert plants at midday may, therefore, be the result of temperature-induced stomatal closure. The possible influence of peristomatal transpiration on stomatal aperture is also discussed. Peristomatal transpiration is directly related to the vapour pressure gradient between the leaf mesophyll and the ambient air which increases with increasing temperatures. 5. Diffusion resistance to water vapour was reduced at high temperatures approaching the limits of heat resistance, due to increased stomatal aperture. This resulted in greater transpirational cooling. 6. Under conditions of increased leaf water stress, diffusion resistance increased, either by sudden stomatal closure at specific threshold values of water stress or through a continuous increase in resistance. This increased resistance is coupled with decreases in transpiration and photosynthesis. 7. In several plant species increased diffusion resistance during the course of the day caused decreased transpiration without a corresponding decrease in photosynthesis. Under these conditions, the ratio of CO2 uptake to transpiration became more favourable as the day progressed. The possibility that this favourable gas exchange response is the result of an increased mesophyll resistance to water vapour loss is discussed.

Notes: Summary Carbon dioxide exchange and transpiration measurements of various wild and cultivated plants were carried out during the dry summer period in 1967 in the Central Negev Desert (Israel). A mobile laboratory used for these investigations is described. Measurements were carried out with conditioned plant chambers which followed either the ambient temperature and humidity or else allowed the experiments to be carried out under constant conditions. The accuracy of the measurements was estimated. The mean error of the determination of the CO2 exchange rate amounts to ±0.07 mg CO2·g-1·h-1. Transpiration rate is measured with an error of ±0.15 g H2O·g-1·h-1. The response time of the instrumentation to reach 90% equilibrium after a change in photosynthesis or transpiration is 7 to 9 minutes. Errors which are caused by changes of quality of incident radiant energy and altered turbulence conditions for the leaves enclosed in the chamber, are discussed.

Notes: Summary The daily course of net photosynthesis and transpiration was measured with temperature and humidity controlled cuvettes at the end of the dry summer season in the Negev Desert. Species studied included: dominant species of the natural vegetation, cultivated plants in the run-off farm Avdat and permanently irrigated plants. An analysis of the influence of single climatic factors on gas exchange was given in part II of this publication. The reactions of the plants to complex changes in all the environmental parameters is the subject of this present study. 1. One-peaked daily courses of net photosynthesis occur in the irrigated species Citrullus colocynthis and Datura metel. After a high rate of net photosynthesis and transpiration before noon CO2 uptake is gradually reduced through stomatal closure even under good soil water conditions. Stomatal closure on C. colocynthis is controlled by the leaf temperatures whereas D. metel closes its stomata due to increasing water stress. 2. Without additional irrigation one-peaked daily courses are only possible with special constitutional adaptations to the extreme climate together with a balanced regulation of water loss. The annual Salsola inermis shows over the whole day no reduction in transpiration. Related to chlorophyll content, CO2 uptake almost attains the rates observed in the irrigated C. colocynthis. It is still unknown what type of water sources are at the disposal of this plant. The perennial chamaephytes Hammada scoparia and Reaumuria negevensis attain a stabilization of net photosynthesis at a lower level through reduction of stomatal water loss and through increased mesophyll resistance to water vapour. This reduces transpiration to a greater degree than CO2 uptake. The stomatal reactions of H. scoparia seem to be mainly controlled by the evaporation conditions in the atmosphere. Related to chlorophyll content in the assimilatory organs, net photosynthesis of both species is higher at noon than in all other chamaephytes. 3. Two-peaked daily courses of net photosynthesis are shown by plants of the run-off farm (Prunus armeniaca and Vitis vinifera). Both have a very high metabolic activity during the morning which is comparable even with that of D. metel and of H. scoparia. At noon, CO2 and H2O exchange is reduced through stomatal closure and falls below the level of all the other plants in the natural vegetation. In apricot, net photosynthesis of vertically oriented leaves does not drop to the compensation point at noon as was found with horizontally oriented leaves which had leaf temperatures some 6–8°C higher. 4. In the natural vegetation, only Noaea mucronata shows gas exchange reactions similar to those of apricot and grapvines. Also N. mucronata has high rates of net photosynthesis in the morning followed by a great reduction of gas exchange through stomatal closure at noon. At a higher water stress this type of an asymmetric two-peaked daily course is changed into a more flat symmetric two peaked curve with low metabolic activity during the morning. In Zygophyllum dumosum and in Artemisia herba-alba the reduction in CO2 uptake at noon is not caused by stomatal closure, but through temperatures above the optimum. The diffusion resistance for water vapour increases steadily during the day. 5. The daily balance of the CO2 exchange is calculated and is compared with the metabolic activity under optimal conditions. The potential photosynthetic capacity of wild plants under optimal conditions is more nearly met under the prevailing desert conditions than in the case with cultivated plants. The effects of the different types of daily courses of gas exchange on the distribution of plants of the Saharo-Arabian region (Reaumuria negevensis, Zygophyllum dumosum) and of plants of the Irano-Turanian vegetation (Hammada scoparia, Artemisia herbaalba) are discussed.