ALBERT

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 A digital registration system used with temperature- and humidity-controlled cuvettes for net photosynthesis and transpiration measurements in the field is described. The associated errors of the measured parameters and calculated data are estimated. The digitalization is based on an analogue registration which is of primary importance in the control of experimental conditions in the cuvettes. The digital system is connected to the analogue registration in series. The error associated with digitalization is 0.1% across 70% of the scale. This error increases to 0.2% between 3 and 30% on the scale due to a minor lack of linearity. The reproducibility of the digitalization is ±0.024%. The error associated with data transfer in the digitalization and the errors of the analogue registration are estimated for temperature and humidity measurements (error of air and leaf temperature is ±0.1° C; error of the dew point temperature is ±1.1° C dew point). The effect of these errors on the calculation of relative humidity and the water vapour difference between the leaf and the air is determined using the progressive error law. At 30° C and 50% relative humidity, the error in relative humidity is ±7.4%, the error for the water vapour difference is ±6.6%. The dependence of these errors on temperature and humidity is shown. The instrument error of the net photosynthesis measurement is calculated to be ±4.2%. Transpiration measurements have an average inaccuracy of ±8.3%. The total diffusion resistance which is calculated from values of transpiration and the water vapour difference has an average error of ±10.9%. The sizeable influence of errors in humidity and temperature measurements on the calculated diffusion resistance is demonstrated. The additional influence of biological errors associated with field measurements is discussed.

Notes: Summary During the dry season in the Negev desert (Israel) Artemisia herbaalba in its natural habitat has a very low water content. It shows values of negative hydrostatic pressure in the xylem down to -163 bars and an extreme of osmotic potential in the leaves of -92 bars. The diurnal water stress does not decrease strongly in the night. Under these conditions Artemisia is still photosynthetically active for a few hours of the day during the whole dry period.

Notes: Summary A temperature- and humidity-controlled plant chamber for CO2 and H2O exchange measurements in the field is described in which the heat exchanger assembly and humidity controlling water vapour trap are separated from the plant cuvette. The shape and construction material of the plant cuvette can vary according to the demands of the experimental conditions and the size and growth form of the plant. The natural illumination field is only slightly altered in this plant cuvette. In the chamber, the temperature and humidity conditions can either be held constant throughout a wide range of conditions or can be programmed to track ambient condition. In this manner, not only temperature and absolute humidity are replicated, but it is also possible to reproduce the natural conditions of water vapour gradient between the evaporating surfaces in the mesophyll and the atmosphere, the relative humidity of the air, and the temperature difference between the leaf and the ambient air. Thus, the chamber appears to be an appropriate instrument to investigate with sufficient accuracy the reactions of individual plants in cultivation or in natural communities under field conditions.

Notes: Summary In a montane beech (Fagus sylvatica) forest the influence of the climatic factors, light and temperature, on net photosynthesis and on the CO2 balance of the ground vegetation was investigated. The total turnover of carbon was calculated. Species studied included: Athyrium filix-femina, Oxalis acetosella, Luzula luzuloides, Deschampsia flexuosa and young plants of Fagus sylvatica. 1. The light compensation point in all spp. is between 300 and 500 lux except for D. flexuosa where it is 2 klx. Light saturation is attained at 2–3 klx for A. filix-femina, at 5–6 klx for O. acetosella, and at 6–7 klx for L. luzuloides and F. sylvatica. The net photosynthesis of D. flexuosa increases linearly upto 12 klx. This plant, therefore, is more closely related to plants with high light requirements than all the other species under experiment. 2. The maximum rates of net photosynthesis in O. acetosella and A. filix-femina are higher than in all the other plants, independent of the reference system. Per unit dry weight they even attain rates of CO2 uptake (22–27 mg CO2/gdw·h) known from herbs under the much better light conditions of an open habitat. F. sylvatica and L. luzuloides exhibit per unit dry weight only 30% of this rate and D. flexuosa 25%. On a leaf surface area and chlorophyll content basis differences are smaller: F. sylvatics attains 75%, L. luzuloides reaches 50% and D. flexuosa only 30% of the maximal rates of net photosynthesis of O. acetosella and A. filix-femina. The higher CO2 uptake of O. acetosella and A. filix-femina points to a better adaptation of their photosynthetic apparatus in comparison to all the other species of the same habitat. 3. At light saturation the temperature optimum of A. filix-femina and O. acetosella covers a smaller range at lower temperatures than was found in the other species. These attain almost maximal rates of net photosynthesis over the whole range of temperatures of their natural habitat. At decreasing light intensities the temperature optimum of O. acetosella changes from 13–18° C at 8–12 klx to a range of even lower temperatures (9–12° C at 1 klx). 4. The respiration of the rhizome and the roots of O. acetosella is per unit dry weight 40% of the dark respiration rate in the above ground material. 5. The daily gain of net photosynthesis per unit dry weight of O. acetosella and A. filix-femina is 4 times as high as in L. luzuloides and in F. sylvatica and 7 times as high as in D. flexuosa. Per unit of surface area and chlorophyll content differences are smaller. The sequence in all cases remains the same. During the night D. flexuosa has the highest relative respiratory loss. Its CO2 gain over 24 hours is very small. 6. The importance of sun flecks on the CO2 balance is small in all species except D. flexuosa. More important is the mean light intensity and the rate of net photosynthesis which is attained under these conditions. The amount of CO2 photosynthetically bound in sun flecks is 6% of the daily balance in A. filix-femina, 16–19% of the daily balance in O. acetosella, L. luzuloides and F. sylvatica, and 27% of the daily balance in D. flexuosa. The existence of D. flexuosa is dependent on the occurrence of sun flecks on the forest floor. The ecological significance of the relative light intensity in the mosaic-like distribution of plants on the forest floor is discussed. 7. The varying success in adaptation to the conditions of the habitat becomes even more evident when compared with the primary production of the beech crown. The daily gain of net photosynthesis of O. acetosella and A. filix-femina per unit dry weight is much larger than in either the sun or shade leaves in the canopy of the same stand. Per unit surface area of the leaves they attain 18–20%, per unit chlorophyll content 32% (L. luzuloides and the young plants of F. sylvatica 16–27%, D. flexuosa 4%) of the gain of net photosynthesis in the beech sun leaves. 8. A comparison with a model of primary production (maximal rates of net photosynthesis under experimentally optimal conditions over the whole day = 100%) shows what effect the different climatic factors of the natural habitat have in limiting the CO2 balance, and to what extend the actual CO2 gain reaches the physiological optinum. On the forest floor the rate of net photosynthesis is reduced primarily through the intense shade of the beech canopy and by dawn and dusk (reduction of the maximal CO2 gain in O. acetosella and in A. filix-femina ca. 50%, in L. luzuloides and F. sylvatica ca. 60% and in D. flexuosa ca. 86%). The effect of additional clouds is smaller (reduction of the maximal CO2 gain in all species 4–6%, maximal 19%). The effect of temperature is very small for L. luzuloides, F. sylvatica and D. flexuosa. Corresponding to the low temperature optimum the influence of the prevailing temperatures is much higher in O. acetosella and A. filix-femina (4–6% reduction of the maximal CO2 gain through temperatures above optimum). 9. For an assessment of the competition potential of O. acetosella compared to L. luzuloides and F. sylvatica for the vegetation period of April to August, the carbon balances of the whole plants were estimated. These are compared with the dry weight increase. Among the three life forms of the rhizome geophyte (Oxalis), the hemicryptophyte (Luzula) and the phanerophyte (Fagus) there are striking differences in the use and in the distribution of the CO2 gain. L. luzuloides invests 65% of the net photosynthetic gain as dry weight increment (O. acetosella only 44%, F. sylvatica 40%). Moreover, the growth of L. luzuloides takes place primarily above ground with the establishment of new leaves. The relative proportion of the growth above ground to the total dry weight increment is for L. luzuloides 63%, as against 57% in F. sylvatica and only 42% in O. acetosella. In respect to the total carbon balance the better use of the CO2 gain gives L. luzuloides in this habitat a higher competition potential than the photosynthetically more active O. acetosella. The constitutional differences in the photosynthetic activity are compensated for by the distribution of the assimilates to shoot and root.

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.

Notes: Summary The stomata of plants growing in the Negev Desert, namely the stomata of the mesomorphic leaves of Prunus armeniaca, the xeromorphic stems of Hammada scoparia, and the succulent leaves of Zygophyllum dumosum, respond to changes in air humidity. Under dry air conditions diffusion resistance increases. Under moist air conditions diffusion resistance decreases. When the stomata close at low air humidity the water content of the apricot leaves increases. The stomata open at high air humidity in spite of a decrease in leaf water content. This excludes a reaction via the water potential in the leaf tissue and proves that the stomatal aperture has a direct response to the evaporative conditions in the atmosphere. In all species the response to air humidity is maintained over a period of many hours also when the soil is considerably dry. The response is higher in plants with poor water supply then in well watered plants. Thus for field conditions and for morphologically different types of photosynthesizing organs the results confirm former experiments carried out with isolated epidermal strips.