ALBERT

Notes: Summary The response of stomata to a gradual increase in temperature at increasing plant water stress was studied in a hot desert habitat (Negev, Israel) in the field, but under controlled temperature and humidity conditions. Four native species (Zygophyllum dumosum, Artemisia herba-alba, Hammada scoparia, Reaumuria negevensis) and one cultivated plant (Prunus armeniaca) were used in these studies. The stomatal response to temperature was compared with the response in well-irrigated plants of the same species. At low water stress, the diffusion resistance for water vapour decreased in response to a gradual increase in temperature. Transpiration increased accordingly. This response was reversible. All species responded in the same way. The opening of stomata with increasing temperature was apparently independent of the stomatal response regulated by atmospheric humidity. At high plant water stress, the stomatal response was reversed, i.e., the stomata closed when temperature was gradually increased. This stomatal closure was also independent of the closure regulated by atmospheric humidity. The plant water potential at which the stomatal response to temperature was reversed, differed among the species investigated.

Notes: Abstract Xylem-tapping mistletoes are known to have normally a higher rate of transpiration and lower water-use efficiency than their hosts. The relationships between water relations, nutrients and growth were investigated for Phoradendron juniperinum growing on Juniperus osteosperma (a non-nitrogen-fixing tree) and for Phoradendron californicum growing on Acacia greggii (a nitrogen-fixing tree). Xylem sap nitrogen contents were approximately 3.5 times higher in the nitrogen-fixing host than in the non-nitrogen-fixing host. The results of the present study show that mistletoe growth rates were sevenfold greater on a nitrogen-fixing host. At the same time, however, the differences in water-use efficiency between mistletoes and their hosts, which were observed on the non-nitrogen-fixing host did not exist when mistletoes were grown on hosts with higher nitrogen contents in their xylem sap. Growth rates and the accumulation of N, P, K, and Ca as well as values for carbon-isotope ratios of mistletoe tissues support the hypothesis that the higher transpiration rates of mistletoes represent a nitrogen-gathering mechanism.

Notes: Summary Large areas of the lower epidermis of full-grown leaves of Polypodium vulgare (and Valerianella locusta) are normally separated from the mesophyll by an extensive subepidermal airspace. Epidermal stripes were prepared for experiments to simulate these conditions in order to investigate stomatal reactions. They were placed with their inner surface in contact with an airspace of uniformly high humidity. The outer surface was treated with air of varying degrees of humidity. The stomatal reactions were observed by microscope and the opening of the guard cells determined photographically. Treatment of the outer side of the epidermis with dry air led to a rapid closing of the stomata, whilst moist air caused opening. This induction of opening and closing movements could be repeated up to 15 times with the same stoma by changing the degree of humidity. Neighbouring groups of stomata showed different apertures according to their individual humidity conditions. The degree of aperture of the stomata depended on the water potential of the ambient air and also on the humidity conditions in the subepidermal airspace. The cause of this stomatal behaviour could lie in the “peristomatal transpiration”. In this way, the guard cells are able to function as “humidity sensors” which “measure” the difference in water potential inside and outside the leaf. Their aperture thus is controlled by their individual transpiration conditions. This controlling mechanism could be very important for the water economy of plants. They would appear to be able to reduce their transpiration through an increase in diffusion resistance of the stomata during decreasing humidity in the ambient air, without changing the water status of the whole leaf.

Notes: Summary Experiments with Prunus armeniaca were carried out under conditions of constant temperature but varying air humidity. Experiments were also contucted with a constant water vapor difference between the evaporating sites in a leaf and the air, but with varying leaf temperature. These served as a basis for predicting the daily course of total diffusion resistance under the natural climatic conditions of a desert. For the simulation, the rsults of the experiments at constant conditions with only one variable factor are fitted with empirical equations which serve as “calibration curves” to predict the change in diffusion resistance caused by a change in humidity and temperature calculated from the meteorological data of a desert day. The simulation shows that for P. armeniaca humidity and temperature are the dominating factors in controlling the daily course of diffusion resistance. For meteorologically very different days the simulation allows the increase in diffusion resistance in the morning to be predicted with an accuracy of 90%–105% as compared to directly observed measurements. In the afternoon, especially after extreme climatic conditions during the morning, the deviation between predicted and observed values of diffusion resistance may be greater, but not more than -20% to -30%. This possibly indicates the existence of an additional factor of significance which was not included in the simulation. The two peaked curves of net photosynthesis and transpiration characteristic of plants living under arid conditions can be explained in this species by the humidity-and temperature-controlled stomatal response. This stomatal regulation leads to a decreasing total daily transpirational water loss on a dry day as compared to a moist one. The significance of this controlling mechanism for the primary production and the water relations of P. armeniaca is discussed.

Notes: Summary CO2-assimilation and leaf conductance of Larix decidua Mill. were measured in the field at high (Patscherkofel, Austria) and low (Bayreuth, Germany) elevation in Europe, and outside its natural range along an altitudinal gradient in New Zealand. Phenology of leaf and stem growth showed New Zealand sites to have much longer growing seasons than in Europe, so that the timberline (1,330 m) season was almost twice as long as at the Austrian timberline (1,950 m). The maximum rates of photosynthesis, A max, were similar at all sites after completion of leaf growth, namely 3 to 3.5 μmol m-2 s-1. Only the sun needles of the Bayreuth tree reached 3.5 to 5 μmol m-2 s-1. Light response curves for CO2-assimilation changed during leaf ontogeny, the slope being less in young than in adult leaves. The temperature optimum for 90% of maximum photosynthesis was at all sites similar between ca. 12–28°C for much of the summer. Only at the cooler high altitude timberline sites were optima lower at ca. 10–16°C in developing needles during early summer. A linear correlation existed between A max and leaf conductance at A max, and this showed no difference between the sites except for sun needles at Bayreuth. Leaf conductance responded strongly to light intensity and this was concurrent with the light response of CO2-uptake. A short-term and a long-term effect were differentiated. With increasing age maximum rates of CO2-uptake and leaf conductance at A max increased, whereas short-term response during changes in light declined. The stomata became less responsive with increasing age and tended to remain open. The stomatal responses to light have a significant effect on the water use efficiency during diurnal courses. A higher water use efficiency was found for similar atmospheric conditions in spring than in autumn. Stomata responded with progressive closure to declining air humidity in a similar manner under dissimilar climates. Humidity response thus showed insensitivity to habitat differences. From the diurnal course of gas-exchange stomata were more closed at timberline (1,330 m) than at lower elevations but this did not lead to corresponding site differences in CO2-exchange suggesting Larix may not be operating at high water use efficiency when air is humid. The main difference between habitats studied was in the time necessary for completion of needle development. Similarity in photosynthesis and leaf conductance existed between sites when tree foliage was compared at the same stage of development. Length of growing season and time requirement for foliar development appear to be a principle factor in the carbon balance of deciduous species. The evergreen habit may be more effective in counterbalancing the effects of cool short summers.

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 The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.

Notes: Summary Tradescantia virginiana L. plants were cultivated under contrasting conditions of temperature, humidity, light quality and intensity, and nutrient status in order to investigate the effect of growth conditions on the water relations parameters of the leaf epidermal cells. Turgor pressure (P), volumetric elastic modulus (ɛ), half-time of water potential equilibration (T 1/2), hydraulic conductivity (L p ) were measured with the miniaturized pressure probe in single cells of the upper and lower epidermis of leaves. Turgor differed (range: 0.1 bar to 7.2 bar) between treatments with lowest values under warm and humid conditions and additional supply of fertilizer, and highest values under conditions of low air humidity and low nutrient supply. The volumetric elastic modulus changed by 2 orders of magnitude (range: 3.0 bar to 350 bar, 158 cells), but ɛ was only affected by the treatments, in as much as it was dependent on turgor. The turgor dependence of ɛ, measured on intact leaves of T. virginiana, was similar to that for cells of the isolated (peeled) lower epidermis, where ɛ as a function of turgor was linear over the whole range of turgors. This result has implications for the discussion of pressure/volume curves as measured by the pressure bomb where changes in “bulk leaf ɛ” are frequently discussed as “adaptations” to certain treatments. The measurements of the hydraulic conductivity indicate that this parameter varies between treatments (range of means: 2.4×10-6 cm s-1 bar-1 to 13.4×10-6 cm s-1 bar-1). There was a negative correlation for L p in cells of intact leaves as a function of turgor which was altered by the growing conditions. However, a correlation with turgor could not be found for cells from isolated epidermis or cells from a uniform population of plants. The large variation in L p from cell to cell observed in the present and in previous studies was accounted for in a study of 100 cells from a uniform population of plants by the propagation of measurement errors in calculating L p . The results suggest that in T. virginiana cellular water relations are changed mainly by the turgor dependence of ɛ.

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 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.