ALBERT

Notes: Summary The photosynthesizing branches of Hammada scoparia, one of the typical dwarf shrubs of the Negev desert, undergo a seasonal change from succulent to xeromorphic anatomy. This trend is accompanied by a marked decrease of water content and of total water Ψ plant and osmotic Ψ π plant potential. Irrigated plants do not show such transitions. The daily courses of Ψ plant and Ψ π plant showed minima around noon and a tendency for maxima before sunrise. Turgor pressure Ψ p plant reached minima around noon and became negative (until ca.-10 bars). Generally, Ψ plant decreases with increasing water vapour concentration difference between plant and air (WD) in the first half of the day, and in the second half the reversal of this trend occurs. Mostly smaller increments of Ψ plant were correlated with larger increases in WD which lead to the conclusion that stomates closed enough to maintain transpiration at a constant value. Non-irrigated and irrigated plants showed different hysteresis loops of relation between Ψ plant and WD. Regulatory reduction of transpiration appears largely independently of Ψ plant which is in spring and with irrigated plants on a high level, with non-irrigated plants in summer on a low level. In summer the continous but decreasing drop of Ψ plant with increasing WD was interpreted as caused by a change in soil or root resistance. Independent of the seasonal state and of the Ψ plant level, H. scoparia regulates its water status within limited ranges of Ψ p plant changes: the irrigated plants on a higher level, the non-irrigated on a lower level of Ψ p plant . The water contents of the tissues of H. scoparia are linearily related to Ψ plant as well as Ψ p plant . Steeper slopes with non-irrigated plants in summer than with spring palnts and with irrigated plants during the whole season signify that in the latter a certain increment in turgor pressure corresponds to a large gain in water content while in the non-irrigated summer plants it varies only little for an identical change in Ψ p plant . This behaviour of non-irrigated wild plants apparently is due to the change of the elastic properties of the tissues in the assimilating branches.

Notes: Summary An empirical model for describing daily courses of net photosynthesis in Hammada scoparia is being developed. The model is based on the functional relationships, by which various environmental factors affect the photosynthetic activity and which can be measured by experiment in the field. In a sequence of steady-states daily courses of net photosynthesis are predicted during a growing season considering the variability of the physiological states and the capacity for regulative adaptations. The rate of net photosynthesis at a certain date is calculated from the maximal rate of CO2 uptake being expected at that season and from the effects of light, temperature, and air humidity which are scaled from 0 to 1. All factors are connected multiplicatively. The light function accounts for the seasonal changes in the light curve, the temperature function is based on the seasonal shift of the temperature optimum, and the humidity function considers the increasing sensitivity of the stomatal humidity response at increasing water stress. The model is built to be a submodel of a general ecosystem model, where various other submodels (i.e. water stress model, phenology model) are supplied. The present model is tested by predicting daily courses at extreme climatic conditions during the year and by comparing the predicted values of gas exchange with values being measured in an independent experimental procedure. The result shows that the model is able to simulate the natural behaviour of Hammada scoparia during the growing and dry season of a desert habitat. The problems of incorporating the influence of water stress, the interaction of the various factors, and the phenological aspect of the photosynthetic activity is being discussed.

Notes: Summary In the Central Negev, the arido-active Hammada scoparia (Chenopodiaceae) is mainly distributed in runnels and loessial plains of the wadis and exists with fewer and smaller individuals on slopes and tops of the neighboring hills. At the end of the dry season, water relations and chloride content of plants from these habitats and of artificially irrigated plants were determined and compared with plant shape and photosynthetic activity. The osmotic potentials and total water potentials of the plants differ characteristically with certain groups of stands. The simultaneously determined range of noon water potentials of the plants within a transect is as high as the annual amplitude of one plant individuum in the wadi (about 45 bars). Plants in the runnel of the wadi show, like the irrigated plants, the highest values of water potentials and their components but markedly lower chloride content than the irrigated ones. Total water and osmotic potentials of plants of the loessial wadi plains are extremely low. Their chloride content is not very high in contrast to that of plants of the hillslope and hilltop. Hill plants, although poorly developed and scarcely branched, have higher water and osmotic potentials than those of the plains. Net photosynthesis of a plant on a natural stand of the wadi plain is, in September, markedly depressed at noon but maximal at noon in an artificially irrigated plant. In contrast to irrigated and nonirrigated wadi plants, a plant of the hillslope shows, already in July, a midday depression of photosynthesis. Whether the relatively low water potentials of the big plants of the loessial plains are the results of a rapid biomass production in the rainy season which might have caused a very strong exhaustion of the soil water reserves for the late summer is discussed. Hill plants do not grow so well and obviously decrease their water exchange even in summer.

Notes: Summary The water relations characteristics of three grass species (Panicum maximum var. trichoglume, Cenchrus ciliaris, Heteropogon contortus), and a legume (Macroptilium atropurpureum) grown in the field were measured using both a modified pressure/volume technique with pressure bomb measurements on single leaves and a dewpoint hygrometry technique applied to fresh and to frozen and thawed leaf discs. The two techniques agreed well in the estimates of osmotic potential at full turgor and the water potential at zero turgor. However, for parameters such as the relative water content at zero turgor, bound water and bulk modulus of elasticity there was a poor correlation between the estimates from the two methods. The pressure/volume technique gave less variable results and is more convenient for field use than the hygrometry technique. The determination of the modulus of elasticity from various functions relating pressure potential to relative water content is discussed.

Notes: Summary Measurements of CO2 and water vapor exchange were performed on Prunus armeniaca L. with humidity- and temperature-controlled chambers under the climatic conditions of a desert habitat. In apricot, the stomatal response to changes in temperature and water-vapor concentration difference between leaf and air (WD) significantly determined the rates of gas exchange during the day (parts I and II). The effect of climate-controlled stomatal response on the transpiration/net photosynthesis (T/P)-ratio was analyzed and simulated using experiments conducted at constant temperature and/or humidity conditions for input parameters. The measured values of the T/P-ratio at naturally varying conditions of humidity and temperature were compared with calculated results of a model in which it was assumed, (1) that stomata and photosynthetic activity are not affected by air humidity and temperature, (2) that the stomata only respond with a constant photosynthetic activity to changes in WD, and (3) that the stomata respond to both, leaf temperature and air humidity with a constant photosynthetic activity. These simulations facilitated an analysis of the naturally observed changes in the T/P-ratio. The calculated T/P-ratios were very small if the simulation assumed that stomata only respond to WD at a constant photosynthetic activity. These low predicted values of the T/P-ratio were not obtained under natural conditions, since an increase in WD during the day was correlated with a temperature rise which tended to open stomata and change the photosynthetic activity. Humidity induced stomatal closure did appear to substantially reduce T/P-ratios. The measured T/P-ratio changed considerably during the year. The lowest T/P-ratios were obtained in the middle of the dry season at a time when stomata responded strongly to air humidity and when optimum of photosynthesis was reached at high temperatures. The daily average T/P-ratio calculated from the daily sum of P and T showed little change during the seasons. A high T/P-ratio was also observed at reduced rates of gas exchange. The T/P-ratios of apricot were compared with different species in different environments.

Notes: Summary Biomass distribution and diurnal CO2 uptake under natural conditions were investigated on Picea abies in a mountainous climate (Solling, Northwest Germany). Spruce has a remarkable variability in leaf characteristics. Even on a single branch in the lower sun crown, needle dry weight and surface area change considerably from the branch base to the tip and accoring to exposure. Only about 18% of the total biomass of the tree was current year's growth, about 40% of the needles were 4 years and older reaching a maximal age of 12 years. The main growing zone was at the border of upper shade and lower sun crown and the main accumulation of dry weight was at a greater tree height than was observed for maximal growth of needle numbers or surface area. The annual, new growth shifted toward the upper sun crown. Maximal daily CO2 uptake was highest in the lower sun crown on days with variable cloud cover when temperatures were moderate and water vapor pressure deficits were low. Also the annual CO2 uptake was highest in the lower sun crown, where 4-year-old and older needles contributed about 35% to the annual CO2 uptake of the tree. Current year growth contributed about 15% of the total CO2 gain. The upper and lower sun crowns produce about 70% of the total carbon gain. The carbon balance of spruce and the distribution of the production process in relation to needle age and crown level are discussed.

Notes: Summary The gas exchange of the apricot (Prunus armeniaca L.) growing in the runoff farm at Avdat (Negev, Israel) was measured during its growing period using temperature- and humidity-controlled chambers. Water potentials of the xylem were measured with a pressure bomb, and the mesophyll internal CO2 concentration was calculated from simultaneous measurements of net photosynthesis and transpiration. The daily changes in water potential Ψ had only little effect on the daily course of stomatal resistance. The early morning peak of CO2 uptake was reached when Ψ had already dropped to very low values. On dry days, Ψ and the relative water content of the leaf were improved at noon during the time of stomatal closure. On humid days, Ψ dropped to very low values (43.5 bar) at a high transpiration rate without causing stomatal closure, as much as on the dry days when stomata where more closed at less water stress. The observed changing sensitivity of the stomata to changes in air humidity during the season is related to the water status in the plant. This change is possibly caused by a long-term effect of stress in this habitat. The daily changes in stomatal diffusion resistance did not consistently correlate with changes of the CO2 concentration in the intercellular air spaces. In the morning a decreasing internal CO2 concentration was even inversely correlated to the stomatal response. In the afternoon the effect of an increasing internal CO2 concentration and the effect of external climate on stomatal response could be additive. However, at the time, when CO2 uptake reached a second peak in the afternoon the same value of diffusion resistance is reached at very different levels of internal CO2 concentration as compared to the morning. For the regulation of the diffusion resistance in apricot under the natural conditions, the effects of plant internal control mechanisms are overruled and/or modified by the external climatic factors of air humidity and temperature. The significance of the climate-controlled stomatal response for the existence and cultivation of this plant species in an arid habitat is discussed.

Notes: Summary Growth and CO2 uptake in the crown of a spruce tree is described and the production processes of this evergreen conifer are compared with those of a deciduous beech. Spruce had 60% lower rates of net photosynthesis per dry weight than beech. But, beech had a 30% shorter growing season and a 84% smaller biomass than spruce. The annual CO2 gain was 40% lower in beech than it was in spruce. An analysis shows the following conclusions for this habitat. (1) The effect of a prolonged growing season is small. The annual CO2 gain of spruce would be reduced only by 9% if the growing season was the same length as for beech. (2) The annual CO2 gain would increase 14% if all needles in spruce were deciduous, because the current year needles have a higher average rate of CO2 uptake than 3-year old and older needles, but a lower average rate than 1- and 2-year old ones. However, the carbon balance of the tree shows that spruce could not afford to produce the existing needle biomass (14 t ha-1) each year. (3) If spruce were to produce the same deciduous foliage biomass during the same growing season as beech then total production by spruce would be reduced 67%. (4) The annual CO2 uptake by evergreen spruce was higher than deciduous beech not because of a long growing season, but because of the longevity of its needles, which during their total life time (an average of 5 years) have a two to three times greater CO2 uptake than a deciduous leaf in one summer season. The relatively small investment in current year needles produces an annually low, but long lasting assimilation of CO2.

Notes: Summary As described earlier, the native arido-active perennial Hammada scoparia and the cultivated Prunus armeniaca exhibit characteristic seasonal shifts of their temperature optimum of net photosynthesis (OP) under desert conditions in the Negev. In the present paper the OP values were compared with the actual tissue temperatures of the experimental plants. During the growing period from March to September the duration of optimal temperatures for net photosynthesis (OP±3°C) experienced by the plants was 32.2% of the total time at light saturation for P. armeniaca and 27.8% for H. scoparia. For optimal photosynthesis the branchlets of H. scoparia are too cold for 66.1% of the time span and too warm for 6.1% of the time. The respective values for the leaves of the apricot are 28.6% and 39.2%. Simulations at changed tissue temperature show, that for P. armeniaca neither a higher nor a lower temperature regime would lengthen the time span for optimal thermal conditions. For H. scoparia, however, an increase of the general temperature level by 6°C would considerably improve the temperature-related photosynthetic efficiency. The natural temperature responses of the plants were compared with simulations using OP values which are supposed not to shift but to stay constant from March through September at their spring minimum, their summer maximum, or at an intermediate value. For P. armeniaca such constant OP values would result in a shorter duration of optimal temperature conditions. With this plant the natural seasonal shift of the temperature characteristics appears to provide an advantage in respect to its photosynthetic capacity. Contrary to this, for H. scoparia a constant OP value at the low spring level or even at the intermediate level during all the season would result in a substantially prolonged period of favourable temperature conditions for photosynthesis. In this case the seasonal change of optimum temperature for photosynthesis with higher OP values in summer signifies a disadvantage with respect to the temperature-related photosynthetic capacity at the habitat in the central Negev. Apparently this C4 plant is adapted to higher temperatures than were present. It appears that “acclimations” of native plants are not always beneficial.

Notes: Summary Previous publications have reported on investigations of CO2 exchange in the desert lichenRamalina maciformis both in its natural habitat in the Negev and in the laboratory. Utilizing laboratory data, net photosynthesis and dark respiration were expressed as mathematical functions of the most important environmental factors. Based on these relationships, a model is developed that allows one to predict CO2 exchange of the plant. Input data are light intensity, temperature, and water content of the thallus, together with a measure of the rate of the seasonal change of photosynthetic and respiratory activity. The validity of the model is tested by comparing simulated daily courses of CO2 uptake and release of the lichen with independent results of CO2 exchange measurements conducted in the field during and after the condensation of dew. The sensitivity of the model is shown by simulating changes in the input data of temperature and water content of the lichen.