U. Buschbom

Stomatal Responses to Changes in Humidity in Plants Growing in the Desert

SummaryThe 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.

The role of air humidity and leaf temperature in controlling stomatal resistance of Prunus armeniaca L. under desert conditions

SummaryThe 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.

Stomatal responses to changes in temperature at increasing water stress.

SummaryThe 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.

Ecophysiological Investigations on Lichens of the Negev Desert: VI. Annual Course of the Photosynthetic Production of Ramalina macifarmis (Del.) Bory1)

Summary Diurnal courses oflight, temperatures, water content, and CO 2 exchange of the thalli of Ramalina maciformis were measured in its natural habitat in the Negev desert from March to September 1971. The measurements of water content of the lichen thalli by means of a Hiltner dew balance were extended over a period of about two years. Water content was the most prominent factor controlling the CO 2 -gas exchange of the lichen. The different types of water uptake by rain-, dew-, or water-vapour moistening and the corresponding daily courses of net photosynthesis are presented. The experimental data, formed the basis of a correlation model for net photosynthesis of R. maciformis by means of which the daily photosynthetic production due to dew and water-vapour uptake was calculated for a, whole year according to the registrations of the thallus water contents, temperatures of a neighbouring weather station, and photoperiod. The net photosynthesis due to rain-water imbibition and the respiration during the night were separately calculated. The photosynthetic CO 2 gain which was yielded by dew on c. 200 days was as high as that calculated for the 29 rain days of the extremely rainy annual period of 1971/72. In years with precipitation near the long term average, as was the subsequent period 1972/73, rain induced production of R. maciformis was only 2/3 of that yielded by dewfall and may be only a small fraction in dry years, whereas dewfall maintains almost always its regular occurrence. Rainfall in the Negev, although being very effective for high photosynthetic productivity, is too scarce to provide life of R. maciformis . The existence of many lichens in the Negev is only possible because of the high frequency and regularity of dew falls. It is discussed whether the calculated amount of the annual photosynthetic gain is representative for the natural production and growth.

Long-term effects of drought on wild and cultivated plants in the Negev desert: I. Maximal Rates of Net Photosynthesis

SummaryThe relation between daily maximal rates of net photosynthesis and plant water status was studied during a dry season on irrigated and non-irrigated, naturally growing, perennial wild plants.Species were examined which differ in phenology, leaf anatomy and morphology: Hammada scoparia, Artemisia herba-alba, Zygophyllum dumosum, and Reaumuria negevensis. Prumus armeniaca which was growing in the run-off farm at Avdat and which has mosomorphic leaves was included in the comparison. All plants differed in their seasonal change in plant water status, and in their seasonal change in daily maximal net photosynthesis. Rates of CO2 uptake were not uniquely related to simultanously measured leaf water potentials. Daily maximal rates of net photosynthesis of non-irrigated plants, and the difference between maximal CO2 uptake of irrigated and non-irrigated plants were examined in relation to pre-dawn water potential. Maximal net photosynthesis rates decreased very rapidly with decrease in pre-dawn water potential or, for Hammada scoparia, they decreased even with a constant level of pre-dawn water potential. Consequently, it was considered necessary to include both time and water potential in a parameter “bar day” describing the accumulated drought stress of the plants. All species showed the same relation between relative maximal net photosynthesis and drought experience as determined by cumulative daily addition of pre-dawn water potentials for the non-irrigated plants since the last rain.

Long-term effects of drought on wild and cultivated plants in the Negev desert. II. Diurnal patterns of net photosynthesis and daily carbon gain.

SummaryThe seasonal change in diurnal patterns of net photosynthesis and daily carbon gain is studied in relation to the plant water status of the irrigated and non-irrigated naturally growing desert species Hammada scoparia, Zygophyllum dumosum, Artemisia herba-alba and Reaumuria negevensis. Comparison is made to cultivated Prunus armeniaca. Under non-irrigated natural conditions Hammada scoparia, a C4 plant, showed one-peaked flat diurnal courses of CO2 uptake which changed into a pattern of a high morning peak of CO2 uptake or slightly two-speaked curves in the late dry season. In contrast, the C3 species Zygophyllum dumosum, Artemisia herba-alba and Prunus armeniaca changed from one-peaked to distinct two-peaked patterns. At the end of the dry season, non-irrigated plants showed respiration only. Reaumuria negevensis had one-peaked curves with a low level of CO2 uptake.There is no general relation between day-time CO2 gain and pre-dawn water potential for the investigated species. In order to characterize the effect of soil drought, the CO2 gain during day-time of non-irrigated plants is expressed as a percentage of the CO2 gain of the irrigated counterparts. After an initial period of minimal drought effect, the relative day-time CO2 gain decreases almost linearly with cumulative water stress as determined by the daily addition of pre-dawn water potentials for the non-irrigated plants since the last rainfall. The slope of decrease differs from species to species. The relation of daily CO2 gain to maximal net photosynthesis is discussed. Initially, at a good plant water status, the daily CO2 gain does not decrease in proportion to the maximal photosynthetic rates as a result of stomatal control at high photosynthetic activity. At increasing water stress the daily CO2 gain decreases more than proportionally to the decrease of the maximal rates.

Extreme water stress and photosynthetic activity of the desert plant Artemisia herba-alba asso

SummaryDuring 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.

The temperature-related photosynthetic capacity of plants under desert conditions

SummaryAs 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.

Plant Production in Arid and Semi-Arid Areas

The determination of phytomass and the assessment of actual and potential primary production of a natural ecosystem has become an important part of modern ecological research. In order to understand the functioning of an ecosystem, to analyse it, and to build valid models (see van Keulen et al., this volume Part 6:B), exact phytomass and production data must be available. This general aim, however, is difficult to approach, since phytomass and primary production are not constant values (see Lieth, this volume Part 6: A). Even for the same habitat they depend upon the total environmental conditions which are changing from year to year. Therefore, the fluctuations of phytomass and production and their relation to the environmental parameters need to be considered. This problem may be of minor importance for ecosystems living in humid environments. It is of great significance, however, in arid and semi-arid areas, where the main factor limiting phytomass and production is the availability of water. It is precisely this factor which in these regions fluctuates to a very large degree from year to year.

Net Photosynthesis, Dry Matter Production, and Phenological Development of Apricot Trees (Prunus armeniaca L.) Cultivated in the Negev Highlands (Israel)

Summary Growth rates, phenological development, and daily courses of net photosynthesis were measured on apricot trees ( Prunus armeniaca L.) together with environmental and physiological parameters in the Negev desert under runoff farming conditions. The investigations were conducted during an entire growing period with two experimental trees. One of them received water only from rain and runoff (non-irrigated tree). The other tree was additionally irrigated starting 11 months prior and during the experiments in order to eliminate water stress (irrigated tree). The irrigated tree opened flowers and leaf buds later, also elongation growth started later, but continued for a longer period of time (time of elongation growth months) than in the non-irrigated counterpart (months). Growth was terminated in both trees at a time when predawn water potential reached —7 to —8 bar and daily minimum water potential reached —35 to —40 bar. Trunk growth continued beyond this stage of water stress until the highest water potential in the soil profile reached —10 bar for the non-irrigated and —6 bar for the irrigated tree. The seasonal development of the ratio dry weight/surface area and the ratio chlorophyll/dry weight of the leaves and their anatomy was identical for both trees. The root system was studied intensively for the non-irrigated tree. It had 3 m long tap roots. Horizontal roots were mainly developed in the range of 2 to 3 m, but some lateral roots extended as far as 8 m. The root system exploited a soil volume of about 44—76 m 3 . The root/shoot ratio was 1.1. Because of earlier leaf development, the non-irrigated tree had higher rates of daily CO 2 uptake m spring than the irrigated counterpart, which, in contrast, had higher rates during the dry season. Per unit leaf dry weight the non-irrigated tree had a higher seasonal CO 2 uptake than, the irrigated one (29.5 versus 25.1 gCO 2 • gdw -1 • year -1 ). But because its larger leaf number the irrigated tree achieved an about 30% higher total CO 2 gain per year than its counterpart (about 75 kg C versus 56 kg). The allocation of carbon for leaf, wood, fruit and root growth showed main differences in root and fruit development. It was estimated, that the irrigated tree allocated about 16% (kg C) of its total carbon gain into fruits but only about 13% into roots, whereas the non-irrigated tree allocated% (kg C) of its carbon gain into fruits and about 52% into its below ground parts. The ecological implications of different strategies in growth, photosynthetic production and carbon allocation in trees of differing water stress are discussed.