M. Evenari

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.

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.

The Influence of the Origin of the Mother Plant on Yield and Germination of Their Caryopses in Aegilops ovata

SummaryDispersal units of Aegilops ovata contain in their spikelets caryopses differing in morphology, weight and germinability according to their position in the spikelets. Plants originating from caryopses of the diverse orders were grown under various photo- and thermoperiodic conditions, and the caryopses harvested germinated under uniform conditions. Under all conditions tested, the relative difference in weight between the caryopses of the various orders was not affected. But plants grown under long days and/or low temperatures produced heavier caryopses than plants grown under short days and/or high temperatures respectively. There was a positive correlation between weight and germinability in caryopses of the various orders derived from one mother plant, and a reverse correlation in caryopses formed on mother plants grown under low and high temperatures. When mother plants arising from various orders of caryopses were grown under long days and low temperatures, they produced caryopses of different germinability, showing that the origin of the mother plant may affect the germination qualities of its offspring. The implications of these results are discussed.

The influence of temperature and light penetration on the abundance of the hypolithic algae in the Negev desert of Israel

SummaryHypolithic algae were found under flint pebbles in the northern part of the Negev desert in Israel. The algae appeared in the contact area between the stone and the loess soil in which it is partially buried. Two types of flint were found in the research area: dark transparent ones and light opaque ones. A significant difference was found in the distribution of algae in these two types: 46.8% of the dark flints bear algae up to a thickness of 40 mm in the main range of 5 to 15 mm, whereas the respective figure for the light flints was 20.9% up to 30 mm only and in the thickness range of mainly 5 to 10 mm. No significant difference could be shown concerning the temperature underneath the two types of flint stones. The dark flint had a higher extinction coefficient than the light flint and this means that a greater amount of light penetrates to a deeper thickness in the dark flint than in the light. It may be assumed that the differences in light penetration explain the wider distribution under the dark flint than the light flint.

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.

Seasonal and Diurnal Courses of Water Relations of the Arido-Active Plant Hammada scoparia in the Negev Desert

SummaryThe 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 Ψplant and osmotic Ψπplant potential. Irrigated plants do not show such transitions. The daily courses of Ψplant and Ψπplant showed minima around noon and a tendency for maxima before sunrise. Turgor pressure Ψpplant reached minima around noon and became negative (until ca.-10 bars). Generally, Ψplant 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 Ψplant 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 Ψplant and WD. Regulatory reduction of transpiration appears largely independently of Ψplant 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 Ψplant with increasing WD was interpreted as caused by a change in soil or root resistance. Independent of the seasonal state and of the Ψplant level, H. scoparia regulates its water status within limited ranges of Ψpplant changes: the irrigated plants on a higher level, the non-irrigated on a lower level of Ψpplant. The water contents of the tissues of H. scoparia are linearily related to Ψplant as well as Ψpplant. 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 Ψpplant. This behaviour of non-irrigated wild plants apparently is due to the change of the elastic properties of the tissues in the assimilating branches.

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.

Seed physiology: From ovule to maturing seed

Abstract1)The future of the seed is partly predetermined by events (flower formation, flowering, nutrient flow from mother plant, etc.) preceding fertilization and the formation of the gametophyte.2)The environmental conditions under which the seed matures affect its final physiological constitution. This faet has mostly been neglected by seed physiologists.3)It is not known how far the triantic nature of the diaspore (seed coat, pulp, etc., 2n of mother plant, embryon of δ +n of Φ, endosperm 2n of Φ +n of δ) affects seed development and germination.4)The integuments of the ovules of some species have stomata. It is not known if they are functional in gas exchange or are constitutional non-functioning relics.5)The causes of the growth-degeneration pattern of the nucellus are unknown.6)During the development of the megaspore mother cell into the mature embryo sac dramatic cellular ultrastructural changes take place. This probably signifies a “change of guards” during which the gametophyte is freed from part of the controls by the ultrastructural units of the mother plant, preparing the ground after fertilization for a new, genetically independent sporophyte.7)Upon closer examination, the seemingly simple processes of fertilization and embryogenesis, as described in textbooks, turn out to be very complex and full of problems. Is the role each male nucleus plays preordained or is it left to chance which male nucleus goes where? What causes the degeneration of the synergids and of the vegetative nucleus, and what protects the other two male nuclei from a similar fate? Which ultrastructural organelles are carried by the generative nuclei into their respective receptor cells and what is their role in them? Why do zygotes in some species develop after fertilization immediately into an embryo whereas in other species the zygote remains dormant for some time? What causes the polarity of the egg cell which, after fertilization, divides into one developmentally most active apical cell (giving rise to the embryo) and into another “lazy” basal cell which develops into the suspensor of “unknown function?”8)In the source-sink relationship between photosynthesizing organs and the maturing seed there is one point at which the photosynthates pass from symplast to apoplast to symplast. The mechanism involved is largely unknown as well as the effect which environmental conditions have on this transport.Zusammenfassung1)Das Schicksal des Samens wird zum Teil durch Ereignisse bestimmt (Blütenbildung, Blühen, Nährstoffzufuhr von der Mutterpflanze u. s. w.), die der Befruchtung und der Bildung des Gametophyten vorausgehen.2)Die Umweltsbedingungne, unter denen der Samen reift, beeinflussen seine physiologische Konstitution. Die meisten Samenphysiologen haben diese Tatsache nicht genügend beachtet.3)Es ist nicht bekannt, wie weit die triantische Natur der Diaspore (Samenschale, Perisperm etc., 2n der Mutterpflanze,n δ + n Φ des Embryos, 2n Φ +n δ des Endosperms) Samenentwicklung und Keimung beeinflussen.4)Die Integumente der Samenanlagen haben bei manchen Pflanzen Spaltöffnungen. Es ist unbekannt, ob diese eine Rolle im Gasaustausch spielen oder ob sie nur konstitutionelle nichtfunktionelle Relikte sind. 1455)Die Ursachen des anfänglichen Wachstums und der folgenden Degeneration de Nuzellus sind nicht bekannt.6)Während der Entwicklung der Embryosackmutterzelle zum Embryosack findet eine fundamentale Änderung der zellulären Ultrastruktur statt. Die Bedeutung dieser Änderung liegt möglicherweise darin, dass sich durch sie der Gametophyt von der Kontrolle durch die ultrastrukturellen Organellen der Mutterpflanze befreit und somit die Basis für einen nach der Befruchtung genetisch unabhängigen Sporophyt gelegt wird.7)Bei näherer Betrachtung zeigt sich, dass Befruchtung und Embryoentwicklung, die in den Lehrbüchern meist als unkomplizierte Vorgänge dargestellt sind, äusserst komplexe und problematische Prozesse sind. Ist die Rolle der beiden generativen Kerne prädestiniert oder ist es dem Zufall überlassen, wohin jeder der beiden Kerne wandert? Was ist die Ursache der Degeneration der Synergiden und des vegetativen Kerns, und was schützt die beiden generativen Kerne vor einem ähnlichen Los? Welche ultrastrukturellen Organellen wandern mit den generativen Kernen in ihre respektiven Rezeptorzellen, und welche Rolle spielen sie dort? Warum entwickeln sich in einigen Arten die Zygoten direkt nach der Befruchtung zu Embryonen während sie in anderen Arten für oft lange Zeiträume im Ruhezustand bleiben? Was verursacht die Polarität der Eizelle, welche sich nach der Befruchtung in eine entwicklungsmässig äusserst aktive apikale Zelle, aus der sich der Embryo entwickelt, teilt, und in eine basale “träge” Zelle, die sich zum Suspensor “unbekannter Funktion” entwickelt?8)An einem bestimmten Punkt der Leitungsbahn, durch die Assimilate der photosynthetisierenden Organe in die reifenden Samen befördert werden, passieren die Assimilate vom Symplast zum Apoplast zum Symplast. Sowohl Transportmechanismus als auch der Einfluss von Aussenbedingungen auf ihn sind im grossen Ganzen unbekannt.