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The article offers information on abscisic acid (ABA) action in plant drought stress responses. It mentions that ABA can accumulate up to 10 to 30-fold in plants under drought stress relative to unstressed conditions. It informs about the hormone's ability to bring out stomatal closure of land plants to limit water loss through transpiration. It discusses how ABA controls membrane transport mechanisms in plants.

The necrosis of the rubber tree is an affection of the stem, being expressed by a deterioration of the cortical tissues on the level of which are located the conducting latex tissues. We studied the water relations in a mature rubber tree plantation (clone PB 260; planted in January 1996), in Côte d’Ivoire (May and September 2004), on «healthy» and on «necrosed» trees: mean height 15.0 m; mean circumference at 1.3 m level 59.6 cm; stand density 333 trees ha-1; leaf area index 3.2; rooting depths 4 m; field capacity, RFC = 412 mm; permanent wilting point, RWP = 225 mm; available water content, RAW = 187 mm. Measurements of water potential allow us to appreciate resistances along the continuum roots - trunk - leaves: the resistances between the trunk and the leaves are identical for «healthy» trees (0.3±0.1 cm3 H20 s-1 bars-1) and «necrosed» trees (0.2±0.1 cm3 H20 s-1 bars-1); on the other hand, concerning the resistances between the roots and the trunk the two types of trees differentiate radically: 0.2±0.1 cm3 H20 s-1 bars-1 for «healthy» trees and 1.1±0.3 cm3 H20 s-1 bars-1 for «necrosed» trees, it is thus on the level of the junction of grafting that resistance is exceptionally high for «necrosed» trees. The results seem to show a difference in functioning between «healthy» and «necrosed» trees in the regulation of transpiration flux. Measurements were made here in optimum conditions for water availability; these measurements would have to be continued in conditions of water shortage for the plant, we could then provide more contrasting results. [ABSTRACT FROM AUTHOR]

Abstract: We seek to identify the depth to which water is extracted by the roots in the soil. Indeed, in an isotopic steady-state condition of leaf water, transpiration introduces into the atmosphere a vapour whose isotopic signature is identical to that of root water. In the isotopic models of atmospheric general circulation, it is classically allowed that the signature of transpiration belongs to the meteoric water line. This supposes that the water taken by the roots has escaped with the evaporation of the soil and comes thus from the deep layers of the soil. At the time of experimentation carried out on maize plants (Nemours, Seine-et-Marne, France), this extraction depth was inferred from the comparison between the signature of the water measured on the level of the first internode of the stems of the plants and the isotopic profile of water in the soil. When the flow of transpiration reaches a maximum value, the plant uptakes water resulting from precipitations and which preserves its non-evaporating character after having quickly infiltrated in the deep layers of the soil. This relates to only 55% of the flux transpired by the canopy, the remainder presenting an evaporating character more or less marked according to ambient conditions. This experiment invalidates the classical hypothesis used in isotopic models of general atmospheric circulation in temperate regions. In fact, only half the amount of water vapour transpired by the canopy during the day presents a signature similar to that of the rainwater sampled in deep soil layers. To cite this article: Z. Boujamlaoui et al., C. R. Geoscience 337 (2005). [Copyright &y& Elsevier]

The rhizogenic activity in Sinapis alba L., subjected to progressively increasing water deficit during growth, shows a sigmoidal pattern of kinetics with three phases: initiation or latent phase, rapid exponential increase, and stationary phase. This pattern remains unchanged if the level of irradiance during growth is reduced. The same number of short tuberized roots is obtained as under high irradiance, although initiation of rhizogenesis appears to be delayed by a few days. However, this delay is not observed if the rhizogenesis is analyzed as a function of the water potential of aerial organs or as a function of the soil water deficit. Since the rate of dehydration depends upon the evaporative demand, which is modulated by high or low irradiance, drought rhizogenesis seems to be directly related to water deficits in the soil‐plant system. The onset of drought rhizogenesis, starting at the end of the growth period of the aerial organs, appears concomitant with the transpiration decline which is induced by water stress at both irradiance levels. [ABSTRACT FROM AUTHOR]

The role of atmospheric oxygen on root water absorption in Helianthus annuus. The effect of atmospheric anoxia on root water absorption was studied. The experiments were carried out on intact young sunflowers in controlled temperature, light and gas environment; roots were kept in aerated nutrient solution at constant temperature. The evolution of root water absorption and transpiration rate was measured continuously. Before the experiment, the plant was preconditioned at a high transpiration rate by illumination or by CO2 free air in darkness. Then the atmospheric oxygen was suppressed for 1 h, after which the normal conditions were restored. In anoxia and darkness, the root water absorption cannot balance transpiration, so that an important water stress develops in the plant; the light compensates this effect through the photosynthetic oxygen. The supply of oxygen, in darkness or in light, immediately removes inhibition of stomatal closure and of root water absorption. Two mechanisms control water absorption by roots: the fast one occurs in the leaves and the slower one cannot develop without the root system. [ABSTRACT FROM AUTHOR]

Metabolism of Zea mays L. cv. INRA F7 × F2 can be measured hourly with the "C23A system", under favourable and constant growth conditions. The photosynthesis is especially stable and is submitted only to a development linked with the leaf surface growth. During the vegetative stage the leaf surface increases regularly both in the day and in the night. The water loss does not change during the diurnal period and remains important during the night if humidity is less than 100%. The leaf respiration is nearly stable. The root respiration, measured with O2 and CO2, fluctuates according to a typical rhythm with two maxima. Day and night mean rates were about the same. The respiratory quotient is about one during the vegetative stage. There is no decrease in the rate of phosphate absorption during the night, and a very small decrease in the rate of nitrate absorption. Ammonium is totally consumed in the first hours after renewal of the nutrient solution. At the same time the potassium consumption is decreased, and then presents a maximum followed by a night reduction. Under our conditions, all of the observations allowed us to assume the presence of an adequate reserve of assimilates and suggest the existence of a precise regulation process, wich can ensure an even day/night functioning of the plant metabolism, but which does not preclude the presence of internal rhythms, as indicated by the oscillation of the root respiration. [ABSTRACT FROM AUTHOR]

Broad-leaved and narrow-leaved mediterranian xerophytes, growing under nearly equal micro-climatic conditions, were the object of a comparative study of transpiration rates and photosynthetic activity during the dry and the rainy season. The measurements, made by improved methods under favourable conditions with cloudless weather, reveal a close relationship between the breadth of the leaves and the physiological behaviour of the plants. During the humid season, the narrow-leaved plants studied show a day-curve of photosynthetic activity which is nearly linear and slightly ascending whereas the one of the broad-leaved species is distinguished by a decided drop beginning in the early morning. During the dry season photosynthesis nearly stops during the afternoon in all species. It was further recorded that the highest internal temperatures of the leaves examined were to be found in broad-leaved species. In general, the species possessing the greatest ability to reduce their total rate of transpiration in summer, also show the most marked reduction of total CO2 absorption. Species having the highest total transpiration rates during the rainy season often had the lowest photosynthetic rates during the same period. The highest daily transpiration rate per surface unit was found in narrow-leaved species. The results were discussed and led to the conclusion that the aptitude of the observed narrow-leaved xerophytes to assimilate during the whole day, during the rainy season, should not be considered a quality of adaptation favorable to life in arid regions and capable of explaining their abundance therin. It is more likely that their special behaviour may be a mere consequence of the structural peculiarities of their leaves. [ABSTRACT FROM AUTHOR]