Showing papers on "Transpiration published in 1970"

Diurnal variations in root diameter.

Abstract: Many aerial plant parts show diurnal patterns of water status, with a decrease in water content and water potential corresponding to the midday peak in evaporative demand (5, 7, 9, 10). Similar diurnal patterns might be expected in root tissues because the water potential in the root xylem should follow closely the xylem water potential of plant tops. The data reported here, obtained at the Auburn Rhizotron (13) where roots in soil may be observed through transparent panels while tops are exposed to field conditions, show that lateral roots do indeed undergo diurnal variations in diameter.

Stomatal Responses to Pressure Changes and Interruptions in the Water Supply of Detached Leaves of Zea mays L.

Abstract: Stomata of Zea mays L respond to changes in hydrostatic pressure in the water supply of the leaves almost instantaneously and in all leaf parts simultaneously Therefore, the leaf is a hydraulic unit The stomata are part of it and their aperture is controlled by the water potential in the water-conducting system Stomatal aperture is not uniquely related to the relative water content of a leaf The relation depends also on the humidity in the air and is different for the upper and the lower epidermis

Inhibition of oxygen evolution in chloroplasts isolated from leaves with low water potentials.

Abstract: Chloroplasts were isolated from pea and sunflower leaves having various water potentials. Oxygen evolution by the chloroplasts was measured under identical conditions for all treatments with saturating light and with dichloroindophenol as oxidant. Evolution was inhibited when leaf water potentials were below -12 bars in pea and -8 bars in sunflower and the inhibition was proportional to leaf water potential below these limits. Inhibition was more severe in sunflower than in pea chloroplasts. In sunflower, it could be detected after 5 minutes of leaf desiccation, and, up to 1 hour, the effect was independent of the duration of low leaf water potential.In high light, the reduction in activity of sunflower chloroplasts paralleled the reduction in CO(2) fixation by intact sunflower plants having low leaf water potentials. Stomatal apertures and transpiration rates were also reduced under these conditions and were probably limiting. In low light, intact sunflowers required more light per unit of CO(2) fixed when leaf water potentials were low than when they were high. This increased light requirement in the intact system was of a magnitude which could be predicted from the reduced oxygen evolution by the isolated chloroplasts. It was concluded that moderately low leaf water potential affects photosynthesis in at least two ways: first, through an inhibition of oxygen evolution by chloroplasts and, second, by closure of stomata in intact leaves.

Temperature and Transpiration Resistances of Xanthium Leaves as Affected by Air Temperature, Humidity, and Wind Speed

Abstract: Transpiration and temperatures of single, attached leaves of Xanthium strumarium L. were measured in high intensity white light (1.2 calories per square centimeter per minute on a surface normal to the radiation), with abundant water supply, at wind speeds of 90, 225, and 450 centimeters per second, and during exposure to moist and dry air. Partitioning of absorbed radiation between transpiration and convection was determined, and transpiration resistances were computed.Leaf resistances decreased with increasing temperature (down to a minimum of 0.36 seconds per centimeter). Silicone rubber replicas of leaf surfaces proved that the decrease was due to increased stomatal apertures. At constant air temperature, leaf resistances were higher in dry than in moist air with the result that transpiration varied less than would have been predicted on the basis of the water-vapor pressure difference between leaf and air.The dependence of stomatal conductance on temperature and moisture content of the air caused the following effects. At air temperatures below 35 C, average leaf temperatures were above air temperature by an amount dependent on wind velocity; increasing wind diminished transpiration. At air temperatures above 35 C, leaf temperatures were below air temperatures, and increasing wind markedly increased transpiration. Leaf temperatures equaled air temperature near 35 C at all wind speeds and in moist as well as in dry air.

The control of water loss in desert tenebrionid beetles.

Abstract: 1. Total water loss in tenebrionid beetles was composed of transpiratory losses from the cuticle and spiracles, water associated with defaecation, and water from the release of defensive quinone droplets or oral fluids. 2. Freshly killed specimens of E. armata, C. muricata and C. verrucosa had higher transpiration rates over long and short exposures than did living animals of the same species. These results may reflect the cessation of active water retention by cuticular and spiracular regulation in dead animals. 3. Cuticular transpiration, although low in absolute rate, was a greater source of water loss than respiratory transpiration in E. armata, C. muricata and C. verrucosa at temperatures from 25 to 42.5 °C and at 0% R.H., suggesting that spiracular control of water loss was of considerable importance in maintaining water balance. 4. A marked increase in respiratory transpiration over previous low rates was observed at 40 °C for E. armata and at 42.5 °C for C. muricata and C. verrucosa , and indicated a temperature-induced breakdown in spiracular water regulation due to increased respiratory activities. In contrast, cuticular transpiration maintained a linear rate of increase over the temperature range investigated. 5. A direct relationship existed between oxygen consumption and estimated respiratory transpiration for E. armata from 25 to 40 °C and at 0% R.H. 6. Evidence for the presence of discontinous respiration and unidirectional tracheal air flow in E. armata was presented. 7. Cuticular transition temperatures were measured for E. armata (40 °C), C. muricata (47.5 °C), and C. verrucosa (50 °C) with results showing that the autumn species ( E. armata ) possessed a lower cuticular breakdown point than either of the two summer species. 8. Much of the success of tenebrionid beetles in desert habitats is due to the development of highly impermeable cuticles and well regulated spiracular control mechanisms for reducing the loss of body water.

Salt Accumulation at the Soybean (Glycine Max. (L.) Merr.) Root-Soil Interface

Abstract: Water soluble salts accumulated at the soybean (Glycine max L.) root-soil interface and in the rhizosphere soil when salts, dissolved in the soil solution, moved to the root surface at a greater rate than they were absorbed by the roots. Salt accumulation increased with increase of salt concentration in the soil solution and transpiration rate. The average salt accumulation decreased with increase in the average age of the roots.

Comparative photosynthesis, growth and transpiration of two species of Atriplex.

Abstract: Throughout a period of 23 days, during which the photosynthesis, growth and transpiration of two species of Atriplex were compared, A. spongiosa, a "C4" species (first products of photosynthesis = 4-C dicarboxylic acids), maintained net rates of leaf photosynthesis as high as, or higher than, those of A. hastata, a "C3" species (photosynthesis exhibiting the Calvin-type characteristics).However, as the experiment progressed, the proportion of photosynthate which was used to produce new leaf material declined progressively in A. spongiosa, so that total plant growth rate, initially more than twice as high as in A. hastata, declined to only 0.8 of the A. hastata value. This result demonstrated clearly that more efficient photosynthesis is only one factor, and in this case a relatively minor factor, in total growth rate.Transpiration rates were consistently lower in A. spongiosa than in A. hastata and the ratio declined slightly during the experiment. In consequence, water-use efficiency, both on a single-leaf and whole-plant basis, was much greater in the C4 species.Levels of mesophyll resistance (r m ) were consistently lower in A. spongiosa and increased from about 0.4-0.6 to 1.2-1.5 s cm(-1) during the experiment. In A. hastata there was more variability in r m levels but little overall trend towards a higher r m , initial and final values being of the order of 2.5-2.6 and 2.6-2.9 s cm(-1), respectively. Levels of stomatal resistance (r l ) were higher in A. spongiosa (about 1.0-1.2 s cm(-1)) than in A. hastata (about 0.7-0.8 s cm(-1)) at the beginning of the experiment and increased to 2.0-2.6 s cm(-1), whereas they remained relatively constant in A. hastata.The combination of relatively low r m levels and relatively high r l levels provide the explanation for the substantially greater water use efficiency in A. spongiosa. The progressive changes in these levels and in the pattern of leaf area development in A. spongiosa provide an elegant example of adaptation to arid conditions by this species.

Interrelations between growth and photosynthesis of salt bush (atriplex halimus l.) grown in saline media

Abstract: Summary Plants of A. halimus were grown in culture solutions to which NaCl or Na2S04 was added to different concentrations. The osmotic potential of the sap, growth, morphology, and leaf gas exchange (transpiration and photosynthesis) were studied. At least three different effects of salinity were identified: (1) an increase of leaf area and succulence induced by relatively low concentrations of salt in the growth medium. This resulted in an increase of the leaf area available for transpiration and photosynthesis (and hence growth); (2) an increase of stomatal resistance to water vapour loss and C02 uptake. This tended to reduce both transpiration and photo­ synthesis per unit leaf area; (3) changes in mesophyll resistance to C02 uptake (r;"). Relatively low concentrations of NaCl reduced r;", but high NaCl concentra­ tions (i.e. external osmotic potentials lower than -9 atm) and all concentrations of Na2S04 brought about an increase in r;". In addition to the above effects, salinity may have caused a hormonal imbalance as was indicated by the increased sprouting of lateral buds. These responses to salinity tend to counteract one another in their overall effect on growth. Which factor will become dominant is determined by the level and type of salinity, and also, possibly, by climatic conditions.

The effect of leaf hairs of verbascum thapsus on leaf energy exchange

Abstract: SUMMARY Energy budgets were determined for normal and hairless leaves of Verbascum thapsus L. in a wind tunnel. Leaves with the dense hair present were warmer than hairless leaves. This was shown to be due to a thicker boundary layer over normal leaves. Boundary layer resistance to heat and water vapour loss was higher for normal than for hairless leaves, resulting in lower convection and transpiration from normal hair-covered leaves. Differences in transpiration resistance were correctly predicted by heat transfer theory. The hairs had little effect on radiation absorption by the leaves.

The role of the mesophyll cell wall in leaf transpiration

Abstract: Evidence is presented which suggests that the mesophyll cell walls of cotton leaves may influence observed rates of transpiration.The net diffusive flux of water vapour, from the upper and lower surfaces of a leaf, was compared with the flux of nitrous oxide through a leaf and evidence obtained of an extra resistance in the water-vapour pathway associated with water transport in the mesophyll cell walls.This extra resistance appeared to be insignificant at low transpiration rates and in turgid leaves, but increased with transpiration rate and dehydration. The most likely explanation for its origin appeared to be a reduction in hydraulic conductivity across the internal cuticle which lines the outer surfaces of the mesophyll cell walls. In turn this served to reduce the relative vapour pressure at the sites of evaporation.The experiments were conducted under conditions where stomatal opening was induced by CO2-free air. Under normal conditions stomatal closure would tend to reduce the development of this extra resistance. Even so, the results throw doubt on the validity of the long-standing assumption that the water-vapour pressure at the evaporation sites is equal to the saturation vapour pressure under all conditions.

Water relations of the desert scorpion, Hadrurus arizonensis.

Abstract: 1. Total water-loss rates for Hadrurus arizonensis (0.028% wt/h at 30 °C in dry air) are comparable to rates for Old World species and are well below rates for other desert arthropods under similar conditions. 2. Cuticular and respiratory transpiration constitute the two major avenues of water loss, cuticular water loss predominating at temperatures up to approximately 38 °C and respiratory transpiration predominating at temperatures above 40 °C. 3. The cuticular transpiration/temperature curve exhibits a two-plateau configuration with abrupt increases in cuticular permeability occurring between 35 and 40 °C and between 65 and 70 °C 4. Cuticular water-loss values in dead scorpions exceed those of total water loss in living scorpions. The increased cuticular permeability after death is interpreted as evidence for the existence of an active cuticular water-retaining mechanism. 5. Water-loss rates are significantly reduced at lower saturation deficits; however, scorpions are unable to absorb significant quantities of water from near-saturated atmospheres or moist substrates, regardless of their hydration state. 6. The importance of water conserving versus water regaining mechanisms are discussed in relation to the total adaptations of these animals to hot, dry environments.

Studies on the Effect of Wind Speed upon the Photosynthesis

Abstract: In the previous paper, the authors described the relationship between the thickness of boundary layer near the leaf surface and wind speed.When the transport of carbon dioxide from atmosphere toward chloroplast depends only on the thickness of boundary layer, the photosynthesis varies in accordance with the wind speed.Generally, when the amount of photosynthesis of single leaf is measured, a narrow assimilation box have been used in order to increase the wind speed in the box. However, it is very difficult to measure the natural photosynthesis, because the thickness of boundary layer of leaf surface is not developed in the box and air stream near the leaf surface differs entirely from that under the natural condition.The authors have constructed a Gottingen type wind tunnel to measure the photosynthesis under the natural condition of air flow. The wind tunnel is control able of wind speed from 15 to 300cm/sec, light intensity up to 0.6cal/cm2 min or up to 85klux, air temperature from 17 to 40°C, relative humidity from 30 to 95%, carbon dioxide concentration up to 15% and the dissolve oxygen concentration in culture solution up to maximum saturation.Cucumber (Cucumis sativas L.) was grown in Hoagland solution in green house of over 20°C minimum temperature. The leaves were cut off except the first and second ones after the second leaf grew over 100 cm2. The gross photosynthesis of the first and second leaves were measured from respiration by dry weight loss in the dark (16 hours) and net photosynthesis by increment of dry weight in the light (8 hours).Under the conditions of 25°C of air temperature, 0.6cal/cm2 min of light intensity and 65% of relative humidity, the gross photosynthesis was measured. The amount of gross photosynthesis, was nicreased with the wind speed and showed the maximum value at 50-60cm/sec of wind speed, and over that wind speed, the amount decreased gradually. Namely, when the wind speed exceeds a certain value, the photosynthesis-wind curve deviates from the photosynthesis-wind curve calculated from the relation between thickness of boundary layer and wind speed. This means that the transpiration is increased and that the content of leaf water is decreased when the boundary layer becomes thinner. In order to know the effect of transpiration, the net photosynthesis was measured under the various relative humidities. When the relative humidity is raised, the maximum value of the net photosynthesis is increased and the wind speed is also elevated showing the maximum value. And the net photosynthesis gradually closes to the photosynthesis-wind curve calculated from the boundary layer. These results mean that the photosynthesis of real leaf is not only dependent on the light intensity but also both the wind speed and relative humidity.

Effects of kaolinite as a reflective antitranspirant on leaf temperature, transpiration, photosynthesis, and water‐use efficiency

Abstract: Transpiration and photosynthesis are influenced by the optical properties of leaves. Application of a uniform white coating of kaolinite (about 225 mg dm−2) to the upper leaf surface produced significant changes in the spectral curves (reflectivity, transmissivity, and absorptivity) of Valencia orange leaves. Photosynthetic light response curves of leaves of orange, lemon, rubber plant (all hypostomatous), and bean (amphibiostomatous) were modified by the reflective material, which at low light intensities decreased photosynthetic rates and increased light saturation levels. The white coating cooled the leaves 3–4°C and reduced transpiration 22–28% under conditions of high light intensity, warm air, low relative humidity, and low wind velocity. However the rubber plant under the same conditions showed a smaller reduction in transpiration (less than 10%), indicating effective self-regulation of its water loss. These experiments demonstrated that water-use efficiency of leaves can be improved, particularly under high light intensities.

Plant gas exchange at high wind speeds.

Abstract: High altitude Rhododendron ferrugineum L. and Pinus cembra L. seedlings were exposed to winds at 15 meters per second for 24-hour periods. Wind-sensitive stomata of Rhododendron seedlings immediately initiated a closing response which resulted in decreased photosynthesis and an even greater reduction in transpiration. Stomatal aperture and transpiration rates of P. cembra were only slightly reduced by high speed winds. However, photosynthesis was substantially reduced because of changes in needle display to available irradiation.

After-effect of Water Stress on Stomatal Opening Potential: II. POSSIBLE CAUSES

Abstract: Experiments were carried out to elucidate the temporary after-effect of a period of water stress on the opening potential of stomata of tobacco (Nicotiana tabaccum) and heans ( Vicia faba). Stomatal aperture was measured on floating leaf discs or epidermal strips after a period of time in light under controlled conditions ; direct effects of leaf-water deficit were thus avoided. The responses of the two species were not qualitatively different. Only a minor part of the post-stress damage was located in the mesophyll in beans, the major part (approximately two-thirds) being located in the guard cells themselves. The C02 compensation point for tobacco leaves showed no after-effect of stress. However, the response of stomata to normal and C02-free air in both species suggested that a minor part of the after effect on stomatal opening, presumably that part arising in the mesophyll, may be explained by elevated COa concentrations in the stomatal cavity. The nature of that part of the after-effect arising within the guard cells themselves was not clarified. Guard-cell starch content was apparently unaffected by stress in both species; in tobacco, epidermal-cell starch content was reduced in the stress and post-stress condition. Stomatal responses to the short-term addition of energy -supplying metabolites (ATP, glycolate, and glucose-1-phosphate) showed no interaction with the post-stress condition. Similarly responses to the addition of stomatal inhibitors (sodium azide, dodecenylsuccinic acid, atrazine, and phenylmercuric acetate) did not indicate the exact nature of the after-effect, although there was a marked increase in the sensitivity to atrazine in post-stress leaf discs. In tobacco leaf discs an examination of the process of elimination of this after-effect, as distinct from studies directly on its nature, suggested that complete recovery involved at least two steps, together lasting 2 to 3 days. One step required time and was independent of light; the other required light of at least 1 klx for a day or more and did not occur in C02-free air. Stomata recovered almost completely in 2 days without light when discs were floated on 0-05 M glucose-1-phosphate.

Measurement of Transpiration in Tropical Trees with Tritiated Water

Abstract: The measurement of transpiration of water by trees in the field with tritiated water as a tracer depends upon a new application of established theory of radionuclide dynamics in steady—state systems. The techniques required are non—destructive to the trees and probably have negligible disturbing effects on transpiration. Average transpiration rates ranging from 1.75 to 372 liters per day per tree were measured by the proposed method on tropical forest trees which ranged from the understory to the canopy in size. Statistical errors range from 12.1% on the largest tree to 6.2% on the smallest for one standard deviation. Non—random sources of error in the method may include (1) loss of tritium from leaves due to rainfall; and (2) possible enrichment of tritiated water in leaves due to differences in vapor pressure and molecular diffusion coefficients between tritiated water and ordinary water. These require further experimental evaluation. The method may be generally applicable to field measurements of trans...

Effect of carbon dioxide, osmotic potential of nutrient solution, and light intensity on transpiration and resistance to flow of water in pepper plants.

Abstract: The rate of transpiration, temperature of the leaves, and relative water content of leaves of pepper plants were measured in a small chamber in which the temperature, relative humidity, and carbon dioxide concentration of recirculated air were controlled and measured. The data reported were obtained by noting the response of pepper plants to all combinations of the following treatments: high light, 1.5 x 10(6) ergs per square centimeter per second; low light, 3.0 x 10(4) ergs per square centimeter per second; three levels of CO(2): 50, 268, and 730 parts per million; nutrient solution osmotic potentials of -0.5, -5.0, -7.5, and -9.5 bars.The rate of transpiration of pepper plants was reduced by a decrease in osmotic potential of the nutrient solution, an increase in CO(2) concentration in the ambient air, and a decrease in light intensity. The response, as measured by transpiration, to the three variables, light, CO(2), and osmotic potential indicated that each variable influenced a different and independent mechanism. A change in a single variable produced essentially the same percentage change at all levels of the other variables. The rate of movement of water from roots to leaves was in response to water potential gradient and not the actual potential in the leaves.The resistance to flow of water through the plants (R) was estimated by dividing the difference between the water potentials of the solution and the leaves by the rate of transpiration. The data indicated an increase in R as the rate of transpiration decreased. The type and size of errors encountered in the estimation of R and location of R within the plant are discussed.

Effect of Light Intensity and Leaf Temperature on Photosynthesis and Transpiration in Wheat and Sorghum

Abstract: Wheat stomata offered less resistance to water and carbon dioxide diffusion than sorghum stomata at light intensities of 0·06 and 0·26 cal cm-2 min-i (400-700 nm) but resistances were comparable at 0·46 cal cm-2 min-i. Consequently, transpiration rates were higher in wheat than in sorghum, except at the high light levels, in leaf chamber experiments described here. Rates of photosynthesis were higher in sorghum than in wheat, with the greatest difference at high light levels. This resulted in a greater efficiency of dry matter production relative to water use in sorghum. Transpiration rate increased with increased temperature in both species. Photosynthesis was independent of temperature in wheat, and in sorghum under low light conditions, but otherwise photosynthesis increased with temperature in sorghum. In both species, efficiency of water use decreased as temperature increased at all light intensities. Water vapour concentration difference between the intercellular spaces and the air was comparable in wheat and sorghum and increased with temperature. The carbon dioxide concentration difference between air and intercellular spaces was substantially greater in sorghum than in wheat and increased with leaf temperature. Maximum values were obtained at the intermediate light level in sorghum.

Effect of drought upon rates of photosynthesis, respiration, and transpiration of seedlings of two ecotypes of Douglas Fir. II. Two-year-old seedlings.

Abstract: Rates of photosynthesis, respiration, and transpiration in Douglas-fir seedlings that grew from seed collected in a wet and a dry site were determined under conditions of variable soil-moisture stress. Photosynthetic rates declined sharply after soil-moisture stress of 1 atm was reached, with no major differences between seedlings from the two sources. At both high and low tensions of soil moisture, respiration and transpiration rates were higher for seedlings from the wet site than for those from the dry site. Seedlings from the xeric environment had higher photosynthetic rates at high relative turgidities and lower at low relative turgidities than seedlings from the mesic sites. Over a wide range of soil-moisture tensions, relative turgidity of seedlings from the xerophytic source varied less than that of seedlings from the mesophytic source.

Calculation of production of potatoes

Abstract: Net photosynthesis in potato crops is discussed. It was possible to calculate DM production even when water supply limited growth, using a modification of the procedure proposed by de Wit (see F.C.A. 19: 515). The relation between transpiration and total DM production was dependent on meteorological conditions during growth and not on specific differences between cv. DM distribution in potato plants is discussed.-F.A.S. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Transpiration and The Waxy Bloom in Brassica Oleracea L.

Abstract: The stomatal and cuticular transpiration rates and quantity of wax per unit area of leaf surface were determined for seven glaucous and non-glaucous sibling lines of B. oleracea. There were no statistically significant differences in the stomatal transpiration rates of the glaucous and non-glaucous lines, but there were highly statistically significant differences between the two classes of lines in terms of cuticular transpiration. Rubbing the surfaces of the leaves to remove lightly adhering wax deposits, such as the waxy bloom, significantly increased the cuticular but not the stomatal transpiration rates of both glaucous and non-glaucous plants. There was no appreciable correlation between the quantity of wax per unit area of leaf surface and the loss of water through cuticular transpiration among either the glaucous or nonglaucous lines. It was concluded that the function of cuticular waxes in limiting cuticular transpiration is a product of the architecture of the deposits and the quantity of wax per unit area of leaf surface.

The effect of nitrogen supply on some indices of plant‐water relations of beans (phaseolus vulgaris l.) *

Abstract: Summary The effect of nitrogen supply on some indices of plant-water relations was studied on potted bean plants. When soil moisture was relatively high, the leaves of N-deficient plants transpired less than those of N-supplied plants, the transpiration rate being closely associated with the chlorophyll content of the leaves of various ages. In detached leaves which were saturated by floating over distilled water, stomatal width was markedly wider in N-supplied than in N-deficient plants. Throughout the available moisture range, the water saturation deficit was higher in N-supplied leaves. In the dry range of soil moisture, chlorotic leaves transpired more than normal green leaves; N-deficient plants failed to exhibit a sharp rise in the content of soluble metabolites in the sap, when approaching the wilting range. The content of ‘cell wall materials’ was higher in N-deficient plants. The relationships between these indices is discussed; it is tentatively concluded that nitrogen deficiency impairs the ability of the plants to adjust their water status to changes in soil moisture by regulation of stomatal transpiration and of sap-solute concentration.

Water potential components in growing citrus fruits.

Abstract: Growing navel orange fruits (Citrus sinensis) 5.4 to 5.7 centimeters in diameter were used as a model system to determine the effects of transpiration and carbohydrate translocation on water and osmotic potentials in fruit tissues. Evidence supported the hypothesis that osmotic potential in the vesicles would be affected little by changes in transpiration or carbohydrate translocation because the vesicles are anatomically isolated from the transpiration stream and are at the end of the carbohydrate translocation pathway. In the mesocarp tissue, which contains a vascular network, osmotic potential decreased during the daytime when environmental conditions favored transpiration and increased at night. Exocarp water potential followed a similar pattern. Girdling of the stem above the fruits 5 days before sampling caused an increase of osmotic potential in the mesocarp but had no effect on exocarp water potential. Neither diurnal changes in transpiration nor girdling of the stem affected the osmotic potential of the vesicles.Osmotic potentials in all tissues of the fruit were in the range of -10 to -15 bars. Measurements of osmotic potential at 16 locations along a longitudinal plant through the fruit axis showed that osmotic potential increased from the stem to the stylar end, but it decreased from the pericarp tissues to the vesicles. As exocarp water potential decreased during a 20-day period after watering, osmotic potential decreased in the vesicles and exocarp. Turgor pressure, calculated as the difference between water and osmotic potentials, decreased with water potential in the vesicles but not in the exocarp. The lack of decrease of turgor pressure in the exocarp may result from a measurement error caused by pectins or from osmotic adjustment related to carbohydrate accumulation at low water potentials.

The effect of nitrogen supply on transpiration and stomatal behaviour of beans (phaseolus vulgaris l.)

Abstract: Summary The effect of nitrogen supply on the transpiration rate and stomatal opening of potted bean plants was studied in a series of experiments. The transpiration rates of N-supplied plants were higher than those of N-deficient plants when soil moisture was relatively high; as soil moisture approached the wilting range, the transpiration rates of N-supplied plants dropped to below those of N-deficient plants. In spite of the marked differences in transpiration rates, as influenced by soil moisture and nitrogen supply, the stomata appeared closed. By coating the upper or lower surfaces of the leaves with a vapour-impervious compound it was shown that stomatal apertures below the limit of microscopic resolution control the rate of transpiration. Under conditions that encourage stomatal opening (covering the plants with transparent plastic bags), the stomata of the N-deficient plants opened to a lesser degree than those of N-supplied plants. There was some evidence that when stomata were visibly open, transpiration rates were regulated by the degree of plant hydration rather than by the degree of stomatal opening. It is concluded that N-deficient plants fail to open their stomata as widely and to close them as tightly as N-supplied plants.