Showing papers on "Transpiration published in 1982"

Simulation of field water use and crop yield

Abstract: SWATR calculates the actual transpiration of a crop and CROPR calculates the actual growth rate of a crop

Stomatal Responses, Water Loss and CO2 Assimilation Rates of Plants in Contrasting Environments

Abstract: Stomatal apertures are the major pathway for the movement of CO2 from the atmosphere into the mesophyll of leaves. The presence of this pathway for the movement of gases also results in water loss from the hydrated surfaces within leaves to the atmosphere. Stomatal aperture appears to be controlled by complex mechanisms which operate to maintain a variable balance between allowing CO2 uptake to proceed, while restricting the loss of water vapor, and preventing leaf desiccation. Recent reviews have examined the physiological bases of stomatal function (Raschke 1979; Allaway and Milthorpe 1976) and stomatal responses to environment (Sheriff 1979; Burrows and Milthorpe 1976; Hall et al. 1976). Analyses which integrated stomatal effects on CO2 exchange, transpiration, and energy balance were developed based upon theory (Cowan 1977), which have led to hypotheses concerning optimal stomatal function (see Chap. 17, this Vol.; Cowan and Farquhar 1977). However, information concerning the simultaneous responses of stomata, water loss, and CO2 assimilation rates has not been reviewed for plants in natural environments.

Increased drought tolerance of mycorrhizal onion plants caused by improved phosphorus nutrition.

Abstract: Onion plants (Allium cepa L, cv. Downing Yellow Globe) grown in pots and infected by the mycorrhizal fungusGlomus etunicatus Becker and Gerdemann were more drought tolerant than were non-mycorrhizal individials when exposed to several periods of soil water stress separated by periods of high water supply, as shown by greater fresh and dry weights and higher tissue phosphorus levels in the mycorrhizal plants. The tissues of stressed, non-mycorrhizal plants were deficient in P, despite the fact that only non-mycorrhizal plants were fertilized with high levels of P (26 mg P per 440 g soil). Differences in plant water relations (leaf water potentials or transpiration rates) and changes in soil P levels which may have affected plant growth were investigated, and discounted as factors important for the results. The P nutrition of plants has been implicated in the ability of plants to tolerate drought and it was concluded that the ability of the mycorrhizal fungus to maintain adequate P nutrition in the onions during soil water stress was a major factor in the improved drought tolerance. Infection of the root by the fungus was found not to be affected by water stress or P fertilization but fungal reproduction, as determined by spore numbers in the soil, was decreased by water stress and by P fertilization.

Resistance of Plant Surfaces to Water Loss: Transport Properties of Cutin, Suberin and Associated Lipids

Abstract: The driving force of transpiration is the water potential gradient between leaf cells and the surrounding atmosphere. The water potential of the atmosphere decreases rapidly with decreasing humidity and may exceed —2,000 bar. This is a hostile environment for plant cells having a water potential of about — 5 to — 50 bar, and in adapting to terrestrial growing conditions, aerial parts of plants have evolved water-saving devices in the form of cuticle and periderm. All primary aerial parts of higher plants and ferns are covered by a cuticle, retained in submerged leaves and stems of the higher water plants (Arber 1920), and there are indications that at least some mosses have a cuticle (Priestly 1943). In secondary organs the cuticle is replaced by a periderm.

The adaptive significance of amphistomatic leaves

Abstract: A clear correlation between the presence of stomata on both surfaces and factors such as habitat, growth form, and physiology has yet to emerge in the literature. However, certain loose trends with these factors are evident, and these are reviewed along with evidence for hypostomaty as the primitive form. It is proposed that the effect of developing stomata on the upper surface as well as the lower is to increase maximum leaf conductance to CO2. Plants with a high photosynthetic capacity, living in full-sun environments, and experiencing rapidly fluctuating or continuously available soil water (as opposed to seasonal or long-term soil water depletion), are identified as deriving an adaptive advantage from a high maximum leaf conductance. The correlation between groups of plants fitting the above conditions and those noted to be amphistomatic is remarkable. Plants not fitting the conditions are found to be largely hypostomatic.

Control of Leaf Expansion by Nitrogen Nutrition in Sunflower Plants : ROLE OF HYDRAULIC CONDUCTIVITY AND TURGOR.

Abstract: Nitrogen nutrition strongly affected the growth rate of young sunflower (Helianthus annuus L.) leaves. When plants were grown from seed on either of two levels of N availability, a 33% decrease in tissue N of expanding leaves was associated with a 75% overall inhibition of leaf growth. Almost all of the growth inhibition resulted from a depression of the daytime growth rate. Measurements of pressure-induced water flux through roots showed that N deficiency decreased root hydraulic conductivity by about half. Thus, N deficiency lowered the steady-state water potential of expanding leaves during the daytime when transpiration was occurring. As a result, N-deficient leaves were unable to maintain adequate turgor for growth in the daytime. N deficiency also decreased the hydraulic conductivity for water movement into expanding leaf cells in the absence of transpiration, but growth inhibition at night was much less than in the daytime. N nutrition had no detectable effects on plastic extensibility or the threshold turgor for growth.

Stomatal Behavior and Water Relations of Waterlogged Tomato Plants

Abstract: The effects of waterlogging the soil on leaf water potential, leaf epidermal conductance, transpiration, root conductance to water flow, and petiole epinasty have been examined in the tomato (Lycopersicon esculentum Mill.). Stomatal conductance and transpiration are reduced by 30% to 40% after approximately 24 hours of soil flooding. This is not due to a transient water deficit, as leaf water potential is unchanged, even though root conductance is decreased by the stress. The stomatal response apparently prevents any reduction in leaf water potential. Experiments with varied time of flooding, root excision, and stem girdling provide indirect evidence for an influence of roots in maintaining stomatal opening potential. This root-effect cannot be entirely accounted for by alterations in source-sink relationships. Although 1-aminocyclopropane-1-carboxylic acid, the immediate precursor of ethylene, is transported from the roots to the shoots of waterlogged tomato plants, it has no direct effect on stomatal conductance. Ethylene-induced petiole epinasty develops coincident with partial stomatal closure in waterlogged plants. Leaf epinasty may have beneficial effects on plant water balance by reducing light interception.

Influence of vesicular‐arbuscular mycorrhizae on water movement through bouteloua gracilis (h.b.k.) lag ex steud*

Abstract: SUMMARY Root growth characteristics and water transport were compared between non-mycorrhizal Bouteloua gracilis and vesicular–arbuscular mycorrhizal Bouteloua gracilis infected with Glomus fasciculatus. Mycorrhizal plants did not have significantly different leaf area or root length from non-mycorrhizal plants, but did have significantly fewer and shorter root hairs. Mycorrhizal plants had 50% lower leaf resistance with no change in leaf or root water potentials; thus transpiration was increased 100% with a 50% reduction in whole-plant, soil-to-root and root-to-leaf resistance. Assuming that the difference in water uptake was due to transport via the mycorrhizal hyphae, estimated fungus-to-root transport was 2.8 × 10−5 mg s−1 per hyphal entry point which compares favourably with evapotranspiration rates measured in other coenocytic fungi. Thus, one of the major factors causing increased water transport and reduced resistance to water through-flow with mycorrhizal infection may be the increased surface area provided by the hyphae.

Phenology and Ecophysiology of the Tropical Tree, Tabebuia Neochrysantha (Bignoniaceae)

Abstract: Phenology and seasonal variations in water stress, as reflected in variations in stem circumference, were monitored throughout 1978 in numerous Tabebuia neochrysantha growing at several dry and wet sites in a lowland deciduous forest in Guanacaste, Costa Rica. The rate of leaf fall during the early dry season was strongly correlated with the decline in soil moisture and increasing water stress of the trees. Without exception, recovery from water stress was required for flowering and shoot emergence. At dry sites rehydration ocurred only after isolated rain showers or irrigation; at wet sites it took place during continued drought, probably as a consequence of the decrease in transpiration during leaf fall. Flowering has been observed to occur at any time between the beginning (December) and end (May) of the dry season. Differences in the timing and intensity of flowering as well as in the timing of leaf fall and shoot emergence can be accounted for by site—dependent differences in the rates of desiccation and rehydration of trees in conjunction with year—to—year variation in the timing and intensity of rainfall. No evidence for the control of seasonal development by other environmental factors, such as variations in temperature of photoperiod, was obtained.

Gas exchange in a 20-year-old stand of Scots pine

Abstract: The rates of net photosynthesis and transpiration of one-year-old shoots were measured in situ in five different positions within the crown of a young Scots pine (Pinus sylvestris L.). Measurements were carried out on south- and north-facing shoots on the third and sixth whorls, respectively, and on an east-facing shoot on the ninth whorl. In another investigation the rates of gas exchange of one-year-old shoots on the third whorl of eight different trees were studied. The measurements were made during June and July, 1977, under non-limiting conditions of soil water. The daily rates of net photosynthesis in whorls three and six followed the light conditions closely, with higher rates for the south side of each whorl and higher for whorl three than six. On whorl nine the shoot had a higher light compensation point and a low rate of photosynthesis at light saturation compared to the other shoot positions. The quantum yield for the shoot on the lowest whorl, as estimated from the linear part of the light response curve, was 50% lower than for shoots on whorl three and six. The variation in transpiration rates was pronounced within the crown as an effect of differences in the absolute value and diurnal course of stomatal conductance. The variation in net photosynthesis was small between different trees while the variation in transpiration was much higher. Thus the variation in water use efficiency was great. It is concluded that it is possible to extrapolate measurements of net photosynthesis from individual trees up to a stand level without introducing large errors in the estimate. More caution must be paid before extrapolating tree transpiration up to stand transpiration. However, before an extrapolation of gas exchange is made from tree to stand level the variation in net photosynthesis and transpiration rate within the crown must be known.

Species distribution and community organization in a Nebraska Sandhills mixed prairie as influenced by plant/soil-water relationships.

Abstract: Plant and soil water relationships in a typical nebraska Sandhills prairie were examined to 1) explain the observed distribution patterns of several dominant grasses along a topographic gradient, and 2) show how spatial and temporal variations in soil moisture are critical to community organization on a sandy substrate. An experimental transect encompassing the major community and soil types along a steep, west-facing vegetated dune was established. Maximum available water was shown to be significantly higher in the fine textured surface soils of the lowland sites than the coarse textured sands of the dune sites. Seasonal (1979) patterns of available soil moisture of the sampling sites on the transect showed that in the upper elevation dune sands, moisture was available in the entire profile with surface depletions not occurring until mid to late summer. In contrast, moisture in the surface 60–80 cm in the fine textured lowland soils was exhausted by early to mid-summer with the entire profile nearly dry by late summer. Deep-rooted, C4 species, Andropogon hallii and Calamovilfa longifolia which are common on upper, coarser sandy soils showed significantly greater water stress on fine textured soils than on dune sands. C3, shallowrooted species, Agropyron smithii, Stipa comata, and Koeleria cristata always experienced lower mid-day and predawn leaf water potentials than the C4 species. The C3 species, with the exception of Koeleria are most abundant on finer textured soils that provide substantial moisture during their peak activity in the spring. It appears that the C4 species show more conservative water use patterns than the C3 species as significantly lower leaf conductances in the C4's were measured when soil water was abundant. The C3 species appear to be opportunistic with available water and rapidly deplete surface soil moisture as a result of high transpiration rates. These data suggest that the temporal and spatial distribution of available water along this gradient controls species distribution according to rooting morphology, photosynthetic physiology, and water deficits, incurred by transpirational losses. Competitive interactions between species that utilize soil moisture differently may be an important factor in community organization.

Water supply and tree growth. Part I. Water deficits

Abstract: Water supply is the most important environmental factor determining distribution, species composition and growth of forests. Net annual primary production of forests varies from as much as 3000 g/m/sub 2/ in wet regions to negligible amounts in dry regions. The water balance of trees has been characterized by visible wilting, tissue moisture content, relative water content, saturation deficit, and water potential. Water deficits develop readily in forest trees, even in trees growing in wet soil, because of excess transpiration over absorption of water. Water deficits adversely affect seed germination and cause shrinkage of leaves, stems, roots, fruits, and cones. Some of the decrease in photosynthesis during drought is the result of increased resistance to diffusion of CO/sub 2/ to chloroplasts and some to decrease in photosynthetic capacity. Water deficits inhibit shoot growth, wood production, and root growth. Yield of fruits and seeds can be inhibited at various stages of reproductive growth such as flower bud initiation, anthesis, pollination, fertilization, embryo growth, and fruit and seed enlargement. Water deficits may also induce leaf scorching and abscission, dieback of twigs and branches, and drought cracks. Severe water deficits often kill trees. Drought tolerance of trees may reflect desiccation avoidance or desiccation tolerance, with the former much more important. Among the most important of these are reduction in number and size of leaves; small, few, and sunken stomata; rapid stomatal closure; abundant leaf waxes; leaf shedding during droughts; extensive root development; capacity for twig and stem photosynthesis; living wood fibers; and strong development of palisade mesophyll.

Effect of barley plants on the decomposition of 14C‐labelled soil organic matter

Abstract: Summary The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with 14C-labelled ryegrass for 1 year. The plants reduced the loss of 14CO2, from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little 14C label being associated with the shoot. Chemical fractionation of the root showed that the 14C was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of 14CO2, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.

Survival of mycorrhiza formed by cenococcum geophilum fr. in dry soils

Abstract: SUMMARY Seedlings and 2-year old saplings of Tilia cordata growing in the shade on sandy-textured soils can survive periods when the water potential of the soil decreases to −18 to −55 bars. Their mycorrhiza formed by Cenococcum geophilum is shown to remain alive. Measurement of rates of water-loss from seedlings in similar conditions decreases from 16.0 to 26.2 mg h−1 when the soil is wet, to 4.1 to 4.6 mg h−1 when the soil is dry. Calculation of the volume of the mycorrhiza shows that its water content would support transpiration for periods not exceeding 1 h. Survival of the mycorrhiza through long periods of drought is apparently related to the ability of the fungal partner to withstand desiccation.

A portable system for measuring carbon dioxide and water vapour exchange of leaves

Abstract: . The apparatus described here is a fully portable, steady-state gas exchange system for simultaneous measurements of the CO2 exchange and transpiration of single, attached leaves. The leaf cuvette provides temperature, humidity, and CO2-concentration control. The system is suitable for either surveys or detailed studies of photosynthetic and stomatal responses to environmental variables. Representative data demonstrate the response time characteristics of the system and constitute the first field evidence of stomatal behaviour consistent with a recent hypothesis concerning the optimum pattern of stomatal conductance for the maximization of water-use-efficiency.

Transpirational supply and demand: Plant, soil, and atmospheric effects evaluated by simulation

Abstract: The assumption that transpiration is the lesser of an atmospheric demand function and a water supply function was tested by simulation with Federer's (1979) soil-plant-atmosphere model. The best estimate of atmospheric demand is called unstressed transpiration, defined as the transpiration that would occur in ambient conditions if stomata were unaffected by plant-water potential. For practical purposes the Penman equation provides a good estimate of unstressed transpiration for short vegetation but not for forests. Even when atmospheric variables and the Penman estimate are held constant among forest canopies, unstressed transpiration can vary by a factor of two because of variation both in the maximum value of leaf conductance and in the ratio of canopy conductance to leaf conductance. The best water supply function incorporates depth variation of soil water potential and of root and soil properties. A more practical supply function uses the ratio of available water in the root zone, W, to maximum available water WM. The maximum available water is soil water held at potentials less than that at which the hydraulic conductivity is 2 mm/d and greater than the critical leaf water potential at which stomata are completely closed. Using a mature hardwood forest as a standard, various parameters were varied to examine their effects on a water supply function defined as a supply constant times W/WM. The supply constant was found to be independent of soil texture and physical properties. Root density and the internal resistance of the plant to water flow were the most important determinants of the supply constant. Reasonable variation of root density and internal resistance produced variation in the constant from 1.9 mm/h, which implies that supply is less than demand only when soil is very dry, to 0.5 mm/hr, which implies that supply cannot meet the demand even when the soil is wet.

Translocation of nitrogen in a vegetative wheat plant (Triticum aestivum)

Abstract: The translocation of nitrogen was studied in vegetative wheat plants (Triticum aestivum L. cv. SUN 9E) grown with a limited supply of nitrogen. The concentration of nitrogen in xylem sap exuding from the excised roots was the same as the nitrogen concentration in the transpiration stream. Translocation of nitrogen to the shoot was, therefore, calculated as the product of the transpiration rate and the concentration of nitrogen in xylem exudates. On the 22nd day from sowing more nitrogen was translo-cated to the shoot than it incorporated, and 56% of the nitrogen translocated to the shoot was retranslocated to the roots. The nitrogen retranslocated to the roots was more than adequate to supply the requirements of the roots for growth, and the balance of the retranslocated nitrogen was reloaded into the xylem stream. Expressed as a proportion of the total increment of nitrogen in the plant on day 22, between 79 and 100% of the nitrogen absorbed by the plant was “cycled'’ in the plant (root shoot root shoot). It is suggested that the size of this mobile reserve of nitrogen may vary depending on the growth requirement of the plant, its nitrogen-uptake capacity and the contribution of nitrogen from mobilisation of leaf protein during senescence.

Leaf conductance as a function of photosynthetic photon flux density and absolute humidity difference from leaf to air.

Abstract: For an entire season of stomatal activity, leaf or needle conductance was observed on four species, each in a different genus: Engelmann spruce (Picea engelmannii Parry ex Engelm.), subalpine fir (Abies lasiocarpa [Hook.] Nutt.), lodgepole pine (Pinus contorta var. latifolia Engelm.), and aspen (Populus tremuloides Michx.). Conductance in the natural environment was described for all species by photosynthetic photon flux density (PPFD) and absolute humidity difference from leaf to air (DAH), as follows: Conductance = b1 (√PPFD/√DAH) + b2 (√PPFD/DAH) + b3 (√PPFD/DAH2). The only data not fitting this relationship were conifer data collected after freezing nights or aspen data collected during a short period in August when water stress occurred. In both cases, leaf conductance was reduced. It is proposed that PPFD and DAH are primary factors controlling stomatal function for plants growing in their native range; secondary factors, such as temperature and water stress, affect conductance intermittently, except when plants are growing outside their normal environmental conditions.

Sap flow rate and transpiration dynamics in the full-grown oak (Quercus robus L.) in floodplain forest exposed to seasonal floods as related to potential evapotranspiration and tree dimensions

Abstract: Sap flow rate and transpiration dynamics were studied in the course of 3 years in a dominant tree species in the floodplain forest,i.e. in the full-grown oak (Quercus robur L.) tree, using the method of trunk heat balance devised by the authors. The investigations were carried out at a period at which regular and marked fluctuation in a relatively high water table usually occurred, culminating in seasonal flooding. High sap flow rate values in the tree were established under conditions of non-limiting water supply in soil (up to 400 kg per day or up to 39 000 kg per vegetation period) and characteristic daily flow curves (rounded with a large amplitude and with the maximum at noon), corresponding to those described theoretically. Relationships were inferred by means of which tree water consumption can be calculated under these conditions on the basis of data measured at meteorological stations. From these equations it follows that the transpiration of the tree canopy amounted to 80% of the potential evapotranspiration. The amount of the used daily tree water reserve was assessed to be 0.4 mm in the seasonal average. The transpiration coefficient reached in climatically distinct years the values of 400 to 700 of the increase in tree dry matter. The area of the so-called effective tree-crown ground plan approximated to the area determined geodetically. The results obtained are useful for both ecophysiological and hydrological studies. Some of the described procedures are convenient for the evaluation of functional tree dimensions and according to them also of the forest stand structure.

Temperature-Induced Water Stress in Chilling-Sensitive Plants

Abstract: Cotton (Gossypium hirsutum) and bean (Phaseolus vulgaris) seedlings wilted when entire plants or roots alone were chilled in the light. The water relations and gas exchange characteristics of these chilling-sensitive species have been compared with a chilling-resistant species, collard (Brassica oleracea), following exposure to a chilling temperature at 5°C. Chilling either whole seedlings or roots alone had little or no effect on leaf water potentials or gas exchange of collard seedlings but induced a rapid response in cotton and bean. Leaf water potentials were reduced below - 1.5 MPa within 1-2 h; however, this response was reduced when seedlings were chilled in the dark or when leaves or roots of bean were pretreated with abscisic acid. Chilling also caused a rapid reduction in photosynthesis but a more gradual decline in transpiration over the first 2-4 h due to the slow closure of stomata. These results suggest that water deficits in chilling-sensitive seedlings when exposed to low temperature in the light are caused by a reduction in the hydraulic conductivity of the root membranes and by the loss of stomatal control in the leaves. The reduced flow of water into the plant through the roots and continued transpiration, because of the slow closure of stomata, bring about a rapid decline in leaf water potential and wilting of primary leaves.

Water Uptake and Flow in Roots

Abstract: The loss of water by evaporation from the leaves of plants (transpiration) is an inevitable accompaniment of the photosynthetic absorption of carbon dioxide from the atmosphere. Water is absorbed from the soil by the roots to make good this transpirational loss. There is thus a flow of water through the plant from the soil to the atmosphere — the so-called transpiration stream — and it is the purpose of this chapter to focus attention on one part of this stream, namely the flow of water from the soil into, and through the roots. However, to abstract this partial process from the whole root-plant-atmosphere system can be misleading and a note of caution is necessary.

Effect of Grass, Legume, and Tree Roots on Soil Shearing Resistance

Abstract: Plants enhance soil stability against downslope mass movement through the removal of soil water by transpiration and by the mechanical reinforcement of their roots. To assess the magnitude of this reinforcement, direct shear measurements were made on 0.25-m diam cylindrical soil columns packed both homogeneously and in layers which simulated water and/or root-impeding horizons. In all cases the matric potential was adjusted to zero before shearing. Twelve plant species were used including seven grasses: Phalaris tuberosa, Lolium rigidium, Dactylis glomerata, Bromus mollis, Sorgum bicolor sudanense, Triticum oestivum, Hordeum vulgare. Two legumes were used: Vicia dascarpa, Medicago sativa. and two trees were used: Pinus ponderosa and Quercus agrifolia. The ratio of the shear resistance at 25-mm displacement of the rooted and unrooted specimens was used as a measure of root reinforcement. Roots of several grasses planted in early fall and sheared the following spring gave about a threefold increase in shear resistance at the 0.3-m depth in homogeneous saturated clay loam. In the same material, roots of oak produced a similar increase only after 3 years' growth. One-year-old alfalfa produced a fourfold increase. At the 0.45-m depth at the interface between soil and a dense gravel-sand mixture simulating weathered rock, yellow pine gave a 1.5-fold increase after 16 months and a 2.5-fold increase after 52 months. Hardinggrass was almost equally effective after only 7 months. In almost all cases where roots increased soil shear resistance, the resistance continued to increase beyond 25-mm displacement so that the selection of 25-mm displacement was conservative, i.e., it may underestimate the root reinforcing effect. Factor of safety calculations for shallow planar slides using measured shear strengths show that plant roots can make large increases in slope stability.

Salt marsh vegetation of the Western Mediterranean desert of Egypt

Abstract: The extensive salt marshes in the plains and depressions of the western Mediterranean desert of Egypt were classified into three habitat types: A with shallow water table and high salinity, B with relatively deep water table and high salinity, and C transitional habitats in which salinity and water table are no controlling factors. Fourteen vegetation types were distinguished, each dominated by one or two species. The dominating life forms are chamaephytes in sites of high salinity, and therophytes in sites of low salinity. Spatial and temporal variations in the standing crop biomass were pronounced. The accumulation of material started during spring and reached a maximum in autumn, when photosynthetic activity was maintained to account for transpiration losses. There was a general trend of increasing salinity and concentration of different ions from habitat type A to habitat type C through habitat type B. The periodical variation in the water table was insignificant, while a significant drop in salinity and the concentration of different ions was detected in spring, which was attributed to the diluting effect of rain water during that season. Most species exhibited clear distribution patterns and their a bundance varied significantly along gradients of different factors. Simple correlations between the compositional gradients or the distributional behaviour of species and salinity or the concentration of individual ions were generally low, while correlations with combinations of ions in the form of ratios (notably sodium and potassium adsorption ratos) were higher. During the early stages of succession the building up of soil and the decrease in salinity are the most important factors while at more advanced stages, soil texture and calcium carbonate content become more decisive.

The effect of vapor pressure on stomatal control of gas exchange in Douglas fir (Pseudotsuga menziesii) saplings

Abstract: Increasing leaf to air vapor pressure deficit (VPD) caused reductions in stomatal conductance of both current year and previous season needles of Pseudotsuga menziesii saplings. The stomata of current year needles were found to be more responsive to changes in VPD than those of previous season needles. The reductions in stomatal conductance of current year needles were not associated with decreases in xylem pressure potential. In fact, the reductions in stomatal conductance of current year needles were sometimes sufficient to reduce transpiration and thus raise xylem pressure potential even though VPD was increasing. There was a decline in stomatal responsiveness to VPD in current year needles between early and late summer. Pressure-volume curves determined for different age needles at different times of the year suggested that differences and changes in stomatal responsiveness to VPD may have been caused in part by differences and changes in needle water potential components. Hexane washes of current year needles during the late summer succeeded in partially restoring their VPD sensitivity, suggesting that changes in the water permeability of the external cuticle during needle maturation may also have played a role in causing the summer decline in VPD responsiveness.

Factors Controlling Transpiration and Photosynthesis in Tamarax Chinensis Lour.

Abstract: Photosynthetic and stomatal responses of Tamarix chinensis to temperature, light, and humidity were investigated in the field in New Mexico and in the laboratory. Transpiration rates for T. chinensis were similar to those of several herbaceous plants and co—occurring phreatophytes. Net photosynthetic rates and water use efficiency of T. chinesis were lower than for other species. Photosynthesis was light saturated at a photon flux density equal to 44% of full sunlight. Carbon dioxide assimiliation was tightly coupled to irradiance below light saturation. Leaf resistances remained low at photon flux densities above one—third of full sunlight, but increased linearly with decreasing photon flux density below that level. Shading for 5 min resulted in a doubling of leaf resistance. The rapid response of stomata to changing light conditions is probably an adaptation to conserve moisture when light is limiting to photosynthesis. Optimal leaf temperatures for photosynthesis were 23°—28° C, which correspond roughly to ambient temperatures during the early part of the day when evaporative demand was relatively low. T. chinensis stomata appeared to respond directly to changes in the leaf—to—air absolute humidity gradient. At constant temperature, leaf resistance increased linearly with increases in the leaf—air humidity gradient. Midday depressions of gas exchange invariably occurred in the field, despite the fact that the plants had an abundant water supply. These depressions resulted from increases in leaf resistance in response to increasing evaporative demand of the air. This response results in improved water use efficiency during the hottest portion of the day. Plant water potential decreased from pre—dawn values of about —0.9 MPa to minimal values of about —2.6 MPa by midmorning. Improvements in bulk water status were often observed during the afternoon when leaf resistances were higher. Diurnal patterns suggested that leaf resistance was largely a function of temperature, light, and humidity, rather than plant water status.

Soil- and atmosphere-induced plant water stress in cotton as inferred from foliage temperatures

Abstract: Foliage temperatures of cotton obtained by means of infrared thermometry, along with air wet and dry bulb temperature measurements, were used to investigate certain relationships existing between the water contents of soil and air and the ability of the crop to maintain transpiration at the potential rate. It was found that as soil water content is progressively depleted following an irrigation, departure from potential transpiration begins to occur at smaller and smaller values of air vapor pressure deficit in a regularly predictable fashion. It was also demonstrated that the plant water potential of cotton transpiring at the potential rate is a function of the air vapor pressure deficit and that the difference between this base value and the tension that develops under nonpotential conditions is a unique function of a newly developed plant water stress index. Finally, an example of the application of this foliage temperature-based index to evaluating the effects of an irrigation event is presented.

Variation in photosynthesis and stomatal conductance in an ozone-stressed ponderosa pine stand: light response

Abstract: The seasonal course (May to October 1977) of gross photosynthesis (from /sup 14/CO/sub 2/ uptake and stomatal conductance) in a stand of ponderosa pine (Pinus ponderosa Laws.) in the San Bernardino National Forest was characterized as a function of light. Nine sapling trees, classified for comparative studies into three chronic injury classes (slight, moderate, severe) had experienced oxidant fumigations from California's South Coast Air Basin for approximately 18 years, since their establishment following fire. The CO/sub 2/-transfer pathway was partitioned into its stomatal and residual (mesophyll, carboxylation, excitation) resistance components, for conditions of light saturation and 20/sup 0/C. Light-saturated gross photosynthetic rates and photochemical conversion efficiencies were highest in the current-year needles and decreased with increasing needle age and oxidant injury. Maximum stomatal conductance and stomatal sensitivity to increasing light during stomatal opening followed a trend similar to that of photosynthesis, except for current-year needles, where conductance parameters were highest in the severely injured trees. This higher conductance may contribute to observed differential ozone sensitivity in ponderosa pine. Premature senesence and abscission of the 1-year (severely injured trees) and 2-year (slight to moderate injury) needles occurred at about the time CO/sub 2/ uptake dropped to 10 percent of the potentialmore » for current needles of slightly injured trees without foliar injury symptoms. The ratio of the stomatal CO/sub 2/ resistance to the total CO/sub 2/ resistance decreased with increasing oxidant injury and needle age, suggesting that loss of photosynthetic capacity was primarily related to the loss of chloroplast function rather than to increased resistance of CO/sub 2/ diffusion through the stomata.« less

Soil Moisture Inferences from Thermal-Infrared Measurements of Vegetation Temperatures

Abstract: Remote sensing methods for the estimation of soil moisture yield direct information only for the topmost layers of soil. Reflected solar, thermal-infrared (IR), and microwave techniques are sensitive to the surface skin, from the surface to about 5 cm, and from the surface to about 10 cm, respectively. When the growth of vegetation is of major interest, soil moisture needs to be infrared at least to the depth of rooting of the plants. Since remote measurement of soil moisture is depth limited, it has been suggested that plant measurements, specifically plant temperatures, may yield information about soil moisture within the root zone. To examine this possibility, three plots of wheat, initially treated similarly, and later irrigated differently, were monitored for vegetation temperature (by infrazed thermometry) and for soil-water content (thrice weekly neutron moisture meter measurements). Vegetation temperatures were converted to a crop water stress index (CWSI). The CWSI was found to be a nonunique function of extractable water. The nonuniqueness was probably caused by inability to adequately specify the root zone and by the fact that plants require a recovery period (five to six days for this experiment) after being stressed before normal water uptake and transpiration proceeds.