Showing papers on "Transpiration published in 1984"

Hydraulic Conductance as a Factor Limiting Leaf Expansion of Phosphorus-Deficient Cotton Plants

Abstract: Suboptimal levels of phosphorus (P) strongly inhibited leaf expansion in young cotton (Gossypium hirsutum L.) plants during the daytime, but had little effect at night. The effect of P was primarily on cell expansion. Compared to plants grown on high P, plants grown on low P had lower leaf water potentials and transpiration rates, and greater diurnal fluctuations in leaf water potential. Hydraulic conductances of excised root systems and of intact transpiring plants were determined from curves relating water flow rate per unit root length to the pressure differential across the roots. Both techniques showed that low P significantly decreased root hydraulic conductance. The effects of P nutrition on hydraulic conductance preceded effects on leaf area. Differences in total root length, shoot dry weight, and root dry weight all occurred well after the onset of differences in leaf expansion. The data strongly indicate that low P limits leaf expansion by decreasing the hydraulic conductance of the root system.

Plant Growth and Water Use With Limited Water Supply in High CO2 Concentrations. I. Leaf Area, Water Use and Transpiration

Abstract: Plants of 16 agricultural and horticultural species were grown from seed in spaced pots in two glasshouses, one with normal and one with twice the present atmospheric CO2 concentration. Water use and leaf area development were measured while soil moisture content declined from field capacity to c. 6%. High CO2 increased leaf area in all but two species, the increase varying from 20 to 75%. However, the water loss per plant followed a similar time course to that of plants in control CO2 concentration because of the reduction of daily transpiration rate per unit leaf area (range 4-39%). Cowpea and sunflower plants rewatered after one soil drying cycle showed 9 and 5%, respectively, lower transpiration rate in high CO2 over a subsequent drying cycle than in the first cycle. Averaging across all species and soil moisture contents, transpiration rate was less reduced by high CO2 (21%) than was stomatal conductance (36%) and this is attributed to the increased leaf temperature caused by reduced stomatal conductance.

Transpiration from a kiwifruit vine as estimated by the heat pulse technique and the Penman-Monteith equation

Abstract: Heat pulse velocities were measured at 4 depths in the stem of a kiwifruit plant (Actinidia chinensis Planchon) throughout one lateautumn day, and sap velocities and total stem sap flux were calculated from physical principles without resorting to empirical calibration. Concurrent measurements of net radiation, air temperature, vapour pressure deficit, wind speed, and stomatal conductance were utilised in a second, independent calculation of transpiration based on the Penman-Monteith combination equation. Results indicated that most flow occurred in the outer ring or rings of the stem xylem tissue, and there was a close relationship between fluxes estimated by the 2 methods. We conclude that heat pulse velocity measurement is potentially a practicable method for measuring transpiration rates in kiwifruit.

Environmental Physiology of Sorghum. II. Epicuticular Wax Load and Cuticular Transpiration1

Abstract: The effect of epicuticular wax (EW) load on the cuticular transpiration rate (Tc of sorghum [Sorghum bicolor (L.) Moench] leaves was studied. Leaves from bloom, bloomless, and sparse bloom isogenic lines and various bloom-type hybrids were collected from field plots after anthesis in 1976, 1977, and 1981. Epicuticular wax was extracted with chloroform and quantified using a colorimetric method. The Tc of detached leaves were measured with a humidity sensor in a closed cuvette or calculated from the mass of water lost per unit time under standardized conditions. The Tc increased as EW decreased over the range of 0.1 to 0.03 g m⁻² when data from all genotypes were pooled for analysis. However, among the normal (bloom) phenotypes there was no clear association between rates of water loss and EW. These data suggest that EW greater than about 0.067 g m⁻² provide an effective barrier to water loss through cuticles of sorghum leaves under most conditions. Although the two methods of measuring Tc produced the same qualitative results, Tc rates calculated from the mass of water transpired under more “realistic” controlled conditions were higher and probably more directly related to rates of water loss under field conditions. Because of its speed and technical simplicity, the water loss method appears superior when large numbers of samples must be evaluated.

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content : I. Species comparisons at high soil water contents.

Abstract: The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.

Stomatal conductance, transpiration, and resistance to water uptake in a Pinussylvestris spacing experiment

Abstract: Stomatal conductance was measured with porometers in two plots of Pinussylvestris L. with markedly different tree spacings (plot 1, 608 stems ha−1; plot 2, 3281 stems ha−1), and hourly rates of transpiration were calculated using the Penman–Monteith equation at intervals throughout one growing season. Stomatal conductance varied little in relation to height or age of foliage. There was a linear decrease in canopy conductance with increasing water vapour pressure deficit of the air. Transpiration rates on both plots increased during the summer (maximum 0.3 mm h−1); rates on plot 1 were always lower (ca. 0.7 times) than on plot 2. Needle water potentials were similar throughout the season and only slightly lower on plot 1 than on plot 2. The mean hydraulic resistance of the trees on plot 1 was 2.4 times that on plot 2. The results support a hypothesis that considers the changes in transpiration rate, conducting cross-sectional area, canopy leaf area, water potential, and hydraulic resistance following thinn...

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species

Abstract: Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO2 partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO2 partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.

Stomatal responses to water stress and to abscisic Acid in phosphorus-deficient cotton plants.

Abstract: Cotton (Gossypium hirsutum L.) plants were grown in sand culture on nutrient solution containing adequate or growth-limiting levels of P. When water was withheld from the pots, stomata of the most recently expanded leaf closed at leaf water potentials of approximately −16 and −12 bars in the normal and P-deficient plants, respectively. Pressure-volume curves showed that the stomata of P-deficient plants closed when there was still significant turgor in the leaf mesophyll. Leaves of P-deficient plants accumulated more abscisic acid (ABA) in response to water stress, but the difference was evident only at low water potentials, after initiation of stomatal closure. In leaves excised from unstressed plants, P deficiency greatly increased stomatal response to ABA applied through the transpiration stream. Kinetin blocked most of this increase in apparent sensitivity to ABA. The effect of P nutrition on stomatal behavior may be related to alterations of the balance between ABA and cytokinins.

Hydraulic resistance of plants. II: Effects of rooting medium, and time of day, in barley and lupin

Abstract: Barley and lupin plants were grown in pots designed to fit inside a pressure chamber. The pots contained sand, soil, or nutrient solution. Transpiration rates were varied over a wide range. At a given transpiration rate, Q, the balancing pressure, p, of a plant was determined; p is the pneumatic pressure that must be applied to the roots in the pressure chamber to have a cut in the xylem of the shoot on the verge of bleeding. The relation between p and Q, p(Q), was non-linear and hysteretic for solution- grown plants, but was remarkably linear for plants grown in sand or soil, i.e. the data for a given plant on a given occasion conformed closely to the equation p =po + rQ, where po and r were constants. Even though p(Q) was linear for the plants grown in sand or soil, po was often much larger than Δπ, the difference in osmotic pressure between the external solution and the xylem of the root, so that the apparent hydraulic resistance of the plants, i.e. (p-Δπ)/Q, depended strongly on Q. Furthermore, po changed diurnally and was typically 100-200 kPa higher in the afternoon than in the morning. These results are discussed in relation to the equations that are commonly used to describe water flow through plants. It is postulated that r represents the true hydraulic resistance of the plant, which is independent of Q in the plants grown in soil or sand but may vary diurnally, and that the discrepancy between po and Δπ represents either an additional and hitherto unrecognized difference in osmotic pressure across the membranes of the root that intercept the transpiration stream, or a pressure required to open valves through which the water has to pass, with the valves possibly being located in the plasmodesmata.

Influence of vesicular–arbuscular mycorrhiza on the hydraulic conductivity of roots of two citrus rootstocks

Abstract: Summary Carrizo citrange (CC) and sour orange (SO) seedlings were grown in a low phosphorus (P) sandy soil and either inoculated with Glomus intraradices Schenck & Smith or fertilized with soluble P. Mycorrhizal seedlings had nutritionally sufficient levels of leaf P, non-mycorrhizal plants of similar size were P-deficient. The root–shoot ratio of both rootstocks was reduced by mycorrhizal colonization, but root hydraulic conductivity per unit root length of mycorrhizal CC and SO was more than twice that of non-mycorrhizal seedlings under well-watered conditions. Mycorrhizal plants also had significantly higher transpiration rates when standardized on a root length basis, and greater transpiration appeared to be related to the increased conductivity of roots. Flow of water to roots via hyphae alone could not account for the greater water uptake by mycorrhizal roots. Apparently, mycorrhizal enchancement of P nutrition was primarily responsible for the greater conductivity of roots since no differences were found between root hydraulic conductivity of mycorrhizal and non-mycorrhizal CC of equal P status under well-watered conditions.

Hydraulic resistance of plants. III. Effects of NaCl in barley and lupin

Abstract: Barley (salt-tolerant) and white lupin (salt-sensitive) were grown in sand in pots designed to fit within a pressure chamber. The sand was irrigated with a nutrient solution to which increasing amounts of NaCl were added daily in increments of 10-25 mol m-3. For a range of transpiration rates (Q), the hydrostatic pressure of the leaf xylem sap of an intact plant was measured by applying sufficient air pressure (p) to the root system to raise the pressure of this sap to zero. The relation between p and Q was linear, i.e. of the form p = po + rQ. Po, the intercept on the p axis, reflects the difference in osmotic pressure across the root, and it is assumed that r, the slope of this relation, gives the hydraulic resistance of the plant. In NaCl-treated barley, r remained constant as the NaCl in the soil solution was increased to 200 mol m-3 over 10 days, and differed little from that of the controls. Po increased by about the same amount as the increase in osmotic pressure of the soil solution. This indicates near-perfect osmotic behaviour by the roots, and consistent with this, osmotic pressure of sap expressed from the cut shoot base generally changed little with increasing NaCl, for a given transpiration rate. In NaCl-treated lupin, by contrast, r increased continually from 25 to 150 mol m-3 NaCI, when it was four times that of the controls. Beyond 75 mol m-3, po increased less than increases in the osmotic pressure of the soil solution, which suggests that salts were then leaking into the root xylem. However, the osmotic pressure of the xylem sap flowing through the petiole did not start to increase until 3 days later when the external solution was over 120 mol m-3, suggesting that substantial amounts of NaCl were being removed from the xylem sap before it reached the petiole.

Effects of Water Stress on Photosynthesis and Carbon Partitioning in Soybean (Glycine max [L.] Merr.) Plants Grown in the Field at Different CO2 Levels

Abstract: The effects of water stress and CO2 enrichment on photosynthesis, assimilate export, and sucrose-P synthase activity were examined in field grown soybean plants. In general, leaves of plants grown in CO2-enriched atmospheres (300 microliters per liter above unenriched control, which was 349 ± 12 microliters per liter between 0500 and 1900 hours EST over the entire season) had higher carbon exchange rates (CER) compared to plants grown at ambient CO2, but similar rates of export and similar activities of sucrose-P synthase. On most sample dates, essentially all of the extra carbon fixed as a result of CO2 enrichment was partitioned into starch. CO2-enriched plants had lower transpiration rates and therefore had a higher water use efficiency (milligrams CO2 fixed per gram H2O transpired) per unit leaf area compared to nonenriched plants. Water stress reduced CER in nonenriched plants to a greater extent than in CO2-enriched plants. As CER declined, stomatal resistance increased, but this was not the primary cause of the decrease in assimilation because internal CO2 concentration remained relatively constant. Export of assimilates was less affected by water stress than was CER. When CERs were low as a result of the imposed stress, export was supported by mobilization of reserves (mainly starch). Export rate and leaf sucrose concentration were related in a curvilinear manner. When sucrose concentration was above about 12 milligrams per square decimeter, obtained with nonstressed plants at high CO2, there was no significant increase in export rate. Assimilate export rate was also correlated positively with SPS activity and the quantitative relationship varied with CER. Thus, export rate was a function of both CER and carbon partitioning.

The effect of nitrogen supply on growth and water-use efficiency of xylem-tapping mistletoes.

Abstract: Xylem-tapping mistletoes are known to have normally a higher rate of transpiration and lower water-use efficiency than their hosts. The relationships between water relations, nutrients and growth were investigated for Phoradendron juniperinum growing on Juniperus osteosperma (a non-nitrogen-fixing tree) and for Phoradendron californicum growing on Acacia greggii (a nitrogen-fixing tree). Xylem sap nitrogen contents were approximately 3.5 times higher in the nitrogen-fixing host than in the non-nitrogen-fixing host. The results of the present study show that mistletoe growth rates were sevenfold greater on a nitrogen-fixing host. At the same time, however, the differences in water-use efficiency between mistletoes and their hosts, which were observed on the non-nitrogen-fixing host did not exist when mistletoes were grown on hosts with higher nitrogen contents in their xylem sap. Growth rates and the accumulation of N, P, K, and Ca as well as values for carbon-isotope ratios of mistletoe tissues support the hypothesis that the higher transpiration rates of mistletoes represent a nitrogen-gathering mechanism.

Transpiration-induced changes in the photosynthetic capacity of leaves

Abstract: High transpiration rates were found to affect the photosynthetic capacity of Xanthium strumarium L. leaves in a manner analagous to that of low soil water potential. The effect was also looked for and found in Gossypium hirsutum L., Agathis robusta (C. Moore ex Muell.) Bailey, Eucalyptus microcarpa Maiden, Larrea divaricata Cav., the wilty flacca tomato mutant (Lycopersicon esculentum (L.) Mill.) and Scrophularia desertorum (Munz) Shaw. Two methods were used to distinguish between effects on stomatal conductance, which can lower assimilation by reducing CO2 availability, and effects on the photosynthetic capacity of the mesophyll. First, the response of assimilation to intercellular CO2 pressure (C i) was compared under conditions of high and low transpiration. Second, in addition to estimating C i using the usual Ohm's law analogy, C i was measured directly using the closed-loop technique of T.D. Sharkey, K. Imai, G.D. Farquhar and I.R. Cowan (1982, Plant Physiol, 60, 657–659). Transpiration stress responses of Xanthium strumarium were compared with soil drought effects. Both stresses reduced photosynthesis at high C i but not at low C i; transpiration stress increased the quantum requirement of photosynthesis. Transpiration stress could be induced in small sections of leaves. Total transpiration from the plant did not influence the photosynthetic capacity of a leaf kept under constant conditions, indicating that water deficits develop over small areas within the leaf. The effect of high transpiration on photosynthesis was reversed approximately half-way by returning the plants to low-transpiration conditions. This reversal occurred as fast as measurements could be made (5 min), but little further recovery was observed in subsequent hours.

Dynamic analysis of water stress of sunflower leaves by means of a thermal image processing system.

Abstract: This paper describes a method for measuring the temperatures of all regions of an intact leaf by using an infrared scanning thermometer at wavelengths between 8 and 14 micrometers combined with a digital image processing system. Pictures obtained every 2 minutes from leaves of sunflower ( Helianthus annuus L. cv Large Russian) plants subjected to increasing water stress showed that water deficit develops first at the margins of leaves, accompanied by stomatal closure and increase in temperature. Finally, the temperature of the entire leaf rises 3 to 5°C above that of nonstressed leaves. When transpiration resumed, it did so first at the leaf margins and these proceeded nonuniformly inward. The results of these experiments indicate that there are significant differences in the temperature and water status of different parts of a leaf. This makes it important to determine causes of such behavior and decide in what part of a leaf the temperature and water status should be measured. The thermo-imaging method can be useful in monitoring short term temperature changes occurring in leaves undergoing water, chilling, and other stresses.

Influence of Elevated Carbon Dioxide on Water Relations of Soybeans

Abstract: Soybean ( Glycine max L. Merrill cv `Bragg9) plants were grown in pots at six elevated atmospheric CO 2 concentrations and two watering regimes in open top field chambers to characterize leaf xylem potential, stomatal resistance and conductance, transpiration, and carbohydrate contents of the leaves in response to CO 2 enrichment and water stress conditions. Groups of plants at each CO 2 concentration were subjected to water stress by withholding irrigation for 4 days during the pod-filling stage. Under well watered conditions, the stomatal conductance of the plants decreased with increasing CO 2 concentration. Therefore, although leaf area per plant was greater in the high CO 2 treatments, the rate of water loss per plant decreased with CO 2 enrichment. After 4 days without irrigation, plants in lower CO 2 treatments showed greater leaf tissue damage, lower leaf water potential, and higher stomatal resistance than high CO 2 plants. Stomatal closure occurred at lower leaf water potentials for the low CO 2 grown plants than the high CO 2 grown plants. Significantly greater starch concentrations were found in leaves of high CO 2 plants, and the reductions in leaf starch and increases in soluble sugars due to water stress were greater for low CO 2 plants. The results showed that even though greater growth was observed at high atmospheric CO 2 concentrations, lower rates of water use delayed and, thereby, prevented the onset of severe water stress under conditions of low moisture availability.

Carbon relations and competition between woody species in a Central European hedgerow : II. Stomatal responses, water use, and hydraulic conductivity in the root/leaf pathway.

Abstract: Responses of stomata to humidity, light intensity, and leaf water status were investigated throughout a growth season under field conditions for five competing woody species dominating in various stages of a Central European hedgerow. Humidity sensitivity of stomata varied between species. Leaf conductance to H2O, g, as measured in steady-state humidity response curves under constant climatic conditions, remained on high summer levels in Prunus and Crataegus, and on a lower level in Acer, but fell from a high spring level to a low summer level in Ribes and Rubus. The effect of partial stomatal closure in dry air on CO2 uptake, A, varied seasonally and between species. Responses of stomata to light, measured by tracking gas exchange in the field but for controlled climatic conditions, were hyperbolic, similar to CO2 assimilation. The relationship between g and A at variable irradiance was always linear and depended on the species investigated and on the leaf age. In no case did stomata respond to short-term (hourly) changes in leaf water potential (up to 13.5 bar).A low leaf conductance level appeared to be correlated with low hydraulic conductivity in the plant, G, whereas high G in most cases coincided with high stomatal conductances. In Ribes and Rubus G and stomatal opening at maximum CO2 uptake varied in parallel during the season at high soil water content, suggesting a root/stomata interaction independent of plant water status. Water use efficiency (WUE, at certain leaf/air water vapour concentration differences), as determined from light and humidity responses of stomata and CO2 assimilation, was high in species of low G (Ribes, Rubus), and low in species of high G (Prunus, Crataegus). Surprisingly, species of low WUE optimized gas exchange in the sense of Cowan and Farquhar's (1977) theory, where stomata maximized CO2 uptake at a certain water loss within a certain period, while species of high WUE did not optimize in this sense. Other control mechanisms at the leaf level independent from optimization may be involved.It appeared that low G and loss of humidity sensitivity of stomata in Ribes limited distribution of this species outside the half shade of the hedgerow, as exposed plant parts desiccated on dry days. However, water use efficiency per se did not determine species-specific competitive ability, as it was highest in species of low competitive strength.

Transpiration- and Growth-Induced Water Potentials in Maize

Abstract: Recent evidence from leaves and stems indicates that gradients in water potential (ψw) necessary for water movement through growing tissues are larger than previously assumed. Because growth is sensitive to tissue ψw and the behavior of these gradients has not been investigated in transpiring plants, we examined the water status of all the growing and mature vegetative tissues of maize (Zea mays L.) during high and low rates of transpiration. The ψw measured in the mature regions of the plant responded primarily to transpiration, while the ψw in the growing regions was affected both by transpiration and growth. The transpiration-induced potentials of the mature tissue formed a gradient of decreasing ψw along the transpiration stream while the growth-induced potentials formed a gradient of decreasing ψw from the transpiration stream to the expanding cells in the growing tissue. The growth-induced gradient in ψw within the leaf remained fairly constant as the xylem ψw decreased during the day and was associated with a decreased osmotic potential (ψs) of the growing region (osmotic adjustment). The growth-induced gradient in ψw was not caused by excision of the tissue because intact maize stems exhibited a similar ψw. These observations support the concept that large gradients in ψw are required to maintain water flow to expanding cells within all the vegetative tissues and suggest that the maintenance of a favorable gradient in ψw for cell enlargement may be an important role for osmotic adjustment.

The effect of increasing elevation on leaf cuticle thickness and cuticular transpiration in balsam fir

Abstract: First-year needles and stems of Abies balsamea were collected at the end of the growing season along an elevational gradient on Mt. Moosilauke, NH. Tissue was sampled from the base (732 m), midslop...

Xylem water flow in a crack willow tree (Salix fragilis L.) in relation to diurnal changes of environment.

Abstract: The diurnal course of the xylem water flow in a solitary Salix fragilis L tree in a wet grassland was measured using the tissue heat-balance method There was considerable variation due to meteorological factors Maximum flow rate was 04 kg h-1 m-2 of crown projection area, or 59 kg h-1 kg-1 leaf dry weight The daily total was 24 kg m-2 day-1 or 36 kg kg-1 day-1 Water flow decreased immediately at the tree base and at the branches after start of rain, and in a branch, after cutting it off: the time constant of the system was 600–700 s in both cases The part of the crown oriented to the sun transpired up to ten times as much as the shaded part Over 70% of the total cross-sectional area of the conductive xylem vessels of the trunk was used by the transpiration flow The water content of the trunk tracked the diurnal changes of the xylem water flow rate with a short time-lag During the day, 1% of the trunk volume was temporarily exploited as water reserve, an amount equalling 3% of daily water loss The stereometric configuration of the crown significantly influenced its water loss During the summer period, about 33 mature (polycormic) trees per ha may drain 100% of water consumed by the present-day sedge-grass marsh

Effects of Humidity on Photosynthesis

Abstract: Mise en evidence chez Helianthus annuus et Chenopodium album d'une reduction des capacites de fixation du CO 2 pour de faibles taux d'humidite. Modifications experimentales des conditions de milieu: la capacite photosynthetique aux differentes pressions de vapeur d'eau est independante de la transpiration et de la conductance stomatique des feuilles

Comparison of Some Crop Water Stress Measurement Methods 1

Abstract: The measurement of crop water stress under field conditions is fraught with technical and logistical problems. Although leaf water potential has become a standard measurement it has spatial and temporal sampling limitations. In the current study, a rice (Oryza sativa L.) crop was used to compare eight measurements indicating crop water status, namely leaf water potential Ww)> stomatal resistance (rj, transpiration rate (T), net photosynthesis rate (PN). canopy temperature (Tc), canopy minus air temperature (Tc — TJ, crop water stress index (CWSI), and visual leaf rolling score (LRS). The eight measurements were compared across seven water stress levels created by a line source sprinkler irrigation system. The methods were compared for accuracy, sensitivity, sampling time, and the destructive-disruptive nature of the sampling procedure. Accuracy was estimated by comparison with ^w and by the interaction between water stress level and time of day. All methods except PN were significantly correlated with laf at the 1% level. However, Tc, Tc — T. and CWSI showed less interaction between water stress level and time of day when total variance was partitioned into its relative components; water stress level, time, and the interaction between water stress level and time of day. All methods, with the exception of PN, were equally sensitive to the water stress gradient when "goodness of fit" response functions across the water stress gradient were compared. The visual LRS was the most rapid while the canopy temperature-based measurements, T,., Tc — T. and CWSI, were nearly three times faster than the gas exchange techniques and about two and a half times faster than lo>{. Leaf water potential sampling was both destructive and disruptive to the crop plant community. The gas exchange methods were nondestructive but repetitive sampling was disruptive. Only the remotely sensed Tn T€ T., CWSI and LRS were neither destructive nor disruptive to the crop. The interpretive value of various measurements is discussed. The CWSI was found to be highly correlated with mean daily PN and represents a significant advancement in crop level detection and measurement of water stress. Additional index words: Rice, Oryza sativa L., Leaf water potential, Stomatal resistance, Transpiration rate, Net photosynthesis rate, Canopy temperature, Crop water stress index, Leaf rolling. T abiJity to quantify plant or crop water deficits is fundamental to research on the response of plant communities to water stress. Great progress toward this end was made in the 1960s and 1970s (Turner, 1981) particularly because of the development of the pressure chamber for measurement of leaf water potential. Because of its portability and lack of variation with temperature, the pressure chamber (Scholander et al., 1965) is well suited to field observations (Ritchie and Hinckley, 1975). However, due to the dynamic diurnal nature of individual leaf, tiller and plant water status, the limited number of observations per hour, and the logistical problems associated with large scale field sampling, adequate sampling of crop level water status with the pressure chamber continues to be problematical. In addition to the difficulty associated with sampling, leaf water potential soon lost its hoped for wide applicability as interpretation of tissue water potential was found to be dependent upon the history of the crop's water relations (Thomas et al., 1976; Hsiao et al., 1984) and growth stage (Morgan, 1977; Sinha et al., 1982). Plant water potential of a single crop species has been found to vary across locations or growth conditions with reference to the apparent degree of stress required to bring about a particular physiological response, i.e., stomatal closure (Boyer, 1976; Thomas et al., 1976). Thus, indirect rapid methods of indicating plant water status have been sought to aid in screening germplasm and measurement of physiological activity per se in large scale field experiments. In recent years leaf rolling and canopy temperature have been suggested as indirect methods of quantifying crop water stress. Leaf rolling scores of cereals has been linearly related to leaf water potential over a range of potentials (O'Toole and Moya, 1978; Jones, 1979; Begg, 1980). Likewise canopy or foliage-to-air temperature difference has been related to leaf water potential (Idso et al., 1981b). 1 Contribution from the Agronomy Dep., The Int. Rice Res. Inst., P.O. Box 933, Manila, Philippines. Received 12 Mar. 1984. 1 Agronomist, visiting scientist, former research assistant, postmasteral fellow, and statistician, Int. Rice Res. Inst. Present address of N.C.T.: CSIRO Dryland Crops and Soils Research Program, Private Bag, P.O., Wembley, WA 6014, Australia. 1122 CROP SCIENCE, VOL. 24, NOVEMBER-DECEMBER 1984 In a recent series of papers (Idso et al., 1981a; Jackson et al., 1981), canopy temperature measurement has been used in the development of a plant or "crop water stress index" (CWSI). Briefly stated, the concept holds that at any given vapor pressure deficit there is a theoretical upper and lower limit of the canopy-to-air temperature differential (To --Ta). When a measure of foliage or canopy temperature (To) is made it can be related to the ratio of actual evapotranspiration to potential evapotranspiration (ETa/ETp) by the prevailing conditions of vapor pressure deficit, and air temperature (Idso, 1981 a). From these inputs the CWSI can be calculated. The index is theoretically analogous to 1--(ETa/ETp) (Jackson et al., 1981). The physiological measurements used to detect water deficits at the leaf level are leaf ex.pansion, stomatal resistance and rates of transpiration and photosynthesis. Although frequently not linearly related to leaf water potential (Turner, 1981) these measurements represent processes of production that can be markedly affectedby water stress. In the present study we sought to compare various methods used in the detection and interpretation of crop water deficits. We sought not simply to compare the ability to detect stress but also the accuracy and speed of measurement, as these factors strongly affect the choice of methods for use in field conditions. MATERIALS AND METHODS The upland rice (Oryza sativa L.) cultivar IRAT 13 was grown in a silty clay loam soil (Typic Hapludoll) at a dryland site on the experimental farm of the International Rice Research Institute (IRRI), Los Bat3os, Philippines (14°13’ N Lat, 121°15’ E Long). Details of the site are given in Cruz and O’Toole (1984). On 3 Feb. 1983 seed was sown in rows --~25 cm apart at a rate of 10 g m-~ to give a final plant population of 35 plants per m of row, i.e., 140 plants m-*. Twenty eight kg ha-~ of N, 6 kg ha-~ of P and 15 kg ha-t of K were applied at seeding and a further 9 kg ha -1 of N and 2 kg ha-t of P were applied 42 days after sowing. The main plot was 16 m long, 2 m wide and had a 2 m border of rice surrounding it. For the first 42 days after sowing the plot was evenly irrigated by overhead sprinklers to keep the soil near field capacity. From 43 to 55 days after sowing, the plot was irrigated with a line source sprinkler system (Hanks et al., 1976; Puckridge and O’Toole, 1981) that applied a continuously decreasing amount of water along the length of the plot. Fifty days after seeding eight water stress measurement techniques were used to sample subplots across the irrigation gradient. Within the main plot seven subplots were positioned perpendicular to the line source irrigation system. Each supblot was approximately 2.3 m long and 2 m wide and received decreasing amounts of irrigation water (Table 1). Plants within these subplots were used for measurement of water stress. The rice rows were grown parallel to the line source irrigation system. To enhance specific measurement comparisons, techniques which required single plant or leaf sampling were confined to one row near the subplot center (Table 1). The following water stress detection measurements were used: leaf water potential (~b~e,f), leaf diffusive resistance (r,), transpiration rate (T), net photosynthesis rate (PN), canopy temperature (To), canopy minus air temperature (To --Ta), crop water stress index (CWSI), and visual leaf rolling score Table 1. Distance from the line source sprinkler of single rows selected for repetitive sampling within each subplot. Changes in amount of water applied, soil water content, and canopy light interception from 42 days after seeding, the last day of uniform irrigation, to 50 days after seeding when the eight stress measurement methods were compared. Data represent changes at four locations on the differential irrigation gradient.

Effect of Prolonged Exposure to NaCl on the Osmotic Pressure of Leaf Xylem Sap from Intact, Transpiring Barley Plants

Abstract: Xylem sap was collected from the leaf tip of intact transpiring barley plants at low and high external [NaCI], by applying pressure to roots of plants growing in sand containing nutrient solution plus 0, 100, 150 or 200 mol m-3 NaCl. Transpiration rates during the collection period were manipulated by varying the light and humidity level around the shoot. The osmotic pressure of the xylem sap (πx) from the oldest leaf of plants without added NaCl increased hyperbolically with decreasing transpiration rate, and rose to values 2-3 times that of the nutrient solution at very low rates of transpiration. A similar pattern applied to plants exposed to high external [NaCl] for several days, except that πx was about 30% higher than that of plants without NaCl, for a given rate of transpiration. However, after a week or more at high external [NaCl], πx of the oldest leaf increased dramatically, and the leaf died a day or two after πx increased by 200-300 kPa. Growth of the youngest leaf was independent of these changes. Simultaneous collections of πx from leaves of all ages showed that πx was lowest in the youngest leaf. We suggest that ions in the transpiration stream build up in the cell walls when ion contents of the cells reach a maximum, and net uptake rates become zero. Such an accumulation would cause loss of turgor then dehydration of cells, and may be the primary cause of death of the old leaves.

Light acclimation in citrus leaves. II. CO2 Assimilation and light, water, and nitrogen use efficiency

Abstract: ~-NetCQ2 assimilation (A) rates of 'Duncan' grapefruit (Citrus paradisi Macr.) and 'Pineapple' orange (c. sinensis L.) seedlingS grown under 3 different photosynthetic photon nux densities (PPFD), were measured in an open gas exchange system under controlled environmental conditions. Apparent quantum yield (If), mesophyll conductance to CO2 (G ), leaf conductance to H20 vapor (G,), transpiration (E) and water use efficiency (WUE) also were m examined. Leaves of both species grown under high PPFD (full sunlight) had the greatest maximum rates of A, but the low PPFD (90% shade) leaves had the highest e. The WUE of low PPFD grapefruit leaves was less than that of the high PPFD leaves but increased within 2 weeks after being moved into full sunlight. Transferring seedlings from low to high PPFD decreased e of grapefruit but not of orange leaves. Changes in A were more strongly correlated to Gm than to G,. Carbon dioxide assimilation rate was positively correlated to total leaf nitrogen content. Citrus leaf photosynthetic characteristics and resources use efficiency not only acclimate to the light regimes under which they expand and mature. but leaves are capable of acclimating to new light regimes, even after full maturation. Bjorkman (2) recently has reviewed how leaves acclimate to from interior positions (15), there is no report of citrus nitrogen changes in the radiation environment to maximize photosyn­ use efficiency. Such information can provide insight regarding thetic efficiency under a particular set of conditions. Leaves resource partitioning (3. 9, 23) during acclimation in tree can­ growing in full sunlight are not only thicker with more densely opy microclimates. packed mesophyll (4, 19) than leaves growing in shade. but also Shade leaves typically have increased quantum yield (jII), an have higher light-saturated CO2 assimilation rates (A) (I, 7, ll, estimate of quantum use efficiency during CO2 fIxation, as shown 20, 22). Furthermore, sun leaves have higher nitrogen use ef­ by greater initial slopes in the quantum yield region of the A ficiency than shade leaves. as estimated by expressing A on a vs. photosynthetic photon flux density (PPFD) response curve total leaf nitrogen (N) basis. NN (5, 6). Although citrus leaves (I, 24). High jII is, of course, an important advantage in shaded from exterior canopy positions have higher N contents than leaves environments (27). Since the majority of leaves in a tree canopy are growing under reduced light, higher 0 of shade leaves allows them to capitalize on existing light microclimates (18). Al­ though the plasticity of 0 in sun and shade leaves has been Received for publication :2 Apr. 198... Florida Agricultural Experiment Stalions Journal Series No. 5511. The assistance of M.L. Smilh. Jr.. is gratefully ac­ described for many crops (20, 24) native shrubs and trees (1, knowledged. The cost of publishing lhis paper was defrayed in part by the 8, 20), there is no available information on how jII varies with payment of page charges. Under postal regulations. this paper therefore must the PPFD environment of citrus trees. This relationship can be hereby marked advertisement solely to indicate this fact J. Amer. Soc. Hort. Sci. 109(6):812-817. 1984'. 812 " . for horticultural purposes. Furthermore, cultivated species of citrus have long lived leaves (up to 3 years) which are not only exposed to gradually changing light environments during normal canopy development. but also can be subjected to immediate increases in light exposure after hedging. Khairi and Hall (14) described how changes in nonstomatal aspects of A enabled citrus leaves to adapt to changing light conditions. The adaptive mechanisms that deal with anatomicaL mOlJ)hological, and physiological changes (4. 13) that can lead \0 to efficient light, water, and nitrogen resource utilization by =' citrus leaves during changing microclimates are not fully under­ ;£ stood. The pUlJ)ose of this study was to determine the photo­ c0 .~ synthetic characteristics of sunand shade-acclimated citrus leaves, s:J and to relate these characteristics to the time course of changes o in resource use efficiency during light acclimation. Materials and Methods ~ Thirty seedlings each of 2 species. Citrus paradisi Macf. ~C .Duncan grapefruit' and C. sinensis L. .Pineapple orange', were ~ grown either outdoors (high PPFD. maximum PPFD = 2300 jJ.mol s t m -2), under 50% shade (intermediate PPFD) or 90% fJ shade (low PPFD). When seedlings were 9-months-old, mature .g leaves within the same age group were tagged for study (26). Half the seedlings of both species in the low and intermediate PPFD were transferred to high PPFD to simulate the contrasting O~ light environment that occurs after hedging. In addition, naIf J-:>the high PPFD seedlings were transferred into 50% shade (in­ q) termediate PPFD) to simulate changes in the light environment ~f that occur during the course of normal canopy development. s:: Thus. there were a total of 6 PPFD treatments: the 3 original :3 growth conditions (high. intermediate, and low PPFD) and 3 transfer treatments, hereafter referred to as low to high, inter­ mediate to high, and high to intermediate. Photosynth~tic CO2 ;sslITrilation and transpiration rates (E) were calculated from CO 2 and H20 vapor fluxes (12), using the tagged leaves in an open gas exchange system. The leaf con­ ductance to H20 vapor (G I ) was calculated from E; G 1/1.56 was used to calculate the leaf conductance to CO~ and the CO, compensation point value of 45 jJ.l I I (16) wa's used as th~ minimum internal CO2 concentration m calculatmg mesophyll conductance to CO2 (Gm ) (12). The gas exchange system con­ tained an Anarad dual gas analyzer that simultaneously mea­ sured CO2 and water vapor in the measurement air stream. Leaves were enclosed in a well-stirred temperature-controlled leaf chamber (25) where boundary layer conductances exceeded 10 cm s I. The chamber was illuminated with a 500 W quartz· iodine lamp. All A. except PPFD response curves, were eval· uated under constant environmental conditions of saturating PPFD (600-800 jJ.mol s -1 m -2), ambient air containing 350 ± 20 jJ.1 I I CO2, leaf temperature of 26 ± 1°C and absolute humidity difference from leaf to air of 6 ± 2 jJ.g cm 3. Preliminary measurements indicated that these standard conditions were nonlimiting and there were no differences in temperature optima for A of leaves growing in the different PPFD conditions. Four seedlings, from each of the high and low PPFD growth conditions, were used to characterize PPFD saturation responses and apparent quantum yield (I. 24), while all other measure­ ment conditions were held constant. The dark respiration of one leaf on each seedling was determined and the linear increase in A was evaluated for at least 4 levels of PPFD, attenuated using cheese cloth filters, between 0 and 100 jJ.mol s t m 2. Apparent J. Amer. Soc. Hort. Sci. 109(6):817.-817. 1984. tion of th~ regression of A vs. incident PP·FD. PPFD satura~i~n responses then were characterized by increasing PPFD in non­ consecutive steps (using filters) to levels where there were no more increases in A. Changes in net gas exchange characteristics that occur during light acclimation were evaluated in a 2nd group of 12 seedlings. -+ from each original PPFD. by measuring A and E of I leaf~/ each under the standard nonlimiting conditions and 2, 4. or 6 wec:ks after transfelTing seedlings into new PPFD. Water use efficiency (WUE, mmol CO2 influx/mol H:O efflux) was cal­ culated for all PPFD treatments. CO2 assimilation rates, E. and WUE of low PPFD leaves were evaluated in a 3rd group of 3 seedlings 0.3,10. 17, and 24 days after transferring to high PPFD in an effort to elucidate more fully the time course of light acclimation in low to high PPFD leaves. Data from these 2 groups of seedlings were used to characterize the relative Importance of G I and Gm in determining maximum A during growth under widely different PPFD conditions and during ac­ climation to changing PPFD. In addition. A was tested for cor­ relation with leaf N and chlorophyll content described previously (26), and the amount of CO2 fixed per unit leaf total N (AN) also was tested for correlation with N using linear regression analysis.

The Effects of Texture on the Resistance to Water Movement within the Rhizosphere

Abstract: A growth chamber study was designed to investigate the relative importance of the resistances involved in water movement within the rhizosphere. The components considered are the soil, the soil-root interface, and the root itself. The plant species used was sunflower (Helianthus annus L.), grown in three soils of different texture. After several weeks of growth all water application was stopped and the soil allowed to dry by transpiration alone. Relevant destructive measurements were carried out during the course of the drying period, one pot being sacrificed for each set. Measurements included transpiration rate, soil and xylem water potential, rooting density, and hydraulic properties of the soils. As the soil dried, total resistance to water movement was found to increase most rapidly in the coarsest textured soil and least rapidly in the finest textured soil. A steady state model was used to partition the total resistance between the various resistive components. This enabled the root plus interfacial resistance to be quantified in terms of soil texture and water content.

The role of capacitance in the water balance of Andean giant rosette species

Abstract: Pith water storage capacity and its role in plant-water relations were studied in seven giant rosette species of the genus Espeletia from the Venezuelan Andes. Readily available water from the pith was calculated to be capable of sustaining mean transpiration for up to 2.5 h. The relative importance of water stored in the pith, however, differed among species. The species that grow in the higher and colder environments tended to have a greater capacitance than the species that grow in the lower and less extreme environments. The pith volume per unit leaf area (PV/LA) was found to be a good indicator of the relative water storage capacity of the adult individuals of each species. Diurnal fluctuations in leaf water potential were not as pronounced in the species with higher PV/LA values. The species-specific PV/LA was highly correlated with the leaf turgor loss point and with the total resistance to water flow from soil to leaves. These results suggested that species-specific capacitance in the genus Espeletia is a response to temperature-limited soil water availability and that cold tropical environments with frequent subfreezing temperatures tend to select for high water storage capacity in giant rosette plants.