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Lenore J. Nash and William R. Graves

Responses of five bottomland tree taxa to drought and flooding were studied to identify those adapted to urban environments. During one experiment, containerized `Franksred' red maple [Acer rubrum L. `Franksred' (trademark = Red Sunset)], sweetbay magnolia (Magnolia virginiana L.), black tupelo (Nyssa sylvatica Marsh.), bald cypress [Taxodium distichum (L.) Rich.], and pawpaw [Asimina triloba (L.) Dunal.] were treated with various irrigation regimes for up to 118 days. Net assimilation rate (NAR) and relative growth rate (RGR) were reduced more by flooding than by drought for plants of all taxa, except pawpaw, which showed similar NAR and RGR during flooding and drought. Only sweetbay magnolia and bald cypress maintained positive NAR and RGR during flooding, and sweetbay magnolia was the only taxon that did not produce significantly less leaf surface area, shoot dry mass, and root dry mass during flooding and drought. Apparent morphological mechanisms of stress resistance included an increase in specific mass of leaves (mg·cm-2) during drought for red maple and bald cypress and a 385% increase in the root: shoot mass ratio for droughted plants of pawpaw. Leaf water relations of drought- and flood-stressed `Franksred' red maple and sweetbay magnolia were determined in a second experiment. Predawn and mid-day leaf water potential (ψ) decreased with decreasing root-zone matric potential for both taxa, and transpiration rate was reduced by drought and flooding. Pressure-volume analysis showed that leaves of `Franksred' red maple responded to drought by shifting symplastic water to the apoplast. Leaves of drought-stressed sweetbay magnolia adjusted osmotically by reducing osmotic potential (ψπ) at full turgor by 0.26 MPa. Our results suggest that sweetbay magnolia and bald cypress will perform well at urban planting sites where episodes of drought and flooding regularly occur.

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Douglas S. Chapman and Robert M. Augé

Understanding physiological drought resistance mechanisms in ornamentals may help growers and landscapers minimize plant water stress after wholesale production. We characterized the drought resistance of four potted, native, ornamental perennials: purple coneflower [Echinacea purpurea (L.) Moench], orange coneflower [Rudbeckia fulgida var. Sullivantii (Beadle & Boynt.) Cronq.], beebalm (Monarda didyma L.), and swamp sunflower (Helianthus angustifolius L.). We measured a) stomatal conductance of leaves of drying plants, b) lethal water potential and relative water content, and c) leaf osmotic adjustment during the lethal drying period. Maintenance of stomatal opening as leaves dry, low lethal water status values, and ability to osmotically adjust indicate relative drought tolerance, with the reverse indicating drought avoidance. Echinacea purpurea had low leaf water potential (ψL) and relative water content (RWC) at stomatal closure and low lethal ψL and RWC, results indicating high dehydration tolerance, relative to the other three species. Rudbeckia fulgida var. Sullivantii had a similar low ψL at stomatal closure and low lethal ψL and displayed relatively large osmotic adjustment. Monarda didyma had the highest ψL and RWC at stomatal closure and an intermediate lethal ψL, yet displayed a relatively large osmotic adjustment. Helianthus angustifolius became desiccated more rapidly than the other species, despite having a high ψL at stomatal closure; it had a high lethal ψL and displayed very little osmotic adjustment, results indicating relatively low dehydration tolerance. Despite differences in stomatal sensitivity, dehydration tolerance, and osmotic adjustment, all four perennials fall predominantly in the drought-avoidance category, relative to the dehydration tolerance previously reported for a wide range of plant species.

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Hava F. Rapoport, Giacomo Costagli, and Riccardo Gucci

Water deficit was applied between 4 and 9 weeks after full bloom by withholding irrigation from 3-year-old Olea europaea L. (`Leccino') plants grown in 2 L containers in a greenhouse. At 6, 8, and 22 weeks after full bloom (AFB), fruit were sampled for fresh weight and volume determinations, and then fixed for anatomical studies. Structural observations and measurements were performed on transverse sections at the point of widest fruit diameter using image analysis. Water deficit applied between 4 and 9 weeks AFB produced a significant decrease in predawn leaf water potential, which reached minimum values of -3.1 MPa. The applied water deficit reduced fruit fresh weight and volume at 8 and 22 weeks AFB. Fruit transverse area of the water deficit treatment was 50%, 33%, and 70% of the irrigated one at the 6-, 8-, and 22-week sampling dates, respectively. Mesocarp growth occurred for both irrigated and water deficit plants between 8 and 22 weeks AFB. At 22 weeks AFB differences between treatments were significant for mesocarp transverse area, but not for endocarp area. Mesocarp cell size, indicated by area, was significantly different between treatments at 8 and 22 weeks AFB. However, the mesocarp cell number was similar for both treatments at all times, and most mesocarp cells were produced by 6 weeks AFB. The growth of endocarp area showed the greatest shift in timing in response to the early water deficit. Ninety percent of endocarp growth had occurred by 8 weeks AFB in the irrigated treatment, but only 40% when the deficit irrigation treatment was imposed.

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Sebastiano Delfine, Francesco Loreto, and Arturo Alvino

Physiological characteristics, growth, and biomass production of rainfed and irrigated bell pepper [Capsicum annuum L. var. anuum (Grossum Group) `Quadrato d'Asti'] plants were measured in the semiarid conditions of a Mediterranean summer to determine if drought stress effects are transient and do not affect plant growth and crop yield or are persistent and adversely affect plant growth and crop yield. A low midday leaf water potential indicated the occurrence of transient drought stress episodes in rainfed plants during the first 2 months of the study. Later on, predawn water potential also increased, indicating a persistent drought stress condition despite the occurrence of some rainfall. Photosynthesis was reduced when stress conditions developed, but the reduction was transient and limited to the central part of the day during the first 2 months. As plants aged, however, the impact of drought stress on photosynthesis was not relieved during the overnight recovery period. Stomatal conductance was reduced both during transient and permanent stress conditions while CO2 transfer conductance (i.e., conductance to CO2 inside the leaf) was only reduced when photosynthesis inhibition was unrecoverable. However, chloroplast CO2 concentration was higher in rainfed than in irrigated leaves indicating that CO2 availability was not limiting photosynthesis. Nonphotochemical quenching of fluorescence increased significantly in rainfed leaves exposed to permanent stress indicating the likely impairment of ATP synthesis. Transient inhibition of photosynthesis did not significantly affect leaf area index and biomass production, but growth was significantly reduced when photosynthesis was permanently inhibited. Fruit dry weight was even higher in rainfed plants compared to irrigated plants until drought stress and photosynthesis reduction became permanent. It is suggested that bell pepper growth without supplemental irrigation over the first part of the vegetative cycle does not impair plant growth and may even be useful to improve yield of early fruit.

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R. Romero-Aranda, T. Soria, and J. Cuartero

High salinity levels in irrigation water available in Mediterranean coastal areas induce a significant loss of yield in greenhouse tomato crops. This loss increases during the spring-summer growing season when high irradiance, temperature, and low humidity occur within greenhouses. This study determined whether salt-induced yield losses could be alleviated by increasing humidity by misting the greenhouse atmosphere. Plants of `Daniela' tomato (Lycopersicon esculentum Mill.), were irrigated with 0 or 50 mm NaCl added to the nutrient solution and grown under natural greenhouse conditions or under applications of fine mist every 8 min during the day. During midday hours, misting reduced greenhouse air vapor pressure deficit 1.0 to 1.5 kPa and reduced greenhouse air temperature 5 to 7-°C. Mist reduced root water uptake from the medium by 40% in nonsalinized plants and by 15% in saline conditions. Foliar concentration of Na was lower in misted-salinized plants than in nonmisted salinized plants. Less negative leaf water potential and higher leaf turgor were recorded with mist at midday, in both salinized and nonsalinized plants. Midday stomatal conductances and net CO2 assimilation rates of salinized-misted plants were 3 and 4 times higher, respectively, than those recorded in salinized-nonmisted plants. Misted plants increased instantaneous water use efficiency 84% to 100%, as estimated from the ratio of net CO2 assimilation to transpiration. Nonsalinized plants grown with mist increased total leaf area by 38%, dry matter by 10%, and yield by 18% over nonmisted plants. Salinized plants grown with mist increased total plant leaf area by 50%, dry matter by 80%, and yield by 100%. Greenhouse misting resulted in a saving of total water input of 31 L/plant under nonsaline conditions and in greater yields and fruit size regardless of salinity. Results suggest that greenhouse misting, during the Mediterranean spring-summer growing season, improves tomato crop productivity both under nonsaline and saline growth conditions.

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David A. Goldhamer, Elias Fereres, Merce Mata, Joan Girona, and Moshe Cohen

To characterize tree responses to water deficits in shallow and deep rooted conditions, parameters developed using daily oscillations from continuously measured soil water content and trunk diameter were compared with traditional discrete monitoring of soil and plant water status in lysimeter and field-grown peach trees [Prunus persica (L.) Batsch `O'Henry']. Evaluation occurred during the imposition of deficit irrigation for 21 days followed by full irrigation for 17 days. The maximum daily available soil water content fluctuations (MXAWCF) taken at any of the four monitored root zone depths responded most rapidly to the deficit irrigation. The depth of the MXAWCF increased with time during the deficit irrigation. Differences relative to a fully irrigated control were greater in the lysimeter than the field-grown trees. Minimum daily trunk diameter (MNTD) and maximum daily trunk shrinkage (MDS) responded sooner than midday stem water potential (stem Ψ), predawn or midday leaf water potential (predawn leaf Ψ and leaf Ψ), or photosynthesis (A). Parameters based on trunk diameter monitoring, including maximum daily trunk diameter (MXTD), correlated well with established physiological parameters of tree water status. Statistical analysis of the differences in the measured parameters relative to fully irrigated trees during the first 10 days of deficit irrigation ranked the sensitivity of the parameters in the lysimeter as MXAWCF > MNTD > MDS > MXTD > stem Ψ = A = predawn leaf Ψ = leaf Ψ. Equivalent analysis with the field-grown trees ranked the sensitivity of the parameters as MXAWCF > MNTD > MDS > stem Ψ = leaf Ψ = MXTD = predawn leaf Ψ > A. Following a return to full irrigation in the lysimeter, MDS and all the discrete measurements except A quickly returned to predeficit irrigation levels. Tree recovery in the field-grown trees was slower and incomplete due to inadequate filling of the root zone. Fruit size was significantly reduced in the lysimeter while being minimally affected in the field-grown trees. Parameters only available from continuous monitoring hold promise for improving the precision of irrigation decision-making over the use of discrete measurements.

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James A. Zwack, William R. Graves, and Alden M. Townsend

Little is known about drought stress resistance of Freeman maples (Acer ×freemanii E. Murray), which are hybrids of red maples (A. rubrum L.) and silver maples (A. saccharinum L.). The objective of our study was to measure plant growth and leaf water relations of `D.T.R. 102' (Autumn Fantasy), `Celzam' (Celebration), and `Marmo' Freeman maples subjected to drought. Plants grown from rooted cuttings were subjected to four consecutive cycles of water deficit followed by irrigation to container capacity. Average stomatal conductance at container capacity for all cultivars was 255 mmol·s-1·m-2 in the first drought cycle and 43 mmol·s-1·m-2 during the fourth drought cycle. Predawn and midmorning leaf water potentials of droughted plants at the end of the fourth drought cycle were 1.16 and 0.82 MPa more negative than respective values for control plants. Osmotic potential of leaves at full turgor was -1.05 MPa for controls and -1.29 MPa for droughted plants, indicating an osmotic adjustment of 0.24 MPa. Root and shoot dry mass and leaf area were reduced similarly by drought for all cultivars, while Celebration exhibited the least stem elongation. `Marmo' treated with drought had the lowest root-to-shoot ratio and the greatest ratio of leaf surface area to root dry mass. Autumn Fantasy had the lowest ratio of leaf area to stem xylem diameter. Specific leaf mass of drought-stressed Autumn Fantasy was 1.89 mg·cm-2 greater than that of corresponding controls, whereas specific masses of Celebration and `Marmo' leaves were not affected by drought. Leaf thickness was similar among cultivars, but leaves of droughted plants were 9.6 μm thicker than leaves of controls. This initial characterization of responses to drought illustrates variation among Freeman maples and suggests that breeding and selection programs might produce superior genotypes for water-deficient sites in the landscape.

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Ursula K. Schuch, Leslie H. Fuchigami, and Mike A. Nagao

The effects of water stress and GA, on breaking dormancy of flower buds of coffee (Coffea arabica L.) were investigated. In the first experiment, water was withheld until the trees reached leaf water potentials (WP) of -1.20, - 1.75, -2.65, or -3.50 MPa. Water potential, ethylene production, and ion leakage of flower buds and leaf disks were examined from release from water stress until anthesis. Trees that had experienced leaf WP of less than - 2.65 MPa, and flower bud WP of about - 4.0 MPa flowered within 9 days after irrigation. In flower buds where dormancy had been broken with water stress, ethylene production was low compared to dormant buds and flowers at anthesis. In the second experiment, O, 50, 100, or 200 mg GA3/liter was painted on branches of nonstressed trees. In experiment three, water was withheld until plants reached leaf WP of -0.6, -1.3, - 2.1, or - 3.0 MPa, then two branches per tree were painted with O, 50, and 100 mg GA3/liter. Gibberellic acid partially compensated for insufficient water stress to initiate flower opening. Ethylene evolution of flower buds was affected by water stress but not by GA3 treatment. Severe water stress treatments and GA, treatment (200 mg·liter-1) increased ethylene evolution of leaf disks. Ion leakage of flower buds and leaf disks was increased by severe water stress. Ion leakage of flower buds was highest at anthesis. After water stress, dormant and nondormant flower buds at the 4-mm stage could be distinguished based on their ethylene evolution. Chemical name used: gibberellic acid (GA3).

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Baolin Zhang and Douglas D. Archbold

The effect of water deficit stress on the foliar solute concentration of Fragaria chiloensis Duch. cv. `BSP14' (FC) and F. virginiana Duch. cv. `NCC85-13V (FV) was studied to assess solute contribution to osmotic adjustment. Plants were stressed for three wilting cycles by withholding water until incipient wilting, followed by a recovery period. The length of each cycle for the FC selection was J-fold that for the FV selection. Wilting of FC plants occurred at a leaf water potential (ψ) 1.1 MPa lower, a leaf osmotic potential (ψπ) 0.45 MPa lower, and a relative water content (RWC) 14% lower than for FV plants. Leaf ψπ at full turgor (ψπ 100) was reduced in the FC selection from 0.11 to 0.27 MPa but not in the FV selection. Water deficit stress increased the total foliar soluble carbohydrate (TSC) concentration from 1.4- to 2.4-fold for FC during each cycle. Glucose and fructose were the primary carbohydrates, comprising >50% of the TSC. Leaf starch concentration decreased appreciably to 4% to 6% of nonstressed (control) levels for FC during each cycle. In the FC selection, the TSC: starch ratio was considerably higher, 30- to 50-fold, in stressed than in control plants. No consistent changes in solute or starch concentration were observed in the FV selection. Total free amino acid concentration increased from 1.8- to 2.7-fold in FC plants in response to stress. Proline accumulation was not detected. The solutes, TSC and amino acids, accounted for <40% of the measured ψπ 100 in control plants, although the average contribution of these solutes to the measured leaf ψπ 100 increased to 40% or more in stressed FC plants. However, the stress-induced increase in solute concentration fully accounted for the reduction of leaf ψπ 100 in the FC selection plants.

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P.C. Andersen, J.G. Norcini, and G.W. Knox

Leaf physiology and plant growth of Rhododendron × `Pink Ruffles' were compared under conditions of 100% sun and under polyethylene shadecloth with specifications of 69%, 47%, and 29% light transmittance. Net CO2 assimilation (A) and stomatal conductance to water vapor (gs) were often reduced for plants in the 100% sun regime, although few differences existed among the 69%, 47%, and 29% sun treatments. Stomatal conductance was very sensitive to leaf to air vapor pressure deficits (VPD), as evidenced by an 85% increase in gs with a decrease in VPD from 3.2 to 2.2 kPa. Light response curves established for plants after 54 days of exposure to 100% and 29% sun were similar, although A was consistently higher at all levels of photosynthetic photon flux for plants in the 29% sun regime. Maximum A was ≈5 and 6 μmol·m-2·s-1 for 100% and 29% sun-grown plants, respectively; light saturation occurred at ≈ 800 μmol·m-2·s-1 Midday relative leaf water content and leaf water potential were not affected by sun regime. The plant growth index decreased with increasing light level. Leaf, stem, and root dry weights; total leaf number and dry weight; total and individual leaf area; dry weight per leaf; and leaf chlorophyll concentration were reduced in 100% sun, yet few differences existed among the 69%, 47%, and 29% sun treatments. Shoot: root ratio and specific leaf weight were proportional to light level. Plants grown in the 100% sun regime were chlorotic and dwarfed, and plants in 29% sun were not sufficiently compact. One year after transplanting to the field under 100% sun, plants of all treatments were chlorotic and failed to grow.