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S.M. Lutfor Rahman, Eiji Nawata, and Tetsuo Sakuratani

Effects of water stress at different plant ages on SOD activities were studied in two tomato cultivars. Water stress treatment decreased the leaf water potential in all stages, but reduction of leaf water potential was more rapid and pronounced in KF than TM at all DSLs (days of seedlings). After withdrawal of water stress treatment, stressed plants of TM increased leaf water potential to the values of control level in all DSLs, but in KF, leaf water potential of stressed plants were much lower than that of control plants. Effects of water stress on relative water content (RWC) of leaves at 20 DSL showed a similar tendency to that on leaf water potential. The SOD activities in both cultivars showed significant increase by water stress treatment at all DSLs, but the increase of SOD by water stress was larger in TM than in KF. This tendency was observed at all DSLs. The results may indicate that SOD activities play an important role in drought tolerance of tomato at various plant ages and suggest a possible use of SOD activities as a criterion for tomato drought tolerance.

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Greg Litus and James Klett

During Summer 2005, green ash (Fraxinus pennsylvanica `Patmore') trees planted at the Colorado State University Agricultural Research Development and Education Center in 1996 were exposed to simulated drought by restricting irrigation for 33 to 41 days. During this period, predawn leaf water potentials in drought-stressed trees progressively dropped to a low of –2.04 MPa, while the control plot was maintained with full irrigation such that predawn leaf water potentials did not fall below –0.5 MPa. On 24 Aug. 2005, 31 days into this drought cycle, mid-day leaf water potentials and stomatal conductance were measured at –3.0 MPa and 22.63 mmol·m-2·s-1, respectively. Measurements in control trees collected at about the same time were –2.0 MPa and 169 mmol·m-2·s-1. The dramatic reduction in stomatal conductance in the drought-stressed trees began at about 10:30 a.m. and continued into the evening. Once irrigation was resumed, drought stressed trees rebounded from depressed predawn leaf water potentials and mid-day leaf water potentials and stomatal conductance and reached levels similar to control trees in 2 to 5 days. Stem flow gauges indicate that, during this period, fully hydrated control trees used about 250 liters/day.

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Mongi Zekri and Lawrence R. Parsons

Abstract

Water relations responses of 21-year-old grapefruit trees (Citrus paradisi Macf.) irrigated by three types of irrigation systems were compared. Drip, undertree microsprinkler, and overhead sprinkler with application levels of 150 and 450 mm of water per year were compared. Leaf water potential, stomatal conductance, and soil water status were measured under field conditions on a deep, well-drained sandy soil in central Florida. In the early part of a dry spring period, there were no differences in midday or early morning leaf water potential, but, by the end of this period, significant differences in leaf water potential were found among all three irrigation treatments. Highest leaf water potential and stomatal conductance values were maintained in the overhead sprinkler blocks. No midday stomatal closure was observed under the conditions of this study. Relationships among diurnal leaf water potentials, vapor pressure deficits, and stomatal conductance showed hysteresis; this affected the correlations among these factors. Greater water stress occurred in trees irrigated with drip than in trees irrigated with overhead sprinkler systems, but responses to microsprinklers were generally intermediate between the overhead sprinkler and the drip treatments. In an area with high rainfall and sandy soils, increased irrigation coverage can reduce leaf water stress.

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Shinsuke Agehara and Daniel I. Leskovar

in 1 m m ABA solution had higher stomatal resistance and leaf water potential than untreated seedlings after transplanting. When irrigation was withheld for 15 h after transplanting to impose water stress, the improved water status by ABA resulted in

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Majken Pagter, Karen K. Petersen, Fulai Liu, and Christian R. Jensen

between 1100 and 1400 hr , and readings were logged every 30 s until stable values of g s were reached. Midday leaf water potential (ψ l ) was measured with a pressure chamber (SKPM 1400; Skye Instruments Ltd, Llandrindod Wells, U.K.). Because of

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Krishna S. Nemali and Marc W van Iersel

400 μmol·mol −1 using a leaf photosynthesis system (CIRAS-1; PP Systems, Amesbury, MA) equipped with an LED light unit. Midday leaf water potential was measured twice during the study using leaf-cutter thermocouple psychrometers (Model 76; J

Open access

Janet C. Henderson and David L. Hensley

Abstract

Studies were conducted to determine the effect of a hydrophilic gel used as a medium amendment or root dip on plant response to moisture deficits. Tomato (Lycopersicon esculentum Mill.) seedling roots were dipped in water or hydrophilic gel solution (7.4 g·liter−1) and planted in sand or a 1 sand : 1 very fine sandy loam mixture (v/v). Seedlings were also planted in the same medium amended with 3 kg·m−3 hydrophilic gel. Leaf water potentials and stomatal resistances were determined at various times after a final irrigation. Significantly greater leaf water potentials occurred in new transplants in sand amended with gel than in control or root-dipped plants. No effect on either leaf water potential or stomatal resistance was apparent for any treatment in finer textured soils. There were no treatment effects on plants exposed to the same gel treatment and allowed to establish for 2 weeks prior to withholding water.

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Mark Rieger and Jeff W. Daniell

Abstract

Leaf water relations and soil-to-leaf resistance were studied in 3-month-old pecan [Carya illinoenis (Wangenh.) C. Koch] seedlings as soil dried progressively to minimum water potentials of −0.3, −0.6, and −1.1 MPa in three separate tests. Leaf conductance, transpiration, and predawn leaf water potential declined with increasing soil water deficits, and only predawn leaf water potential fully returned to pre-stress levels after rewatering. Reduced levels of leaf conductance following water stress were apparently caused by internal factors other than leaf water potential. Leaf conductance of well-watered seedlings decreased logarithmically and with increasing leaf-to-air vapor pressure gradient. Soil-to-leaf resistance to water flow varied diurnally and generally increased following water stress at minimum soil water potentials of −0.6 and −1.1 MPa. Osmotic adjustment and changes in the distribution of water between the apoplast and symplast in leaves did not occur in response to soil water potentials of −0.6 MPa.

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D. C. Elfving and M. R. Kaufmann

Abstract

Leaf water potential of ‘Valencia’ orange trees was influenced by soil water availability, soil temp, and vapor pressure deficit (VPD) of the atmosphere. Low soil water availability prevented full nighttime recovery from water stress. Soil temp below 15°C decreased root permeability for water absorption and contributed to low daytime water potentials (high water stress), but at night when transpiration was minimal, complete recovery occurred. VPD effects on leaf water potential were also most noticeable during the day. Stage II fruits exhibited a recurring diurnal size change during daylight hr. This reversible shrinkage was correlated with simultaneous changes in fruit and leaf water potential and fruit diffusion resistance. However, the irreversible nighttime growth rate could not be correlated with day or night water potentials or air temp, or with night VPD. While differences between fruit and leaf water potentials occurred, no evidence presented here or elsewhere permits an unequivocal statement that transpiring fruits can behave as a midday reservoir of water for leaves.

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Cristos Xiloyannis, K. Uriu, and G. C. Martin

Abstract

Water potential, diffusive resistance, and abscisic acid (ABA) were measured at 10-12 day intervals from May to October in leaves from irrigated and non-irrigated peach (Prunus persica L. cv. Fay Elberta) trees, and measurements were taken at intervals from sunrise to sunset on September 8. Leaf water potential, before sunrise, was between −5 and −8 bars in irrigated trees during the entire season whether drip irrigated at 100% evapotranspiration (ET) or 50% ET. Non-irrigated trees showed a decrease in pre-dawn leaf water potential with time, following a pattern similar to that of decreasing soil moisture. Leaf water potential values taken during the afternoon were not associated with soil moisture and did not reflect the stressed condition of the trees. In non-irrigated trees stomatal resistance at mid-day increased rapidly after mid-summer as leaf water potential decreased. ABA concentration in leaves from irrigated trees ranged from 30 to 80 ng/g fresh wt during the entire season. In non-irrigated trees the ABA concentration increased sharply after mid-summer; this was associated with an increase in leaf diffusive resistance and a decrease in leaf water potential. Diurnal variations in leaf water potential were associated with changes in soil moisture, air temperature, relative humidity, and stomatal resistance. Leaf diffusive resistances were similar for all treatments until 1100 hr after which a notable increase occurred with increasing stress, ultimately leading to stomatal closure. ABA concentrations in leaves from irrigated and non-irrigated trees increased as leaf diffusive resistance increased; however in stressed trees, high levels of ABA in the morning were not associated with closed stomata.