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Carol M. Foster and William R. Graves

Desiccation damage in ornamental plant species is of particular concern to the nursery and landscape industry. Species in two genera, Acer and Alnus, display fundamental differences in how drought affects leaves. The same soil moisture content that causes foliar desiccation and abscission in Alnus maritima (Marsh.) Nutt. (seaside alder) causes neither response in Acer rubrum L. (red maple). Understanding molecular mechanisms associated with plant response to drought stress can be an im portant factor in developing strategies for improved sustainability in urban landscapes. Our objective was to characterize expression of drought-induced dehydrin genes in leaves of `Red Sunset' red maple (desiccation-resistant) and seaside alder (desiccation-sensitive) in response to dehydration and rehydration. Potted cuttings grown in a glasshouse were subjected to four cycles of drought and rehydration. Stomatal conductance and volumetric moisture content of rooting medium were used to determine when drought cycles ended. During the second and fourth cycles, leaves were sampled for RNA and protein extraction. Dehydrin probes were generated from genomic DNA of both species by using PCR with primers designed from conserved regions in dehydrin genes. Southern blot analyses revealed the presence of dehydrin genes in seaside alder and red maple genomes. Reverse transcriptase (RT)-PCR was used to isolate desiccation-induced dehydrin cDNAs from total RNA extracted from drought-stressed leaves. The cDNA clones show 61% to 66% identity at the nucleic acid level with dehydrin genes of soybean, sunflower, radish, and potato. Accumulation of dehydrin transcripts and proteins in leaves in response to dehydration and rehydration are being studied through northern and western blot analyses, respectively. Our results may lead to a rapid screening technique for seedlings with improved mechanisms of drought resistance.

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Timothy M. Spann and Holly A. Little

Numerous physiological functions are affected by drought stress in citrus trees ( Gómez-Cadenas et al., 1996 ). Stomatal closure has been reported to occur within 2 h in response to severe drought stress in citrus ( Tudela and Primo-Millo, 1992

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Ricardo Cesped-Ruiz* and Bingru Huang

The American cranberry often undergoes drought stress during the summer. However, the physiological response of this species to drought is not well understood. This study was designed to determine the effects of drought on two commercial cranberry cultivars of high potential yield, `Ben Lear' and `Stevens', during a vegetative stage. The plants were subjected to drought for 15 days in a greenhouse. Soil water content, leaf water content, leaf photosynthetic rate, stomatal conductance, transpiration, differential leaf-air temperature, photochemical efficiency (Fv'/Fm') and the actual PSII efficiency (deltaF/Fm') decreased in those plants subjected to drought. Drought reduced differential leaf-air temperature at day 6 of treatment and stomatal conductance and transpiration starting at day 9 and photosynthetic rate at day 13. Drought decreased leaf water content at day 14 and Fv'/Fm' and PSII efficiency at day 15. Our results indicated that cranberry plants in vegetative stage were sensitive to drought for both cultivars and stomatal conductance was the most sensitive parameter among those examined for both cultivars.

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D.R. Edwards and M.A. Dixon

Six-year-old trees were repeatedly conditioned by withholding irrigation until Ψpd (predawn) thresholds of either –0.9 (“mild”) or –1.4 MPa (“moderate”) were attained. After conditioning, trees were exposed to severe drought (Ψpd –2.0 MPa) and then to 10 days of well-watered conditions. Throughout the investigation, osmotic potential (Ψπ), leaf RWC, transpiration, and total water potential (Ψx) were measured. Water stress was quantified by integrating Ψx. Conditioning caused a significant, but modest, degree of osmotic adjustment (0.08 to 0.28 MPa), which persisted after a brief relief from stress and transpiration rates were reduced 35% to 50%. Osmotic adjustment was not significantly enhanced by more than one stress exposure or conditioning beyond the mild threshold of stress. During severe drought, the moderate group maintained less negative Ψx and lower transpiration rates (38%). After prolonged stress relief, Ψx was similar among all treatments and daily transpiration rates and Ψx gradually recovered. Thuja occidentalis appears to rely on increased stomatal resistance more than osmotic adjustment to tolerate drought stress.

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Robert M. Augé, Ann J.W. Stodola and Brian D. Pennell

The influence of irradiance and drought on osmotic and turgor adjustment was examined in leaves of rose (Rosa hybrida L. `Samantha'). Plants cultured under full ambient light in the greenhouse were placed in shade chambers and, after 2 weeks of acclimation, exposed to drought for 21 days. Treatments consisted of a water stress factor (well-watered and drought-stressed) and an irradiance factor (100%, 70%, and 30% of ambient irradiance). Pressure-volume analyses of leaves indicated that osmotic potentials at full turgor were decreased 0.42, 0.36, and 0.23 MPa by drought in the 100%, 70%, and 30% irradiance treatments, respectively. Plants stressed under 100% and 70% irradiance exhibited similar osmotic adjustments. Plants under 30% irradiance had higher osmotic potentials at full turgor under well-watered conditions than plants in the other two irradiance treatments and showed only 55% as much adjustment to drought. In each irradiance treatment, drought induced an increase in elastic modulus and a decrease in relative water content at zero turgor. Turgor pressures were higher across a range of relative water contents in plants in the two higher irradiance treatments under both soil moisture treatments. Turgor also was higher at any particular water potential at 100% and 70% irradiance than 30% irradiance, within each soil moisture treatment. Heavy, but not mild, shading inhibited osmotic and turgor adjustments in leaves during drought.

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Avinoam Nerd and Peter M. Neumann

Hylocereus undatus [(Haworth) Britton and Rose] is a vine cactus from central America that has been established as a new fruit crop (pitaya) in many tropical and subtropical countries. In order to develop improved irrigation practices, the relationships between water parameters and growth were studied in rooted stem cuttings growing in pots with sandy soil under well-watered and drought-stressed conditions, in a controlled environment. Soil water potential rapidly decreased from -0.02 to -1.5 MPa during the first 5 days of drought. However, growth of new stems emerging from the succulent mature stems only decreased significantly after 3 weeks of drought. After 3 weeks of drought, the water content of growing stems decreased by 2% (P < 0.05) and their water potentials by 0.05 MPa (P > 0.05), as compared with the irrigated controls. At the same time, water content in drought-treated mature stems decreased by 4% (P < 0.05) and water potentials by 0.25 MPa (P < 0.05). Several lines of evidence indicated that active phloem supply of assimilates and associated water reserves from mature stems was the mechanism that allowed developing stems of H. undatus to maintain growth under well-watered and drought conditions: 1) Girdling the phloem of growing stems rapidly inhibited stem elongation. 2) Secretion of sucrose-containing nectar by growing stems was maintained during drought. 3) The water potential gradient was in the wrong direction for xylem transport from mature to young growing stems and axial hydraulic conductivity in young stems was either zero or comparatively low.

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Emma L. Locke*, Cecil Stushnoff, Joyce C. Pennycooke and Michelle Jones

Salinity, drought and temperature frequently limit crop productivity. Transgenic Petunia ×hybrida cv. Mitchell with altered endogenous raffinose family oligosaccharides (RFO) due to over-expression (sense) or under-expression (antisense) of the tomato α-galactosidase gene show that antisense increases in RFO are associated with greater tolerance to freezing stress (Pennycooke et al., 2003). Because vegetative propagules of these antisense lines rooted and established more quickly than their sense counterparts, we hypothesized that antisense lines would also respond to salinity and wilting stress. Salinity treatment plants were exposed to 50-200 mm NaCl graduated 25 mm every 3 days and held at 200 mm for 13 days. Dry-down treatments were watered to pot capacity, then not watered until the onset of wilting. This was repeated in cycles for 26 days. Data were collected on plant growth, root/shoot ratios, and leaf water potential. Fresh and dry weights in four of the six antisense lines exceeded the wild type and sense lines. Osmotic potential for salinity and dry-down plants was 160% to 220% higher than control plants. Pearson correlations revealed that higher osmotic potential was partially associated with higher fresh weight (r = 0.7214, P = 0.02) and root/shoot ratios (r = -0.7414, P = 0.02) in salinity stressed plants. In the dry-down drought stressed plants, osmotic potential was not associated with fresh weight (r = 0.3364, ns) nor root/shoot ratio (r = -0.0431, ns). Salinity stress reduced root mass compared to control and dry down plants. Sense plants grew slowly and were highly variable.

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Genhua Niu and Denise S. Rodriguez

Salvia greggii (salvia) and Dalea frutescens (dalea) are two popular shrubs. However, little information is available on their drought tolerance. The objectives of this study were to investigate the effect of various degrees of water stress on growth and to characterize the dynamics of water relations to root substrate water content for developing best irrigation management. Salvia and dalea plants in 12-L plastic containers were grown in a greenhouse and pruned to one node at the base of the soft shoots for salvia or at the same height for dalea prior to the start of the experiment. There were three irrigation regimens: plants were irrigated daily (control), or irrigation was withheld until moderate or severe water stress signs exhibited. After several weeks of intermittent cyclic dry-down irrigation regimens, total shoot number per container was reduced by 40% to 50% for salvia and 35% to 40% for dalea. Average shoot length was reduced by 35% to 45% for salvia and 50% to 65% for dalea in moderate and severe stressed treatments compared to the control. Drought stress resulted in less shoot elongation and fewer new shoots in both species. To examine the relationship between plant water status and substrate water content, a dry down test was performed on five well-watered plants by withholding irrigation until midday water potential dropped to below –4 MPa. As substrate water contents in both species reached 8%, the predawn water potentials did not recover from the midday water potential of the previous day, indicating there was no available water in the substrate for roots to take up. The drought tolerance of these two species needs further study using various growing media.

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Krishna S. Nemali and Marc W. van Iersel

We have developed a completely automated irrigation system that measures and maintains substrate volumetric water content (θ) at a target level for any length of time. Advantages of this system include complete automation of irrigation and simulation of precise levels of drought stress for potted plants. This system uses ECH2O moisture sensors interfaced with a CR10X datalogger and solenoid valves connected to the datalogger by a SDM CD16 AC/DC controller. The datalogger measures the θ of the substrate hourly. When the θ of the substrate drops below the set point, the datalogger opens the solenoid valves, which results in irrigation. Substrate θ is maintained at a constant level as the datalogger is programmed to increase θ by 2% to 3% during each irrigation. When the system was validated for its accuracy, we determined that the θ measured in the substrate within the range of 0.15 to 0.35 m3·m-3 was close (2% to 3%) to the θ determined by the conventional volumetric analysis. The daily average θ maintained in the substrate was slightly higher (within 3%) than the target level. Using this system, we were able to maintain four distinct levels of substrate θ for a prolonged period (40 days), regardless of differences in plant size and environmental conditions. Significant increases in number of irrigations, total water-use, and transpiration rate of impatiens, salvia, vinca, and petunia were noticed with increasing target θ of the substrate. For all species, highest and lowest water-use efficiency (WUE) were seen at 0.09 and 0.32 m3·m-3, respectively, while WUE was not different between 0.15 and 0.22 m3·m-3.

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Jeffrey H. Gillman, Mark W. Rieger, Michael A. Dirr and S. Kristine Braman

Two experiments were conducted to determine the effect of drought stress on the susceptibility of Buddleia davidii Franch. `Pink Delight' to the two-spotted spider mite (Tetranychus urticae Koch). In the first experiment, drought stress was imposed by withholding water until predawn xylem pressure potential fell below -1 MPa. Shoot growth was 75% less in drought-stressed than in nonstressed plants. Mite population densities were not affected, but noninfested leaf area was 14% higher, and degree of mite damage was lower, in nonstressed plants. Evidently, the greater amount of new growth in nonstressed plants leads to lower spider mite densities by diluting populations. In a second experiment, nonstressed B. davidii `Pink Delight' plants were watered every 1 to 2 days and drought-stressed plants were watered every 3 days. Spider mite populations were monitored by sampling newly expanded and mature foliage. Mite populations on mature foliage were not affected by stress, but stressed plants grew less and had larger spider mite populations on their newly expanded foliage than did nonstressed plants.