Abstracts

Carroll, A.B., Pallardy, S.G., et al. 2001, 'Drought Stress, Plant Water Status, and Floral Trait Expression on Fireweed, Epilobium Angustifolium (Onagraceae)', American Journal of Botany, vol. 88, no. 3, pp. 438-446.

In a controlled environment, we artificially induced drought during flowering of Epilobium angustofolium, an animal-pollinated plant. Leaf water potential (Ψ₁) and floral traits were monitored over a 12-d period of soil moisture depletion. Soil moisture depletion induced drought stress over time, as revealed by significant treatment x day interactions fro predawn and midday Ψ₁. Nectar volume and flower size showed significant negative responses to drought stree, but nectar concentration did not vary between treatments. Floral traits were more buffered from drought than leaf water potentials. We used path analysis to examine direct and indirect effects of Ψ₁ on floral traits for plants in well-watered (control) vs. drought treatments. According to the best-fit path models, midday Ψ₁ has significant positive effects on flower size and nectar volume in both environments. However, for controls midday Ψ₁ also had a significant negative effect on nectar sugar concentration. Results indicate that traits influencing floral attractiveness to pollinators in E. angustofolium vary with plant water status, such that pollinator-mediated selection could indirectly target physiological or biochemical controls on Ψ₁. Moreover, under mesic conditions selection for greater nectar sugar reward may be constrained by the antagonistic effects of plant water status on nectar volume and sugar concentration.

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Dichio, B., Xiloyannis, C., et al. 2005, 'Osmotic Regulation in Leaves and Roots of Olive Trees During a Water Deficit and Rewatering', Tree Physiology, vol. 26, pp. 179-185.

We evaluated the osmotic adjustment capacity of leaves and roots of young olive (Olea europaea L.) trees during a period of water deficit and subsequent rewatering. The trials were carried out in Basilicata (40°24' N, 16°48' E) on 2-year-old self-rooted olive plants (cv. 'Coratina'). Plants were subjected to one of four drought treatments. After 13 days of drought, plants reached mean predawn leaf water potentials of -0.45 ± 0.015 MPa (control), -1.65 ± 0.021 (low stress), -3.25 ± 0.035 (medium stress) and -5.35 ± 0.027 MPa (high stress). Total osmotic adjustment increased with increasing severity of drought stress. Trees in the high stress treatment showed total osmotic adjustments ranging between 2.4 MPa at 0500 h and 3.8 MPa at 1800 h on the last day of the drought period. Osmotic adjustment allowed the leaves to reach leaf water potentials of about -7.0 MPa. Active osmotic adjustment at predawn decreased during the rewatering period in both leaves and roots. Stomatal conductance and net photosynthetic rate declined with increasing drought stress. Osmotic adjustment in olive trees was associated with active and passive osmotic regulation of drought tolerance, providing an important mechanism for avoiding water loss.

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Fernández, J.E., Díaz-Espejo, A., et al. 2006, 'Water Relations and Gas Exchange in Olive Trees under Regulated Deficit Irrigation and Partial Rootzone Drying', Plant and Soil, vol. 284, pp. 273-291.

It is widely believed that partial root drying (PRD) reduces water losses by transpiration without affecting yield. However, experimental work carried out to date does not always support this hypothesis. In many cases a PRD treatment has been compared to a full irrigated treatment, so doubt remains on whether the observed benefits correspond to the switching of irrigation or just to PRD being a deficit irrigation treatment. In addition, not always a PRD treatment has been found advantageous as compared to a companion regulated deficit irrigation (RDI) treatment. In this work we have compared the response of mature 'Manzanilla' olive trees to a PRD and an RDI treatment in which about 50%of the crop evapotranspiration (ET_c) was supplied daily by localised irrigation. We alternated irrigation in the PRD treatment every 2 weeks in 2003 and every 3 weeks in 2004. Measurements of stem water potential (Ψ_stem), stomatal conductance (g_s) and net CO₂ assimilation rate (A) were made in trees of both treatments, as well as in trees irrigated to 100% of ET_c (Control trees) and in Rain-fed trees. Sap flow was also measured in different conductive organs of trees under both PRD and RDI treatments, to evaluate the influence of alternating irrigation on root water uptake and tree water consumption. We found small and random differences in Ψ_stem, g_s and A, which gave no evidence of PRD causing a positive effect on the olive tree performance, as compared to RDI. Stomatal conductance decreased in PRD trees as compared to Control trees, but a similar decrease in g_s was also recorded in the RDI trees. Sap flow measurements, which reflected water use throughout the irrigation period, also showed no evidence of g_s being more reduced in PRD than in RDI trees. Daily water consumption was also similar in the trees of the deficit irrigation treatments, for most days, throughout the irrigation period. Alternating irrigation in PRD trees did not cause a change in either water taken up by main roots at each side of the trees, or in the sap flow of both trunk locations and main branches of each side. Results from this work, and from previous work conducted in this orchard, suggest that transpiration is restricted in trees under deficit irrigation, in which roots are left in drying soil when water is applied by localised irrigation, and that there is no need to alternate irrigation for achieving this effect.

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Girona, J., Mata, M., et al. 2006, 'The use of Midday Leaf Water Potential for Scheduling Deficit Irrigation in Vineyards', Irrigation Science, vol. 24, pp. 115-127.

Midday leaf water potential (Ψ_md) was monitored for 3 years at a commercial vineyard (cv. Pinot Noir) under four irrigation strategies. Three treatments were established based on irrigating vines with 4-6 mm/day, when daily measured Ψ_md was more negative than the predefined threshold. After the first experimental year, thresholds were adjusted for each treatment as: (1) Control (C), irrigated when Ψ_md was less than -0.6 MPa at the beginning of the season and gradually fell to -0.8 MPa at about mid-June, after which the threshold was maintained at -0.8 MPa until harvest. (2) Control-Deficit (CD), irrigated as C from bud-break to mid-June (around the middle of Stage II of fruit growth), and from then until harvest when Ψ_md decreased below -1.2 MPa. (3) Deficit-Deficit (DD), irrigated when Ψ_md was less than -1.0 from bud break to mid-May (about the middle of fruit growth Stage I), and after that time the Ψ_md threshold became -1.2 MPa until harvest. A fourth treatment was applied following a soil water budget approach (WB). All treatments were replicated five times but irrigation in the Ψ_md-based treatments were independently applied to each of the replicate plots, whereas irrigation for WB was applied equally to all replications. The more site-specific information obtained from Ψ_md thresholds in C provided substantial advantages for yield homogeneity and repeatability of results with respect to WB, thus demonstrating the method's greater ability to account for spatial variability. Average applied water for the 3 years in C, CD, and DD was 374, 250, and 178 mm, respectively, while the yields were 11.8, 9.2, and 6.1 kg/vine, respectively. The CD treatment produced better juice quality than C, and was superior in other quality parameters to both C and DD. However, over the study period, an important carryover effect was observed in the yields and the grape size of CD, which tended to diminish from year to year relative to C.

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Guerrero, J., Moriana, A., et al. 2006, 'Regulated Deficit Irrigation and the Recovery of Water Relations in Pistachio Trees', Tree Physiology, vol. 26, no. 1, pp. 87-92.

Recovery of water status in water-stressed pistachio trees (Pistacia vera L. cv. Kerman) was investigated by subjecting trees to regulated deficit irrigation (RDI) (60% of crop evapotranspiration rate, ET_c) during stages I and II of fruit development (FD) followed by full irrigation during FD stage III (kernel-filling). Trees irrigated at 100% ET_c throughout FD stages I, II and III served as controls. Water-stress severity was characterized by changes in soil water content and midday stem water potential (Ψ_md). Midday leaf conductance (g₁) and trunk diameter variation (TDV) were also measured. In RDI trees, the lowest Ψ_md value, -1.8 MPa, occurred at the end of the RDI period. The corresponding value for the control trees was around -1.1 MPa. Although the RDI treatment affected gas exchange later than Ψ_md, the greatest reductions in gas exchange (60% of control values) also appeared at the end of the RDI period. There were significant differences in TDV between control and RDI trees at the end of the RDI period. Although plant water status recovered within 20 days of resuming irrigation, the TDV values indicated a longer period might be necessary for complete recovery. Recovery of g₁ was faster than that of Ψ_md, although differences in TDV between control and RDI trees indicated that gas exchange recovered later than Ψ_md. The slow recovery of pistachio trees during FD stage III from water stress imposed during FD stages I and II suggests that irrigation should exceed 100% ET_c during FD stage III or that more extensive irrigation should commence before the end of FD stage II.

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Hubbert, K.R., Beyers., J.L., et al. 2001, 'Roles of Weathered Bedrock and Soil in Seasonal Water Relations of Pinus Jeffreyi and Arctostaphylos patula', Canadian Journal of Forest Research, vol. 31, pp. 1947-1957.

In the southern Sierra Nevada, California, relatively thin soils overlie granitic bedrock that is weathered to depths of several metres. The weathered granitic bedrock is porous and has a plant-available water capacity of 0.124 m³ m^-3, compared with 0.196 m³ m^-3 for the overlying soil. Roots confined within the bedrock joint fractures access this rock-held water, especially during late summer when overlying soils are dry. We sought to determine seasonal soil and bedrock water changes in a Jeffrey pine (Pinus jeffreyi Grev & Balf.) plantation and to examine concurrent effects on the water relations of Jeffrey pine and greenleaf manzanita (Arctostaphylos patula Greene). In 1996, plant-available water in the 75 cm thick soil was depleted by late June, with soil water potential (Ψ_soil) <-2.2 MPa, but below 75 cm, bedrock water potential (Ψ_bedrock) was still >-2.2 MPa. Thus, the bedrock, not the soil, supplied water to plants for the remainder of the dry season. Higher values of, and smaller fluctuations in, seasonal predawn pressure potential (Ψ_predawn) for Jeffrey pine indicated that it is deeply rooted, whereas active roots of greenleaf mananita were interpreted to be mostly within the upper 100 cm. The extra rooting volume supplied by weathered bedrock is especially important to pine relative to manzanita.

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McClenahan, K., Macinnis-Ng, C., et al. 2004, 'Hydraulic Architecture and Water Relations of Several Species at Diverse Sites Around Sydney', Australian Journal of Botany, vol. 52, pp. 509-518.

Seasonal comparisons of leaf water potential, root biomass, hydraulic architecture, xylem embolism and xylem dimensions were made for eight woody species in four diverse habitats (mangroves, coastal heathland, ridge-top woodland and river-flat woodland). In most comparisons, pre-dawn and minimum leaf water potentials were lower in winter than in summer, a result attributed to lower rainfall and a smaller root biomass in winter than in summer. Branch hydraulic conductivities (per unit transverse area, sapwood area or leaf area) were generally larger in summer than in winter across all species in all habitats. An inverse relationship between Huber value and conductivity was observed across all four habitats. Increased solar radiation and evaporative demand in the summer was associated with an increased percentage loss of xylem conductance arising from embolism, compared with winter. These results are discussed in the context of patterns and relationships among water relations, microclimate and hydraulic architecture.

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O'Grady, A.P., Eamus, D., et al. 2005, 'Comparative Water Use by the Riparian Trees Melaleuca argentea and Corymbia bella in the Wet-dry Tropics of Northern Australia', Tree Physiology, vol. 26, pp. 219-228.

We examined sources of water and daily and seasonal water use patterns in two riparian tree species occupying contrasting niches within two riparian zones throughout the wet-dry tropics of northern Australia: Corymbia bella Hill and Johnson is found along the top of the levee banks and Melaleuca argentea W. Fitzg. is restricted to riversides. Patterns of tree water use (sap flow) and leaf water potential were examined in four trees of each species at three locations along the Daly River in the Northern Territory. Predawn leaf water potential was higher than -0.5 MPa throughout the dry season in both species, but was lower at the end of the dry season than at the beginning of the dry season. Contrary to expectations, predawn leaf water potential was lower in M. argentea trees along the river than in C. bella trees along the levees. In contrast, midday leaf water potential was lower in the C. bella trees than in M argentea trees. There were no seasonal differences in tree water use in either species. Daily water use was lower in M. argentea trees than in C. bella trees. Whole-tree hydraulic conductance, estimated from the slope of the relationship between leaf water potential and sap flow, did not differ between species. Xylem deuterium concentrations indicated that M. argentea trees along the riverbank were principally reliant on river water or shallow groundwater, whereas C. bella trees along the levee were reliant solely on soil water reserves. This study demonstrated strong gradients of tree water use within tropical riparian communities, with implications for estimating riparian water use requirements and for the management of groundwater resources.

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O'Leary, S.J.B. and von Aderkas, P. 2006, 'Postpollination Drop Production in Hybrid Larch is not Related to the Diurnal Pattern of Xylem Water Potential', Trees, vol. 20, pp. 61-66.

The hourly production of postpollination drops in the ovules of three hybrid larch trees (Larix x marschlinsii Coaz) was examined and compared with the measured diurnal pattern of xylem water potential of the same trees under non-stressed conditions during two reproductive seasons. There was no consistent relationship between xylem water potential and ovular drop production in hybrid larch. Individual trees that showed a diurnal drop production in one year did not follow a similar pattern the other year. One tree produced drops in over half of its ovules hourly, regardless of the measured xylemwater potential. Variation in number of drops produced per ovule and drop mass was observed between the three trees both years, and between years for two of the trees. This variationwas not in response to xylem water potential or environmental conditions. Here we report the first simultaneous measurements of branch water status and the production of ovular secretions for any conifer. We conclude that the postpollination drop production of hybrid larch is not regulated by a tree's overall water status, but is under the control of localized structures such as the cone or the ovule.

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Thompson, R.B., Gallardo, M., et al. 2007, 'Using Plant Water Status to Define Threshold Values for Irrigation Management of Vegetable Crops using Soil Moisture Sensors', Agricultural Water Management, vol. 88, pp. 147-158.

Thresholds of soil matric potential (SMP) and available soil water content (AWC) required to prevent water limitations between irrigations were determined for bell pepper, melon, and spring and winter tomato grown in Mediterranean-type greenhouses on the south-eastern coast of Spain. Thresholds were identified by measuring the divergence of leaf water potential of un-watered plants from that of well-watered plants. Soil matric potential thresholds were -58 kPa for pepper, -35 kPa for melon, and -38 to -58 kPa for tomato. In general, SMP thresholds were more negative under lower evaporative demand conditions such as during autumn and winter months. Available soil water content thresholds, for a given crop and drying cycle, differed appreciably depending on soil depth and the method used to calculate the values. For the four crops studied, AWC thresholds calculated at 0-40 cm were 13-15% higher than those calculated at 0-20 cm. Each AWC threshold for 0-20 cm depth was 21-29% lower when AWC was based on laboratory rather than field determinations of field capacity and permanent wilting point. For a given method of calculating AWC, AWC threshold values were similar for different crops and drying cycles, suggesting limited sensitivity of the AWC approach. Using the manufacturer's calibration, the capacitance sensor used for SWC measurements overestimated SWC by an average of 36%. An in situ calibration provided generally good agreement with the actual SWC between 0.15 and 0.22 cm³ cm^-3; however, for higher SWC values, the in situ calibration underestimated SWC. The results of this study demonstrated the uncertainty of using recommended fixed AWC threshold values for irrigation management, using SWC sensors, because of issues related to the definition of rooting depth, measurement of FC and PWP, sensor calibration, and sensor accuracy across the relevant range of water contents. These data suggest that SMP thresholds are much more reliable than AWC thresholds for scheduling irrigations in greenhouse-grown vegetable crops. Technical issues regarding onfarm measurement of SMP and SWC are discussed.

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Trifilò, P., Raimondo, F., et al. 2004, 'Drought Resistance of Ailanthus altissima: Root Hydraulics and Water Relations', Tree Physiology, vol. 24, pp. 107-114.

Drought resistance of Ailanthus altissima (Mill.) Swingle is a major factor underlying the impressively wide expansion of this species in Europe and North America. We studied the specific mechanism used by A. altissima to withstand drought by subjecting potted seedlings to four irrigation regimes. At the end of the 13 week treatment period, soil water potential was -0.05 MPa for well-watered control seedlings (W) and -0.04, -0.08 and -1.7 MPa for drought-stressed seedlings (S) in irrigation regimes S1, S2 and S3, respectively. Root and shoot biomass production did not differ significantly among the four groups. A progressively marked stomatal closure was observed in drought-stressed seedlings, leading to homeostasis of leaf water potential, which was maintained well above the turgor loss point. Root and shoot hydraulics were measured with a high-pressure flow meater. When scaled by leaf surface area, shoot hydraulic conductance decreased by about 20% in S1 and S2 seedlings and by about 70% in S3 seedlings, with respect to the well-watered control value. Similar differences were observed when root hydraulic conductance was scled by root surface area, suggesting that roots had become less permeable to water. Anatomical observations of root cross sections revealed that S3 seedlings had shrunken cortical cells and a multilayer endodermal-like tissue that probably impaired soil-to-root stele water transport. We conclude that A. altissima seedlings are able to withstand drought by employing a highly effective water-saving mechanism that involves reduced water loss by leaves and reduced root hydraulic conductance. This water-saving mechanism helps explain how A. altissima successfully competes with native vegetation.

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