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Ben van Hooijdonk, David Woolley, Ian Warrington and Stuart Tustin

limits root growth and/or cytokinin biosynthesis and consequently the amount of root-produced cytokinin supplied to the scion in the xylem vasculature. In support of this hypothesis, cytokinins were identified in the xylem sap of apple trees ( Jones, 1973

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Eleanor W. Hoffman, Dirk U. Bellstedt and Gerard Jacobs

budbreak is correlated with an increased concentration of cytokinins in the xylem sap just before budbreak ( Tromp and Ovaa, 1990 ; Van Staden and Davey, 1979 ). Furthermore, exogenous application of cytokinins can promote budbreak during late dormancy

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Brandon R. Smith and Lailiang Cheng

; Gonzalez-Vallejo et al., 2000 ; Larbi et al., 2001 ). It has been proposed that bicarbonate uptake increases the pH of xylem sap and leaf apoplast and interferes with foliar Fe utilization ( Mengel et al., 1984a , 1984b ), but this is not well-agreed on

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Nauja Lisa Jensen, Christian R. Jensen, Fulai Liu and Karen K. Petersen

potential reaches −1.0 MPa ( Sruamsiri and Lenz, 1986 ). To date, no published work has been reported on chemical signaling in xylem sap of Fragaria × ananassa during soil drying, DI, or PRD. PRD irrigation is based on the hypothesis that PRD plants

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

determined by dividing total leaf area with total number of leaves. Specific leaf area (SLA) was calculated as total leaf area (square centimeters) per total DW of leaf biomass (grams). Collection of xylem sap and determination of xylem sap (ABA

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Wen-Shaw Chen

The changes in cytokinins and gibberellins in xylem sap of lychee (Litchi chinensis Sonn. cv. Heh yeh) trees were investigated at the stages of leaf expansion, dormant bud (when apical leaves are dropped), 30 days before flower bud formation, flower bud formation, and full bloom of grafted field-grown lychee trees. Also; the diffusible IAA and ABA in diffusate from shoot tips were examined at the successive stages of development. High gibberellin was found in the xylem sap at the stage of leaf expansion. A constant level of IAA was maintained through the five growth stages. At 30 days before flower bud formation, ABA increased dramatically. Concurrently, total cytokinin content increased in the xylem sap, reaching a maximum during flower bud formation and full bloom. Gibberellin content in the xylem sap was at a low level 30 days before flower bud formation and through the stage of flower bud formation.

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Ben-Hong Wu, Shao-Hua Li, Marta Nosarzewski and Douglas D. Archbold

abundant carbohydrate reserve in the xylem sap, with levels at a maximum early in bud development and then declining over time ( Hansen and Grausland, 1973 , 1978 ; McQueen et al., 2004 ). These carbohydrate reserves are remobilized to support initial

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J.G.M. Cutting, D.K. Strydom, G. Jacobs, D.U. Bellstedt, K.J. Van Der Merwe and E.W. Weiler

Xylem sap was vacuum-extracted weekly from 1-year-old apple shoots from trees treated with dinitro-o-cresol (DNOC) oil, hydrogen cyanamide, or untreated controls. Sampling began 1 week before treatment and continued until 2 weeks after budbreak had occurred in the control trees. Sorbitol, calcium, magnesium, potassium, and zeatin-type cytokinin concentrations were determined by enzymatic, atomic absorption, and immunoassay methods, respectively. The rest-breaking treatments resulted in earlier and more intense budbreak. Xylem sap cytokinin concentrations increased rapidly in response to the rest-breaking chemicals and peaked just before or at budbreak. The rapid increase in cytokinin was closely followed by increases in calcium and magnesium concentrations in the sap. Potassium concentration appeared to be unaffected by rest-breaking treatment. Sorbitol levels dropped rapidly as a result of the rest-breaking treatments and appeared to be used rapidly in budbreak and early bud growth.

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E. Peterlunger and B. Marangoni

ABA implication in root signals of water stress has been suggested by several authors. To verify this hypothesis in grapevines, this experiment has been carried out. One-year-old own rooted cuttings of grapevine cultivar Cabernet Sauvignon were exposed to water stress. After three months of growth, water was completely withdrawn for nine days, till the plants reached the wilting point. The plants were then rewatered. During the whole period, root hydraulic conductivity was measured with a pressure bomb; xylem sap samples were collected, as well as leaf and root samples. ABA concentration in these samples was measured using Radio Immuno Assay with DBPA1, a monoclonal antibody for ABA. The concentration of xylem sap ABA was 68.2 mg m-3 at the start of the experiment. After eight days of stress it was 1863.6 mg m-3, 27 × higher. On the ninth day the plants were rewatered, and the xylem sap ABA decreased at 100.2 mg m-3, keeping this level for eight more days. Leaf ABA showed high levels of this inhibitor, with a peak in correspondence with the maximum stress. A similar behaviour was attained by roots. In grapevine, ABA seems to be involved in a water stress root signal directed to the canopy.

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Maria G. Janssen and Albert H. Markhart III

Tepary beans (Phaseolus acutifolius Gray) are more drought tolerant and have stomata that are more sensitive to low leaf water potentials (ψ w) than common beans (P. vulgaris L.). This study was designed to examine the role of ABA in controlling stomatal behaviour in these species. Comparison of the bulk leaf ABA content does not explain why tepary stomata are more sensitive to low leaf ψ w compared to common bean (at -1.4 MPa ABA content increased 40-fold in common bean and 25-fold in tepary). We hypothesize that the greater sensitivity of tepary stomata to low leaf ψ w is related to a higher concentration of ABA in the xylem sap, and/or to a greater sensitivity of tepary stomata to ABA. Xylem sap of well-watered and water stressed plants is analyzed to determine the concentration of ABA, and whether ABA is a putative candidate serving as a chemical root signal in response to water stress in Phaseolus. To test stomatal sensitivity to ABA, epidermal strips and detached leaves are exposed to a range of ABA concentrations. The relationship between stomatal aperture and different ABA concentrations is discussed.