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The effects and interactions of water stress and nutrient solution on water relations and concentrations of amino acids, organic acids and sugars in xylem fluid of ‘Methley’ plum (Prunus salicina Lindl.) and ‘Carolina Beauty’ crape myrtle (Lagerstroemia indica L.) during midday were determined. Container-grown plants were irrigated with water or nutrient solution (i.e., osmolarity = 138 mM) for 15 days, then irrigation was either continued or terminated for the next 5 days. The experiments were analyzed as factorial designs for each species separately, with the nutrient solution and irrigation status the last 5 days as the main factors. Xylem fluid tension increased ≈ 2to 3-fold and leaf conductance to water vapor and transpiration were reduced ≈ 10-fold by withholding irrigation for both species; plant water relations of L. indica were also influenced by the nutrient solution. For both species, the osmolarity of xylem fluid was not altered by withholding irrigation. The predominant organic compounds quantified in both species were amides (i.e., glutamine and asparagine), arginine, and citric and malic acids. Sugars represented a small proportion (i.e., generally ≤ 1%) of total osmolarity. Irrigation altered the chemical profile of amino acids and organic acids to a greater degree than the nutrient solution. Water stress induced a 3-fold increase in total organic acids in xylem fluid of both species. The osmolarity and the concentration of most organic compounds in xylem fluid of P. salicina were not significantly affected by the nutrient solution. Arginine increased markedly in concentration by withholding irrigation or with the application of nutrient solution for L. indica. The concentration of most organic compounds did not vary greatly in response to variations in soil water or nutrient status. In conclusion, soil water-or nutrient-mediated changes in plant water relations exceeded changes in xylem fluid chemistry. Xylem vessels serve as a major conduit for the delivery of inorganic ions and certain organic compounds to plant tissues. Xylem fluid is extremely dilute (i.e.,>95% water) and consists primarily of amino acids, organic acids and inorganic ions (Andersen and Brodbeck, 1989a-c, 1991; Andersen et al., 1989, 1992; Clark et al., 1986). Although the concentration of solutes in xylem fluid is low, a large quantity of solute is transported in xylem fluid due to the large amount of water typically transpired by mesophytic plants. Despite the importance of xylem fluid to plant nutrient status and to leaf and shoot physiology, there have been relatively few attempts to characterize the influence of environmental stresses on the chemistry of xylem fluid. Soil water and soil fertility status may drastically alter the concentration of organic compounds in foliar tissue (Boggess et al., 1976; Good and Zaplachinski, 1994; Rabe 1990; van Rensburg et al., 1993; Voetberg and Sharp, 1991). Rabe ( 1990) reviewed the influence of numerous abiotic and biotic stresses on the composition of N-containing compounds in plants. The amino compounds most often accumulated in foliar tissue as a function of stress include glutamine, asparagine, arginine, proline, citrulline, and ornithine. It is not known, however, to what extent the chemical or publication 15 Apr. 1994. Accepted for publication 8 Aug. 1994. orida Agricultural Experiment Station Journal series no. R-03757. Use mes does not imply endorsement of products named nor criticism of ucts not mentioned. The cost of publishing this paper was defrayed in payment of page charges.Underpostal regulations, this paper therefore eby marked advertisement solely to indicate this fact. of horticulture. ologist. of entomology. compounds derived from xylem fluid contribute to stress-induced changes in the chemical profile of foliar tissue. Evidence derived from plants exuding xylem fluid under positive pressure has been consistent with the concept that the composition of xylem fluid is regulated by the plant and is not greatly influenced by the composition of the rhizosphere. Fertilization during the dormant season had a minimal influence on the chemistry of xylem fluid during budbreak of several Vitis species. (Andersen and Brodbeck, 1991; Roubelakis–Angelakis and Kliewer, 1979) and kiwi (Clark et al., 1986). Andersen and Brodbeck (1991) found no difference in the osmolarity of xylem fluid of bleeding V. rotundifolia despite irrigation with 3x Hoagland’s solution, although total water and solute flux were increased 3-fold compared to plants irrigated with water. Few differences in the concentration of organic and inorganic constituents in xylem fluid of Vitis spp. before budbreak occurred after the application of fertilizer in the field (Andersen and Brodbeck, 1991; Roubelakis–Angelakis and Kliewer, 1979). The potential contribution of solutes derived from the xylem fluid during periods of growth and positive root pressure is relevant to studies involving nutrient partitioning or stress-induced osmotic adjustment in plant tissue. These data may also be useful for developing and understanding C and N budgets. Our objectives were to test the effects and the interaction of water stress and of soil applied nutrients on plant water relations and the concentration of amino acids, organic acids, and sugars in the xylem fluid of two woody species Prunus salicina a n d Lagerstroemia indica, which were chosen since they are in different families and have xylem fluid with markedly different profiles of organic compounds (Andersen et al., 1989). J. AMER. SOC. HORT. SCI. 120(1):3642. 1995. Materials and Methods Plant material. In March 1989, P. salicina ‘Methley’ and L. indica ‘Carolina Beauty’ were potted in 12-liter containers with media consisting of 3 pine bark : 1 Canadian sphagnum peat : 1 sand. One cubic meter of medium was amended with 6.1 kg dolomite, 2.2 kg superphosphate, 0.9 kg Micromax (12S–0.lB– 0.5 Cu-12Fe–2.5 Mn-.5Mo-12Zn), and 5.9 kg Osmocote 18-N2.6P–10K (Grace Sierra Corp., Milpitas, Calif.); initial medium pH was 5.6. Plants were supplied with overhead irrigation twice daily until 14 July, at which time they were transported to a rainproof shelter covered with 6 ml clear polyethylene. Twentyeight plants of each species were selected for uniform size. From 14 to 28 July, one-half of the plants was irrigated with 500 ml tap water or with triple-strength Peters soluble fertilizer 20N–8.2P– 16.6K (Grace Sierra Corp.) twice daily. The nutrient solution was composed of ammonia (30 mM), nitrate (47 mM), urea (38 mM), superphosphate (14 mM), and potash (30 mM). Total osmolarity was 138 mM. On July 28, irrigation was terminated for one-half of the plants for 5 days. On 2 Aug., plant water status was determined and xylem fluid was extracted for quantification of amino acids, organic acids, and sugars. Thus, the four treatments were 1) irrigation with water for 20 days (well-watered treatment); 2) irrigation with nutrient solution for 20 days (nutrient solution treatment); 3) irrigation with nutrient solution for 15 days which was then discontinued for 5 days (nutrient solution/water-stress treatment); 4) irrigation with water for 15 days which was then discontinued for 5 days (water-stress treatment). Plant water relations. Leaf conductance to water vapor (g,) and transpiration rates (E) were measured on two recently expanded leaves of each plant with a steady-state porometer (model 1600M; LI–COR, Lincoln, Neb.) between 1100 and 1400 HR. Air temperature was 29 to 33C and relative humidity was 70% to 80% during the measurement period. Leaves were placed in the leaf chamber for 1 minor until near steady-state conditions of H2O vapor exchange were achieved. Water vapor exchange was measured on abaxial leaf surfaces since adaxial g1 was negligible. Measurements were recorded when photosynthetic photon flux exceeded 1000 μmol·m-2·s-1. Several minutes after measurements of leaf H2O vapor exchange, xylem fluid tension was measured on one or two 25to 35-cm stem segments of each of seven plants per treatment with a pressure chamber apparatus (model 600; Plant Moisture Stress Corp., Table 1. The influence of irrigation and nutrient solution on plant water re Prunus salicina. Treatment designation. t = irrigated with water for 2 20 days (nutrient solution treatment),+= nutrient solution for 15 days treatment), -= irrigated with water for 15 days which was terminated Dependent Irrigation (I) + + variable Nutrient solution (N) – + Xylem tension (MPa) 0.66 0.74 Leaf conductance (mol·m-2·s -1) 0.203 0.198 Transpiration (mmol·m -2·s -1) 6.58 5.24 Total osmolarity (mM) 26.0 28.0 Amino acids (mM) 2.32 2.67 Organic acids (mM) 1.18 0.93 sugars (mM) 0.15 0.16 Total organic compounds (mM) 3.64 3.75 Organic N (mM) 3.69 4.42 Organic N : C 0.258 0.308 ‘s,** *,’’* Nonsignificant or significant at P ≤ 0.05,0.01, or 0.001, respectiv J. AMER. SOC. HORT. SCI. 120(1):3642. 1995. Corvallis, Ore.) (Scholander et al., 1965). Tissue exterior to the xylem was removed from the 2to 4-cm stem section protruding from the chamber before measurement of xylem tension and collection of xylem fluid. Collection of xylem fluid and chemical analyses. After recording the xylem tension, the pressure was increased 0.5 MPa higher than the balance pressure for 90 sec (Andersen et al., 1993; Berger et al., 1994). Xylem fluid was collected in 1.5-ml Eppendorf tubes, sealed, and placed on ice. Total osmolality of xylem fluid was determined with a vapor-pressure osmometer (model 5500; Wescor, Logan, Utah). Osmolality and osmolarity are virtually identical for dilute samples; hence, to facilitate comparison, concentrations of organic compounds will be expressed as osmolarity (mM). Preliminary experiments conducted on shoots of P. persica (L.) Batsch and L. indica have shown that the osmolarity of xylem fluid was not significantly altered in samples collected from 0 to 2 min after pressurization. Samples were stored at –20C for several mont

journal of the american society for horticultural science

Water Stress- and Nutrient Solution-mediated Changes in Water Relations and Amino Acids, Organic Acids, and Sugars in Xylem Fluid of Prunus salicina and Lagerstroemia indica