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Diurnal variations in the chemical composition of xylem fluid have been established for many plant species exhibiting positive root pressure; similar patterns have not been well documented in transpiring plants. Diurnal changes in plant water status and xylem fluid chemistry were investigated for `Flordaking' peach [Prunus persica (L.) Batsch], `Suwannee' grape (Vitis hybrid), and `Flordahome' pear (Pyrus communis L.). Xylem tension was maximum at 1200 or 1600 hr and declined to <0.5 MPa before dawn. Xylem fluid osmolarity ranged from 10 to 27 mm and was not correlated with diurnal patterns of xylem tension. The combined concentration of amino acids and organic acids accounted for up to 70%, 45%, 55%, and 23% of total osmolarity for irrigated P. persica, nonirrigated P. persica, Vitis, and P. communis, respectively. The concentration of total organic compounds in xylem fluid was numerically greatest at 0800 or 0900 hr. For irrigated P. persica the osmolarity of xylem fluid was reduced by 45% from 0800 to 1200 hr, 1 h after irrigation, compared to only a 12% reduction from 0800 to 1200 hr for nonirrigated trees. Asparagine, aspartic acid, glutamine, and glutamic acid were mainly responsible for diurnal changes in the concentration of total amino acids and organic N for P. persica; the diurnal variation in organic N for Vitis was due to glutamine. Arginine, rather than the amides, was the primary source of organic N in xylem fluid of P. communis, and there was no consistent diurnal change in the concentration of amino acids or organic N. The predominant organic acids in all species examined were citric and malic acids. No consistent diurnal trend occurred in the concentration of organic acids or sugars in xylem fluid.

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 ≈ 2- to 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.