DIURNAL-VARIATIONS IN TENSION, OSMOLARITY, AND THE COMPOSITION OF NITROGEN AND CARBON ASSIMILATES IN XYLEM FLUID OF PRUNUS-PERSICA, VITIS HYBRID, AND PYRUS-COMMUNIS

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.