Cation Pretreatment Effects on Nitrate Uptake, Xylem

Week-old wheat seedlings absorbed at least 40% NO3from NaNO3 when preloaded with K+ than when preloaded with Na+ or Ca2". Cultures of Tnticum vulgare L. cv. Arthur were grown for 5 days on 0.2 ,M CaSO4, pretreated for 48 hours with either 1 mM CaSO4, K2SO4, or Na2SO4, and then transferred to 1 mM NaNO3. All solutions contained 0.2 mM CaSO4. Shoots of K+-preloaded plants accumulated three times more NO3than shoots of the other two treatments. Initially, the K+-preloaded plants contained 10-fold more malate than either Na+or Ca2+-preloaded seedlings. During the 48-hour treatment with NaNO3, malate in both roots and shoots of the K+-preloaded seedlings decreased. Seedlings preloaded with K+ reduced 25% more NO3than those preloaded with either Na+ or Ca2+. These experiments indicate that K+ enhanced NO3uptake and reduction even though the absorption of K+ and N03were separated in time. Xylem exudate of K+-pretreated plants contained roughly equivalent concentrations of K+ and NO3-, but exudate from Na+and Ca2'-pretreated plants contained two to four times more NO3than K+. Therefore K+ is not an obligatory counterion for NO3transport in xylem. According to the nitrate transport hypothesis of Ben Zioni et al. (1, 2), K+ may act as a counterion for the transport of N03from root to shoot, and for the transport of malate (produced stoichiometrically with NO3reduction) from shoot to root. The malate in the root may be decarboxylated, providing HC03for exchange for an external N03-. Considerable NO3may be absorbed in excess of K+ uptake; this has been observed (2, 8, 9). Despite a wide range ofNO3-/K' uptake ratios, equal concentrations of K+ and NO3were found in xylem exudates (2, 8). This suggests that K+ is necessary for NO3translocation. We predicted that plants preloaded with K+ should absorb and translocate more NO3than controls, even in the absence of K+ in the NO3solution. The experiments reported here were designed to test this hypothesis. MATERIALS AND METHODS Seeds of wheat (Triticum vulgare L. cv. Arthur) were cultured according to the methods of Blevins et al. (4). The seeds were soaked for 24 hr in continuously aerated deionized H20. The water was changed three times during the 24-hr period. Fifteen seeds/culture were then placed on cheesecloth, suspended by concentric plastic rings over 1-quart jars containing 870 ml of 0.2 'This research was supported by Project No. K-014 from the Maryland Agricultural Experiment Station. Scientific article No. A2319, Contribution No. 5325 of the University of Maryland Agricultural Experiment Station. 2 Present address: Department of Agronomy, Curtis Hall, University of Missouri, Columbia, Missouri 65201. mM CaSO4. The jars had previously been painted black, then silver. All cultures were continuously aerated with filtered air throughout the experiments. After 2 days in the dark, the cultures were culled to 10 plants each and placed in a growth chamber. They were given 16 hr of light (37,000 lux, 25 C) and 8 hr of darkness (20 C) alternately throughout the experiment, unless otherwise noted. The CaSO4 solution was changed every other day during the growing period. Plants used for cation preloading experiments were grown 5 days, then transferred to jars containing 870 ml of 0.5 mm K2S04 + 0.2 mm CaSO4; 0.5 mm Na2SO4 + 0.2 mM CaSO4; or 0.5 mM CaSO4. Preloading was continued for 48 hr, solutions being changed at 24 hr. After preloading, the seedlings were moved to jars containing 870 ml of a 1 mm NaNO3 + 0.2 mm CaSO4 solution to begin treatment. The seedlings were harvested after 0, 24, and 48 hr of treatment. Solutions were changed at 24 hr. The volume and pH of remaining solution were taken at the time of harvest. A sample of the culture solution was frozen at -80 C for later analyses. At harvest, the roots were excised from the shoots, rinsed thoroughly with deionized H20, blotted dry, weighed, and immediately frozen at -80 C. Shoots were also weighed and frozen. Extraction of plant tissue consisted of refluxing twice with 80%Yo ethanol and once with H20. The combined extract was blown dry, then redissolved in 10 ml of H20, filtered, and frozen at -80 C. Potassium, Na+, and Ca2+ concentrations were determined by atomic absorption spectrophotometry. Nitrate was analyzed by a modification of the automated method of Litchfield (7). A Technicon autoanalyzer connected to a Gilford spectrophotometer was used to assay both plant samples and culture solutions. The dialyzer used by Litchfield (7) was found to be unnecessary for the solutions and diluted plant samples. No detectable N02 was found in any sample. Malate concentration in extracts was assayed spectrophotometrically by a modification of the procedure of Williamson and Corkey (12). The attached seeds were discarded; preliminary experiments showed that changes in K+, N03-, and malate in the seed were small. Results are expressed on a per 10 plant basis, in order to eliminate any effects of growth during the experiments. All experiments were replicated three or six times. Where results are described as significantly different, analysis of variance (not shown) indicated that a difference of K+ treatment versus (Na+ and Ca+) treatments differed at the 1% level.