Cation pretreatment effects on nitrate uptake, xylem exudate, and malate levels in wheat seedlings.

Week-old wheat seedlings absorbed at least 40% NO(3) (-) from NaNO(3) when preloaded with K(+) than when preloaded with Na(+) or Ca(2+). Cultures of Triticum vulgare L. cv. Arthur were grown for 5 days on 0.2 mm CaSO(4), pretreated for 48 hours with either 1 mm CaSO(4), K(2)SO(4), or Na(2)SO(4), and then transferred to 1 mm NaNO(3). All solutions contained 0.2 mm CaSO(4). Shoots of K(+)-preloaded plants accumulated three times more NO(3) (-) than shoots of the other two treatments. Initially, the K(+)-preloaded plants contained 10-fold more malate than either Na(+)- or Ca(2+)-preloaded seedlings. During the 48-hour treatment with NaNO(3), malate in both roots and shoots of the K(+)-preloaded seedlings decreased. Seedlings preloaded with K(+) reduced 25% more NO(3) (-) than those preloaded with either Na(+) or Ca(2+). These experiments indicate that K(+) enhanced NO(3) (-) uptake and reduction even though the absorption of K(+) and NO(3) (-) were separated in time. Xylem exudate of K(+)-pretreated plants contained roughly equivalent concentrations of K(+) and NO(3) (-), but exudate from Na(+) and Ca(2+)-pretreated plants contained two to four times more NO(3) (-) than K(+). Therefore K(+) is not an obligatory counterion for NO(3) (-) transport in xylem.