Effect of Time, Water Flow, and pH on Centripetal Passage of Radiophosphorus across Roots of Intact Plants.

The effects of time, rate of the water flow, and ambient pH on centripetal passage of radiophosphorus across intact bean roots to the xylem were studied. Isotope which completed passage and entered the xylem stream, as well as amounts delivered to the plant top, served to measure centripetal passage.Centripetal passage of radiophosphorus increased parabolically reaching a maximum after 1 hr and maintained this level during the 2nd hr. This pattern was consistent for all conditions studied. The curve suggested that passage did not progress as an abrupt front, but rather that it occurred through a phosphorus pool before reaching the xylem.Differences in rate of water flow through test plants, accomplished by adjusting the humidity of the foliage environment, did not significantly affect centripetal passage of radiophosphorus. Water flow did, however, profoundly influence composition of the xylem stream by altering the solvent to isotope ratio.Centripetal passage of radiophosphorus was not affected by solution pH in the acid range (pH 4.8, 5.2, 6.4), but was inhibited in the more alkaline range (pH 7.0, 7.5, 8.0). The similarity of these findings to those in the literature for phosphorus uptake by individual cells suggests that cell uptake may constitute the primary rate-limiting step in the over-all process of ion passage to the xylem.