Diurnal cycles in centripetal passage, residual uptake, and xylem sap concentration of radiophosophorus in roots

Abstract

The following measurements were made on intact bean plants over a consecutive 16 hr photoperiod and an 8 hr dark period: (a) centripetal passage of ³²p across the root from the external medium to the xylem, (b) residual uptake of isotope by the root mass, and (c) concentration of isotope in the xylem stream. Centripetal passage and residual uptake followed a similar diurnal pattern, being maximal during the photoperiod and minimal in the dark period. The possibility that this similarity reflected a common rate‐limiting mechanism for both processes was discussed. Concentration of the xylem stream, in turn, was dominated more by the dilution effect of water than by passage of phosphorus to the xylem. Stream concentration was minimal during the photoperiod (high transpiration) in spite of high phosphorus influx, and maximal in the dark period (low transpiration) with a diminished phosphorus influx. Further, concentration of the xylem stream was more dilute than the external solution at all times in the experiment. These data indicated a greater proportionate flux of water to the xylem than of phosphorus throughout the diurnal period.

Key words:

• Diurnal
• centripetal passage
• residual uptake
• xylem sap