The Transpiration Stream and Upward Translocation of Mineral Ions

Ca and P were supplied simultaneously to young Phaseolus vulgaris L. seedlings grown in solution. After short periods of uptake, the Ca/P ratios in opposite leaves and opposite leaflets were very similar. This relationship also held when Ca and Cs were paired. Ca/P ratios in lateral halves of the same leaf were in good agreement, also. However, the basal halves of leaves always had higher Ca/P ratios than terminal halves. There was a negative acropetal gradient of Ca/P ratio in the stem, but the ratio in the trifoliate leaves was higher than in the primary leaves. When roots were removed prior to supplying the radioisotopes, the Ca/P ratios in the foliage and stems were lower than in plants with roots intact. The data obtained support the en masse flow concept of ion movement through the stem. The usual mechanism proposed for the upward translocation of mineral ions in plants is visualized as a bulk transport of these ions in the transpiration stream. This concept has led to considerable investigation attempting to ascertain the precise dependency of salt uptake and distribution on the movement of water through the plant. Several investigators have shown that an increase in transpiration rate is accompanied by an increase in salt uptake and movement to the shoots (Hylmo, 1953, 1955, 1958; Brouwer, 1954, 1956; Kylin and Hylmo, 1957; Smith, 1957). However, the mechanism whereby transpiration influences the overall p No. 681 from the Department of Botany and Plant Pathology, The Ohio State University. Manuscript received April 17, 1964. THE OHIO JOURNAL OF SCIENCE 65(6): 357, November, 1965. 358 JAMES w. O'LEARY Vol. 65 movement of salts from the outer surface of the roots to the shoots is not agreed upon. There are, basically, two opposing views at present. The point of conflict lies in explaining the transfer of salts from the root surface, across the cortex, to the stele of the root. One view, the "passive transfer" hypothesis, visualizes the movement of ions as a mass flow through the apparent free space of the root. The opposing view, the "active transfer" hypothesis, visualizes the movement of ions across the root as an active process dependent upon the release of energy through metabolic processes. The pros and cons of these hypotheses have been reviewed by Russell and Barber (1960). In spite of differences of opinion as to how ions reach the xylem of the root, it is generally agreed that the subsequent movement of ions to the shoots occurs as a flow en masse in the transpiration stream. If this is so, it follows that the ratio of ions accumulating in similar leaves or leaflets after transpiration periods of short duration should be relatively uniform, assuming that the pathways traversed by these ions to the respective leaves are also relatively uniform. For test areas at similar levels of insertion on the plant, such as opposite leaves or opposite leaflets, this assumption seems reasonable, and any significant disagreement among ratios at these comparable test sites should, therefore, indicate that a mechanism of upward solute movement visualized as a flow en masse in the transpiration stream does not adequately describe this process. The investigation reported here tests this mass flow hypothesis. Ca and P (or Ca and Cs) were supplied simultaneously to plants for short periods. It was assumed that after short term transpiration periods, the relative uptake and distribution of two concurrently supplied radioisotopes could be ascertained and be accurately interpreted as material which entered the plant only during these limited time periods. MATERIALS AND METHODS Asgrow's Black Valentine bean (Phaseolus vulgaris L.) was used as the experimental plant in all investigations described herein. Seedlings were raised in a modified Hoagland solution (Meyer et al., 1955) in a controlled environment room operating on a 14-hr photoperiod. The temperature was maintained at 76 =*= 1 C during the light period and 72 ± 1 C during the dark period. Relative humidity was 60 to 65 per cent during the light period and 90 to 100 per cent during the dark period. The light intensity was approximately 1000 ft-c at the base of the plants. All experiments were conducted in this room, also. Radioisotopes were supplied in pairs to the plants as constituents of an aerated solution of the following composition: 10~ M KN03, 10~ 3 M MgSO4, 3 x 10~ 4 M Ca(NO3)2, and 2 x 10~ 4 M KH2PO4. After an absorption period of about 2 hr, when a suitable amount of activity could be detected in the plant parts designated for analysis, the plants were harvested and prepared for assay. Plant parts were sliced into small pieces, dried overnight at 50 C, and wet ashed at a temperature of 85 C in a mixture of three parts concentrated nitric acid and one part 60 per cent perchloric acid. The digestion solutions were refluxed until clear, and then evaporated to dryness, resulting in the deposition of a clear precipitate in the bottom of each beaker. The precipitate was dissolved in 1 ml of 0.1 N HC1, transferred to a glass planchet, and dried under 300-w flood lamps mounted 14 inches directly overhead. This procedure ensured the deposition of clear, uniform samples, which was essential for valid self-absorption corrections. Samples were counted with an ultra-thin end window gas flow detector. A sufficient number of counts was totalized in all cases to insure less than 1 per cent error at the 95 per cent confidence level. Counts were corrected for background and coincidence losses in all cases and, in the case of Ca, for self-absorption as well. The counts for Ca were separated from those of P or Cs by the technique of differential absorption (Comar, 1955). Samples were counted first in the normal manner and then recounted with an aluminum absorber (42.2 mg/cm) No. 6 TRANSPIRATION AND TRANSLOCATION 359 inserted between the geiger tube and the sample. The partial counts for the respective radioisotopes were then calculated by substitution of the total counts (Co and Ct) in the following set of simultaneous equations: