Changes in Ion Transport in Spring Wheat during Ontogenesis

Abstract

AbstractThe influence of plant age on free space uptake to the root, rate of continuous uptake and translocation of potassium and sulphate was investigated during about 100 days in intact, high‐salt plants of spring wheat (Triticum aestivum L. cv. Svenno). The plants were grown in a green‐house in complete nutrient solution. For the short term uptake experiments, the test solutions were labelled with 36Rb+ and 35S‐sulphate.Free space uptake to the roots increased during the entire growth period. The SO2‐4 free space uptake was divided into a Water Free Space (WFS) fraction and a labile‐bound fraction. The labile‐bound SO2‐4 was considered to be constant during development, and the WFS fraction of SO−24 could then be computed. WFS increased from 2% of total cell volume in 1‐day‐old plants to 30% in 100‐day old plants, apparently due to an increasing proportion of freely permeable root cells. As the WFS fraction of the free space uptake was known, the binding capacity (BC) of K+(86Rb−) of the cell walls and at the cytoplasmic surfaces could be computed. It is suggested that the increasing BC for cations with age was due to an increasing proportion of soluble pectate in the cell walls.Except for the initial 20 days, the continuous ion uptake rate decreased during development. It is suggested that the low uptake rate in young plants is limited by the energy supply to the roots and that the decreased uptake in older plants is due to the increasing proportion of metabolically inactive and collapsed roots. At the end of the cultivation period the ion uptake rate increased at the same time as there was a shift from active to passive ion uptake. This was shown by uptake experiments with 2,4‐dinitrophenol (2,4‐DNP).By changing the air humidity around the shoots and using 2,4 DNP, it was shown that ion and water uptake were closely linked to root activity in young plants but that transpiration pull became gradually more important for water uptake with age.