Longitudinal Water Movement in the Primary Root ofZea mays

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Abstract
The rates of transfer of tritiated water (THO) along lengths of excised primary roots of Zea mays have been measured under a variety of conditions. The following values of ‘apparent diffusion coefficients’ for THO in the root tissue have been evaluated: 1.5±0.1×10-5 cm2 sec-1 in roots boiled for 3 min before use,0.5±0.03×10-5 cm2 sec-1 in roots poisoned with 10-2 M NaF,0.9±0.07×10-5 cm2 sec-1 in roots poisoned with 10-2 M NaN3,and 2.1±0.2×10-5 cm2 sec-1 in normal roots. The bathing medium in all cases was 1.0 mM KCl/0.1 mM CaCl2 with the addition of the inhibitors where appropriate.The fourfold increase in the rate of THO transfer in normal roots compared with poisoned ones is attributed to the existence of a long-distance convective flow in the first case, which is terminated by the addition of inhibitors. Since experiments show that this convective flow must occur both acropetally and basipetally with equal velocity, it is thought to occur in the phloem.By assuming the ‘streaming transcellular strands’ model for phloem transport, the rate of movement required to give the observed transfer has been computed as approximately 4.5×10-2 cm sec-1 (160 cm h-1).The earlier report of the existence of a highly impermeable barrier surrounding the xylem vessels has been further substantiated by the experiments reported here.

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