EFFECT OF CYANIDE ON RATE OF EXUDATION IN EXCISED ONION ROOTS

PROCEDURE.-Straight roots without laterals were grown by the water culture method using one-fourth Hoagland solution enriched with microelements and ferrous sulphate. Data were obtained from individual roots 62-65 mm. in length. Each root was almost totally immersed in a relatively large volume of solution confined in a potometer which permitted no evaporation. The potometers were partially immersed in a water bath within a closed chamber; a humid atmosphere prevailed in the enclosed space 1 Received for publication November 27, 1943. Grateful acknowledgment is made to Dr. Harold Webber (National Cotton Council of America, Memphis, Tenn.) for unpublished data on the effect of cyanide on oxygen consumption and to my research assistant, A. A. Horak. above the bath. The apparatus permitted filling and draining the potometers without removing them from the chambers. Details of the potometers are described elsewhere (in press). KCN solutions were made up fresh for each experiment. The pH used was within the range of phenol red (pH 6.8-8.4). Aerated solutions were used throughout. Experiments were run with an initial control period during which an individual root was exposed to tap water, distilled water, or a solution of potassium bicarbonate. This was followed by a period of exposure to cyanide and, finally, a "recovery" period. After exposure to cyanide each potometer was thoroughly flushed to remove all traces of cyanide. In a previous paper (Rosene, 1941a) attention was called to an increase in rates of water absorption and exudation with time after excision. In the present investigation the cyanide solution was usually applied after a five-hour control period whether or not maximum absorption rates had been reached. Figures 1 and 2 show rates of exudation observed throughout the initial control period; the remaining figures show only those rates observed during the latter part of the control period. REVERSIBLE CYANIDE INHIBITION OF EXUDATION.When cyanide solutions of suitable concentrations replace cyanide-free solutions, the rate of exudation falls gradually to an inhibition level. Percentage inhibition is greater when the initial cyanide-free rate of exudation is higher. Figures 1 and 2 show typical results obtained with .01 M KCN.' The inhibition level in both roots was reached in 60 minutes. Comparison of the initial maximum and minimum rates of exudation during absorption of cyanide solution shows a reduction of 63 per cent in one root (fig. 1) and 78 per cent in the other (fig. 2). With certain exceptions, each circle in these curves and those that follow represent the average rate of exudation determined during a ten-minute time period. The exceptions are averages for longer time periods as indicated by the abscissae. Replacement of .01 M cyanide solution by tap water after a relatively short period of exposure results in prompt recovery (fig. 1). With long periods of exposure at this concentration, maximum exudation rates fail to reappear during the subsequent recovery period. With relatively long exposures to cyanide solution of suitable concentrations, root tissue continues to transfer water at the inhibition level (fig. 2 and 3). At lower concentrations of cyanide the time to reach the inhibition level is greater. In figures 3 and 4 the time is approximately 140 minutes; in figure 6 the time is over three hours. Figure 4 shows the effect of a second exposure of an individual root to .005 M cyanide. The inhibition level was not reached during