Chloride Accumulation as a Homeostatic System: Set Points and Perturbations: THE PHYSIOLOGICAL SIGNIFICANCE OF INFLUX ISOTHERMS, TEMPERATURE EFFECTS AND THE INFLUENCE OF PLANT GROWTH SUBSTANCES

It has previously been observed that Cl^–influx falls withincreasing internal CI^–concentration. This paper attemptsto show that this is a bona fide negative feedback involvedin a homeostatic CI^–accumulation system. First, predictions are made of how the final steady concentrationto which Cl^–is accumulated (Cl]₁) would be expected tovary as external Cl^–concentration (Cl]₀) or temperature,change. This is done for the cases where influx has (a) no feedbackcontrol, or is (b) under error-actuated or (c) under reciprocalfeedback control. Secondly, Cl^–influx and Cl]₁are measured in carrot andmaize root tissue over a range of Cl]_o and temperatures forcomparison with the prediction. Cl)₁varies by only about 20%over a range of Cl]₀ and temperatures which have large immediateeffects on Cl^–influx. This is consistent only with error-actuatedfeedback control. Changes in Cl]₀ and temperature are bestregarded simply as perturbations of Cl^–influx since theyhave no marked effect on Cl)₁. In such a homeostatic system the feedback signal is a functionof (C_R – Cl], ), where C_R represents the value of aninbuilt set point. C_R must be constant with Cl]_o and temperature,and its nature is briefly discussed. It is further predictedthat the accumulated level can only be altered in a controlledmanner by a change in C_R, and that this would alter influx bythe same proportion. The effects of abscisic acid in increasingboth Cl]₁ and Cl^–influx are consistent with an effectprimarily on C^R. These conclusions imply that when varieties or species differin levels accumulated, they do so not because influx isothermsdiffer but rather because their set points differ. Key words: Homeostasis, Chloride, Accumulation