I. IntroductionThe influence of acid and alkaline reactions of the growth medium on iron relationships in plants has been known, or at least suspected, for over a century. The internal conditions of the plant in their relation to iron metabolism and chlorophyll production are not at all well understood. A number of investigations (22,23,20,18,16,14,10,3,1) have been carried out in which the general conclusion is that the reaction of the growth medium governs, to a large extent, the quantities of iron available to the plant. These observations are in keeping with the results obtained by PATTEN and MAINs (24). Working with HCl solutions containing definite concentrations of iron, they found that, beginning with a pH of 3.5, iron was precipitated in increasing amounts up to pH 6.0, above which point practically no iron remained in solution. This was accomplished by adding, in separate experiments, ammonium hydroxide, sodium hydroxide, and hydrogen sulphide to the hydrochloric acid solutions containing definite quantities of iron. The range of iron precipitation under these conditions thus extends from pH 3.5 as the lower limit to pH 6.0 as the upper limit.All of this work deals with the influence of the reaction of the growth medium on the availability of iron. Very little has been said regarding the possible influence of internal reactions on the availability and mobility of iron within the plant. Investigations (21,12,13,17) in which actual quantitative measurements were employed in determining iron in the plant tissues generally indicate that plants chlorotic from lack of available iron contain as much, or more, total iron per unit weight of tissue than do normal green plants of the same species. From this it follows that certain internal conditions may render unavailable to the chlorophyllous cells the iron already in the plant.INGALLS and SHIVE (15) found that not only does the H-ion concentration of plant tissue fluids fluctuate over a 24-hour day and night period, but that the soluble (filterable) iron content also fluctuates, and, in most cases, is directly proportional to the H-ion concentration of the respective tissue fluids. The pH of fleshy succulents fluctuates much more than does that of thin-leaved or non-succulent forms, the H-ion concentration in all
