On the Mechanism of Ammonium Ion Uptake by Maize Roots

{1) NH4+-ion uptake experiments are performed with intact maize plants in continuously-flowing water cultures. The NH4~ uptake-concentration curve fits best with a Langmuir adsorption equation. However, no strict saturation effect at the higher concentrations is observed. This can be explained by assuming an everincreased participation of deeper cell layers in the NH4+-ion uptake process at the higher NH4+ concentrations. An agreement of the data with Langmuir’s adsorption equation, however, does not imply that NH4+-ion uptake is a non-specific adsorption reaction. The same equation also holds for specific binding reactions such as enzyme reactions. In the present study, a first binding to specific carriers is proposed. Some of the properties of the assumed carriers can be studied in intact living roots, because no measurable diffusion resistance exists between the place of first binding and the external medium. The uptake-concentration curve shows that the NH4+-ion uptake reaches its saturation value at about 10 p.p.m. NH4+. Its “half-value” concentration lies at 0.23 p.p.m. NH4+. This very low half-value concentration points to a great affinity between the carrier and NH4+ ions. No measurable difference in the rate of NH4+-ion uptake between pH = 6.0 and pH = 4.6 is observed. Therefore, between these limits, pH does not affect the carrier mechanism nor does the H+ ion compete with theNH4+ion for the same sites in the carrier. Arguments are put forward that the NH4+-ion uptake is a specific and active uptake, independent of the uptake of other species of ions. (2) The well-known physiologically acid reaction caused by NH4+-ion absorption is quantitatively studied at a constant pH, again using the flowing culture solutions by which the excess of H+ions produced is continuously removed. It appears that in vigorously growing maize plants at low NH4+ concentrations (below 3 p.p.m.) a nearly quantitative exchange ratio of 1 : 1 of H+ ions for NH4+ ions exists. At higher NH4+ concentrations, about 80 % H+ ions are released for NH4+ ions absorbed. At still higher NH4+ concentrations (about 20 p.p.m.) the H+-ion release tends to decrease gradually, probably due to an ever-increasing anion uptake which has not yet reached its saturation value. At pH = 4.6 the ratio of NH4+-ion uptake to H+-ion release is not measurably different from that at pH 6.0. In older maize plants the H+-ion release is markedly lower, because there exists an exchange of NH4+ ions of the medium for K+ ions of the root. The K+-ion release can be considerable. In addition, there is a lower NH4+-ion uptake due to a decreased capacity to synthesize in older plants. Arguments are put forward in favour of the view that not only NH4+-ion uptake, but also H+-ion release, is linked to active metabolic processes. (3) Respiratory changes due to NH4+-ion uptake are studied in excised maize roots with the standard Warburg manometric technique. These experimentsespecially those with NH4+-bearing exchange resins—suggest a cation-induced respiration, i.e. a salt respiration caused solely by an active NH4+-ion uptake. Moreover, it is shown that the respiratory response depends primarily on a salt deficiency induced by protracted washing or starvation in distilled water, dilute salt solutions, or tap water. In fact, these responses can be obtained more specifically by making plants deficient in one particular mineral requirement: addition of the missing element—provided there is a sugar reserve—will immediately give a respiratory response. Salt respiration can be initiated in roots suspended in relatively strong, oneelement deficient, salt solutions by addition of the missing mineral element. Thus, a “low-salt” condition or a deficiency in a particular ion is the prerequisite for a particular related respiratory response. As both processes (metabolism as well as ion uptake) are dependent on a deficit of a specific ion, it seems probable that the connexion between salt uptake and salt respiration would be of an indirect nature, i.e. the salt respiration would not be directly linked with ion transport. Nitrogen and phosphate starvation lead to considerable depression of the respiration rate, even when the carbohydrate content of the root tissue is high. Subsequent addition of nitrogen or phosphate salts markedly increases the respiration rate, while other ions do not affect the respiration to any degree. However, potassium starvation in maize roots produces an increase of respiration rate which is again specifically reduced by potassium salt addition. Once again arguments seem to be more in favour of a connexion of salt respiration with metabolism rather than with ion transport. The opinion is expressed that the effect of an ion on respiration is in some way connected with the role and the fate of that particular ion in metabolism. (4) Although the results of the present study do not strictly disprove Lundeg&rdh’s “anion respiration” hypothesis, they can be completely understood by the assumption of an “active” cation uptake and “cation respiration”. In fact, if no other results were known, this would be the obvious conclusion. In principle, three different mechanisms of active salt uptake could be concieved: (a) an active anion uptake and a passive cation uptake, (b) an active cation uptake and a passive anion uptake, (c) an active anion as well as an active cation uptake. A priori, no special preference can be given to any one of these mechanisms. On the basis of the evidence discussed in Chapter V, Lundegardh preferred concept (a). The tendency of this thesis is, however, to emphasize that concept (c) is surely not less acceptable.