sis or decomposition of organic acids accomplishes such regulation in response to nonstoichiometric an-
ion and cation absorption.
When plants are transferred to a salt solution with K 2 SO 4 , they usually absorb an excess of Kover
SO 42 equivalents. The excess of positive charge in the cytosol is compensated by proton excretion, which
results in a transient elevation of cytosolic pH. An elevated cytoplasmic pH induces respiratory CO 2 to
form bicarbonate; this then enhances phosphoenolpyruvate carboxylase activity and consequent malic
acid synthesis. The consumption of phosphoenolpyruvate enhances its replenishment, via glycolytic for-
mation of phosphoglyceric acid from triose phosphate. Thus, replacing KOH with K 2 -malate [177] (Fig-
ure 2A) adjusts the elevated cytosolic pH.
Excess uptake of anionic equivalents may occur when plants are presented with Ca(NO 3 ) 2. The tran-
sient acidification of the cytoplasm by cotransported protons is regulated by activation of malic enzyme
and malic acid decomposition [176] (Figure 2B).
352 JACOBY AND MORANFigure 2 Charge compensation and pH regulation during cation (A) and anion (B) uptake by plant tissues
when the uptake is not balanced by a counterion. U, uniport; S, symport; NAD, nicotinamide adenine dinu-
cleotide; PGA, phosphoglyceric acid; PEP, phosphoenolpyruvate; OAA, oxaloacetic acid. Details in text.