SummaryThe epidermal bladders of several Atriplex species contain high concentrations of ions. Chloride was secreted from the solution or the lamina to the bladders, against a concentration gradient. Transfer of 36CI to the bladders was strongly light stimulated, but uptake to the lamina was much less sensitive.Electrical potential measurements showed that the vacuole of the bladder cell was highly electronegative with respect to the bathing solution. Switching from dark to light and vice·versa resulted in transient changes in potential. In some instances the potential settled to a level which was more negative in the light than in the dark. These observations suggest that uptake of chloride into the bladders is an active process.Autoradiographs of intact and sectioned bladders after exposure to K.35S04 and K36CI showed that radioactivity was concentrated in the stalk cell and peripheral cytoplasm of the large vacuolated bladder cell. Electron microscopy showed that the stalk cell and peripheral cytoplasm of the bladder cell contained chloroplasts, numerous mitochondria, much endoplasmic reticulum, and many small vesicles. The stalk cell has the submicroscopic characteristics of a salt gland and, as it is con· nected to the bladder cell and the epidermal cells by plasmodesmata, may secrete ions from the leaf symplasm to the bladder cell.

Abstract. Beet discs aged in 0.5 mM CaS04 develop a capacity to absorb K+ and Cl-from solutions of low concentration. The initial influx of these ions is described by a hyperbolic relationship with concentration in the range 0.01 to 0.5 mM KCI, which is identical with the system 1 absorption isotherm found in other tissues. A second hyperbolic isotherm, attributable to system 2, i's found at higher concentrations (1-50 mm Ka).When the transport of labeled ion to the vacuole is studied by wash-exchanging the bulk of the cytoplasmic label following the absorption period, it is noted that in the range of system 1, isotope influx to the vaouole increases with time as the concentration of labeled ions in the cytoplasm increases, while in die range of system 2, influx to the vacuole is oonstant from the beginning. Diminution of the cytoplasmic specific activity during radioisotope absorption by prefilling the cytoplasm with the analogous unlabeled salt, markedly reduces subsequent radioisotope uptake to the vacuole only in the range of system 1. These experiments suggest that the cytoplasm serves as a mixing chamber, and that the plasma membrane controls ion uptake to the tissue at 'low concentrations, indicating thazt the system 1 isotherm reflects ion movement into the cytoplasm through the plasma membrane. Flux experiments support this conclusion, showing that development with age of the system 1 isotherm corresponds to a quantitatively similar increase in plasma membrane influx in 0.2 mM KC1.At higher concentrations the outer membrane no longer rate-limits entry of ions to the vacuole. Isotope influx under these conditions, described by the system 2 isotherm, presumably reflects movement across the tonoplast.The initial rates of absorption of labeled ions by plant tissues over a wide range of concentrations show 2 or more hyperbolic isotherms which presumably represent carrier mechanisms having different apparent K. values (5). Torii and Laties (23) recently suggested a spatial separation of the 2 principal ion absorption svstems, which differ in their apparent affinities for ions by almost 3 orders of magnitude. On the basis of a comparison of the ion absorption isotherms in largely non-vacuolated root tips and proximal vacuolated cells, they proposed that system 1 (0.01-0.5 mM) reflects ion transport across the plasma membrane, whereas system 2 (1-50 mM) reflects transport across the tonoplast. -Other distinctions, with regard to inhibitor sensitivity '(12), counter ion effects (10) and organic acid synthesis )(24), have supported this separation. The dual mechanism of ion transport, developed so extensively by Epstein (5, 6) was further explored in relation to symplastic transport

SummaryThe leaves of AtripZe:c and eight related genera of the Chenopodiaceae from semi-arid Australia oontained high levels of oxalate. Most of the oxalate was in the soluble form and was correlated with high cation content generally rather than with calcium level. In water-culture experiments calcium absorption and oxalate synthesis varied independently.Other oulture experiments with various levels of salts or over a time course showed that oxalate produotion was geared to the excess cation content of leaf tissue (:E Na++K+-CI--NO s )' Oxalate balanced 75% of this cation exoess in A. 8pongio8a leaves. Changes in the oation excess and insoluble nitrogen content of growing leaves were similar suggesting that cation exoess arose principally from the metabolic incorporation of nitrate. Soluble nitrate levels remained low in growing leaves but exudation experiments showed that nitrate transport to the shoot was consider!Lble, implying that nitrate was incorporated into organio form in leaf tissues.These experiments indicate that oxalate is synthesized in Atriplex leaves in response to the cation exoess created by metabolic incorporation of nutrient anions, suoh as nitrate, during growth. It is likely that oxalate and oations are accumulated to the vacuole of leaf cells.

SummaryLeaf tissue cut into disks was unsuitable for quantitative ion-absorption studies because cells in the interior did not equilibrate with the external solution. Ion entry was restricted to the cut surface and diffusion was too slow to permit equilibration of the whole disk (apparent diffusion coefficient for sodium at O· 5°C was 1 X IO-L 3 X 10-7 cm2 sec-I). However, leaf slices 0·5 mm in width permitted rapid access of electrolyte to all cells and were used to study uptake of monovalent cations and oxalate.Cells of mature leaves were near to flux equilibrium for potassium, but not for sodium, and large changes in the ratio of monovalent cations could be induced in single salt solutions. Thus, when leaf slices were placed in N aCI, sodium was taken up largely in exchange for potassium lost from the vacuole; any small net influx of monovalent cation was balanced by chloride absorption. This monovalent exchange uptake was sensitive to low temperature and to carbonyl cyanide m-chlorophenylhydrazone (CCCP). Sodium influx and potassium efflux were reduced by low concentrations of calcium.The kinetics of the uptake of externally supplied oxalate were consistent with its transfer across the tonoplast to a slowly exchangeable compartment. This transfer was reduced by low temperature, which also reduced the concurrent metabolism of oxalate to carbon dioxide, and by CCCP at 25°C, which did not interfere with oxalate metabolism.

SummaryMalate and aspartate, labelled in the C-4 position, were the only stable products of short-term dark 14C02 fixation in Atriplex spongiosa leaves. Label was subsequently transferred to citrate, glutamate, and other amino acids. Oxalate was slowly labelled during dark fixation and was a major labelled product after 18 hr. The accumulation of label in oxalate and other acids was inhibited by 20 mM malonate suggesting that oxalate stemmed from tricarboxylic acid cycle intermediates. Oxalate was slowly labelled during metabolism of [U-14C]glucose and [1,5-14C]citrate, but [1_14C]glyoxylate was readily oxidized to oxalate. These data are consistent with oxalate production from glyoxylate derived from isocitrate during dark acid metabolism. Phosphoenol pyruvate carboxylase, isocitrate lyase, and glycolate oxidase activity in cell-free extracts of A. spongiosa leaves provided further evidence for this pathway.The significance of these dark reactions which, in photosynthetic tissue, conserve a two-carbon unit and lead to net acid synthesis, is discussed.