The absorption and translocation of sodium by maize seedlings

Potassium supply promotes the mitigation of NaCl-induced effects on leaf photochemistry, metabolism and morphology of Setaria viridis

Soil salinity has the potential to severely affect crop performance. To maintain cell functioning and improve salt tolerance, the maintenance of K⁺ homeostasis is crucial in several plant metabolism processes. Besides, potassium fertilization can efficiently alleviate the perilous effects of salinity. We characterized impacts in Setaria viridis exposed to NaCl and KCl to underlying photochemistry mechanisms, K⁺ and Na⁺ shoot contents, enzymatic activity, electrolytic leakage, and morphological responses focusing on non-stomatal limitation of photosynthesis. Plants were exposed to sodium chloride (NaCl; 0, 150 and 250 mM) and potassium chloride (KCl; 0, 5, 9 mM). The exposure to NaCl affected S. viridis leaves morphological and physiologically. Plants submitted to 150 mM showed reductions in performance indexes (PI_abs and PI_total; JIP-test), and the presence of positive K- and L-bands. Plants exposed to 250 mM exhibited blockage in electron flow further than Q_A within 48h and permanent photoinhibition at 96 h. The presence of 9 and 5 mM of KCl counteracted the effects of NaCl on plants submitted to 150 mM, concomitant with increases in K⁺ accumulation and cell turgidity conservation, causing positive effects in plant growth and metabolism. Neither KCl concentrations were effective in reducing NaCl-induced effects on plants exposed to 250 mM of NaCl. Our results support the conclusion that greater availability of K⁺ alleviates the harmful effects of salinity in S. viridis under moderate stress and that application of KCl as means of lightning saline stress has a concentration and a salt level limit that must be experimentally determined.

Ion measurements by X-ray microanalysis in unfixed, frozen, hydrated plant cells of species differing in salt tolerance

A technique is described for X-ray microanalysis of unfixed, frozen, hydrated higher plant cells using a scanning electron microscope in conjunction with a cryostage. Freezing in liquid N2 is the only preparative step required. Using this method, ion distribution was compared in the roots of Zea mays L. (termed a salt excluder) and Hordeum vulgare L. (which is rather more tolerant), both grown in the presence of NaCl. Distinct differences were observed between the two species in Na, K and Cl distribution. Evidence is presented to support the hypothesis that reabsorption of Na from the xylem sap in the mature regions of the root may occur in salt-sensitive glycophytes such as Z. mays.

Measurement of profiles of potassium activity and electrical potential in the intact root

Profiles of potassium activity and electrical potential in the vacuoles of cells across the intact root of Helianthus annuus have been measured. No gradient in either potassium activity or potential difference was detected from epidermis to pericycle. The trans-root electrical potential was found to be made up of two components, a large P.D. between the outside solution and the outer cells and a small P.D. at the pericycle-xylem interface. The results indicated that all the living cells of the root have the same capacity to actively accumulate potassium.

[Effect of K(+) on Na (+) fluxes and transport in barley roots: K(+)-stimulated Na (+) efflux in the root cortex]

Barley roots grown on a nutrient solution containing 1 mM Na(+) but no K(+) are capable of a considerable Na(+) transport via the symplasm of the root and the xylem vessels. K(+) added to the medium surrounding the root cortex severely inhibits this transport after a lag period at a high rate constant (Fig. 3).It is likely that the fluxes of Na(+) are changed drastically during this transition from low to high K(+) status. Although originally limited to steady state fluxes, the extended method of efflux analysis for excised roots (Pitman, 1971) has been applied to the non-steady fluxes which occur upon the addition of K(+) to the roots. It is shown that besides other changes the efflux of (22)Na(+) through the cortex of barley roots is stimulated instantaneously (Fig. 5) by the addition of K(+) and presumably by an influx of K(+) ions. From this a transient, K(+)-stimulated Na(+) efflux at the plasmalemma of the cortical cells can be estimated. It amounts to 10.9 μ moles/g fw · h compared to the control efflux of 3.3 μ moles/g fw · h without K(+).The stimulated efflux is attributed to a Na(+) efflux pump at the plasmalemma and is thus related to the K-Na-selectivity of barley plants. The inhibition of the Na(+) transport by K(+) is probably a consequence of this increased efflux of Na(+) from the symplasm through the root cortex.