An in Vivo Nuclear Magnetic Resonance Investigation of Ion Transport in Maize (Zea mays) and Spartina anglica Roots during Exposure to High Salt Concentrations

Abstract

l h e response of maize (Zea mays 1.) and Spartina anglica root tips to exposure to sodium chloride concentrations in the range O to 500 mM was investigated using 23Na and "P nuclear magnetic resonance spectroscopy (NMR). Changes in the chemical shift of the pH-dependent 31P-NMR signals from the cytoplasmic and vacuolar orthophosphate pools were correlated with the uptake of sodium, and after allowing for a number of complicating factors we concluded that these chemical shift changes indicated the occurrence of a small cytoplasmic alkalinization (0.1-0.2 pH units) and a larger vacuolar alkalinization (0.6 pH units) in make root tips exposed to salt concentrations greater than 200 mM. The data were interpreted in terms of the ion transport processes that may be important during salt stress, and we concluded that the vacuolar alkalinization provided evidence for the operation of a tonoplast Na+/H+-antiport with an adivity that exceeded the activity of the tonoplast H+ pumps. l h e intracellular pH values stabilized during prolonged treatment with high salt concentrations, and this observation was linked to the recent demonstration (Y. Nakamura, K. Kasamo, N. Shimosato, M. Sakata, E. Ohta [1992] Plant Cell Physiol 33: 139-149) of the salt-induced activation of the tonoplast H+-ATPase. Sodium vanadate, an inhibitor of the plasmalemma H+-ATPase, stimulated the net uptake of sodium by maize root tips, and this was interpreted in terms of a reduction in adive sodium efflux from the tissue. S. anglica root tips accumulated sodium more slowly than did maize, with no change in cytoplasmic pH and a relatively small change (0.3 pH units) in vacuolar pH, and it appears that salt tolerance in Spartina is based in part on its ability to prevent the net influx of sodium chloride.High extemal salt (NaC1) concentration is detrimental to many plants both as a result of the decreased extemal water potential, which inhibits cell expansion, and the build-up of sodium and chloride in the cytoplasm, which results in the inhibition of metabolic processes. Because salinity is often a problem in imgated soils and can lead to severe decreases in

Intracellular pH stability in the aquatic resurrection plant Chamaegigas intrepidus in the extreme environmental conditions that characterize its natural habitat

“…Thus the response of C. intrepidus again emphasizes the effectiveness of the pH regulatory mechanisms in this species, and it could be argued that this might be a property of desiccation-tolerant and salt-tolerant plants. In support of this conclusion, pH cyt in the salt-tolerant grass Spartina anglica was much less affected by salt stress than maize root tips (Spickett et al, 1993), and similarly pH cyt was unaffected by PEG treatment in the desiccation-tolerant alga Prasiola crispa (Bock et al, 1996).…”

“…These results contrast with those of Spickett et al (1992), who found that hyperosmotic shock caused a small alkalinization of both the cytoplasm and the vacuole in maize root tips. Suspending maize root tips in solutions with a water potential of k1n35 MPa caused an increase in pH cyt of between 0n05 and 0n1 pH units within 3 h of reducing the water potential of the suspending medium to k1n35 MPa with several non-ionic osmotica, and a similar effect was observed in response to salt stress (Spickett et al, 1993). The constancy of pH cyt in the submerged leaves of C. intrepidus during exposure to reduced water potential also contrasts with the cytoplasmic acidification of 0n2-0n3 pH units observed in leaf discs of Valerianella locusta incubated in hyperosmotic sorbitol (Daeter & Hartung, 1990).…”

“…Hyperosmotic shock can also lead to pH changes, with maize root tips showing a small increase in both pH cyt and the vacuolar pH (pH vac ) (Spickett, Smirnoff & Ratcliffe, 1992) and Valerianella locusta leaf discs showing a decrease in pH cyt (Daeter & Hartung, 1990). By contrast, the desiccation-resistant Antarctic alga Prasiola crispa showed no change in pH cyt in response to a hyperosmotic shock (Bock et al, 1996) and root tips of the salt-tolerant grass Spartina anglica maintained a constant pH cyt during salt stress (Spickett, Smirnoff & Ratcliffe, 1993). This summary suggests that the intracellular pH values in C. intrepidus should be relatively insensitive to changes in both the pH and the water potential of the surrounding medium.…”

“…normally be expected in the spectra of roots (Spickett et al, 1992(Spickett et al, , 1993 and leaves (Loughman, Ratcliffe & Southon, 1989). A 4-h acquisition time was usually required to define the position of the cytoplasmic inorganic phosphate (P i ) signal in the whole plant spectra, which is much longer than the 5-30 min that is commonly used for the spectra of root tips, and this was attributed to the low cytoplasmic volume of the mature tissues of the C. intrepidus plant and the difficulty in maximizing the packing of the sample in the detectable volume of the NMR tube.…”

“…A capillary containing a 2 % (v\v) aqueous solution of the tetraethyl ester of methylene diphosphonic acid was used to provide a reference signal at k22n49 ppm relative to the signal from 85 % (v\v) orthophosphoric acid. Estimates of pH cyt and pH vac were obtained from the chemical shifts of the cytoplasmic and vacuolar P i signals using the calibration curves described by Spickett et al (1993).…”