Water‐extractable sulfhydryl content of spinach leaf discs increased up to four‐fold when they were incubated with sulphate (10–100 mM) for 20 h in light or darkness. The accumulated sulfhydryl compound was reduced glutathione. An increased glutathione content did not result in a higher frost‐tolerance of the spinach leaf discs. Both freezing temperature and time of exposure to freezing, determined as the point at which 50% of the cells were killed, remained unchanged after incubation with sulphate. These observations suggest that a sulfhydryl compound as glutathione does not play a direct role in protection of plants against freezing injury.

Of the two Taraxacum microspecies used. Taraxacum sellandii Dahlst. usually occurs in grasslands with a high nutrient level; Taraxacum nordstedtii Dahlst. is generally restricted to undisturbed and mineral‐poor habitats. Growth response curves for internal N and P were established, based on relative yield of (whole) plant tissue water and (whole plant) internal mineral concentration on a tissue water basis. Critical nutrient concentrations of N and P were determined from the response curves derived. For both macroelements, T. nordstedtii showed lower critical nutrient concentrations. The difference in critical N concentrations coincided with differences in internal NO3‐3 concentrations between the microspecies. Finally, we discuss the use of tissue water as a (whole) plant growth parameter and internal mineral concentration on tissue water basis as a parameter describing the mineral status.

The work reported here forms part of a larger research program on the physiological aspects of adaptation of grassland species, in particularly Plantago species, to different habitats.Erdei and Kuiper I-3] have already published the results of a comparative study concerning the effects of saline conditions on growth, ion-content and translocation in three ecologically different Plantago species. The investigated species were: Plantago media, a glycophyte; Plantago maritima, a halophyte and Plantago coronopus, which occurs in habitats which may vary in salinity.The most important conclusions of these experiments were that (1) Plantago media was sensitive to 25 mM NaC1, while Plantago coronopus and Plantago maritima could grow in 150 and 300 mM NaC1 respectively, and that (2) the three species did accumulate Na § in the shoot and maintained a relatively low Na § level in the root. K § Mg 2+ and Ca 2 § levels of shoot and roots decreased with increasing salinity.Our results correspond with those of Erdei and Kuiper 1-3] for Na § uptake in the roots and further translocation to the shoot in the three species in the presence of 150mM NaC1 (Fig. 1). Na + uptake rates in the roots of Plantago media and Plantago maritima were similar, while Plantago coronopus showed a higher rate (Fig. 1A). Translocation ofNa § to the shoot of Plantago media started after a lag period of about two hours (Fig. 1B). Na § translocation in Plantago coronopus also showed a lag period, although to a lesser extent. Translocation in Plantago maritima responded without delay. This difference in translocation

The etTect of chilling was studied in leaf dises from the ehillingsensifive Cucumis sativus L. by measurement of the electrolyte leakage from the discs, by oxygen uptake and by uncoupling of respiration with 2,4-dinitrophenol. Short periods of chilling are characterized by minor significant increases of electrolyte permeability, of respiration and of preserved ability to be uncoupled by 2,4-dinitrophenol. A longer period of chilling resulted in a strongly increased electrolyte permeability, in reduced oxygen uptake and in disappearance of uncoupling by 2,4-dinitrophenol. In general the indueed ehanges in permeability and respiration were reversible within 4 days of ehilling, if the discs were plaeed at 25 °C after chilling.

Mg2+‐ and Ca2+‐uptake was measured in dark‐grown oat seedlings (Avena sativa L. cv. Brighton) cultivated at two levels of mineral nutrition. In addition the stimulation of the ATPase activity of the microsomal fraction of the roots by Mg2+ was measured. Ca2+‐uptake by the roots was mainly passive. Mg2+‐uptake mainly active; the passive component of Mg2+‐uptake was accompanied by Ca2+‐efflux up to 60% of the Ca2+ present in the roots. In general Mg2+ ‐uptake of oat roots was biphasic. The affinity of the second phase correspond well with that of the Mg2+‐stimulation of the ATPase activity, in low‐salt roots as well as in high‐salt roots and in roots of plants switched to the other nutritional condition. Linear relationships were observed when [phase 2] Mg2+‐uptake was plotted against Mg2+‐stimulation of the ATPase activity of the microsomal fraction of the roots. In 5 days old high‐salt plants 1 ATP (hydrolysed in the presence of Mg2+ J corresponded with active uptake of a single Mg2+ ion, but in older high‐salt roots and in low‐salt roots more ATP was hydrolysed per net uptake of a Mg2+ ion. The results are discussed against the background of regulation of the Mg2+‐level of the cytoplasm of root cells by transport of Mg2+ by a Mg2+‐ATPase to the vacuole, to the xylem vessels, and possibly outwards.