Ion concentration and saturation water content were measured in various aged leaves of halophytes growing in saline soils east of lake Neusiedlersee (Austria).All species investigated showed a substantial sodium accumulation within the maturing organs accompanied by a considerable potassium decline. In most species chloride concentration rises distinctly with increasing leaf age, too, whereas concentration shifts of alkaline earth ions and of sulfate (except in Plantago maritima, Lepidium crassifolium and Crypsis aculeata) are of comparably less importance.Saturation water increases markedly in succulent species (Suaeda maritima, Chenopodium glaucum, Spergularia media, Lepidium crassifolium) and to a less degree in xerophytic monocotyledons (Puccinellia distans, Crypsis aculeata, Bolboschoenus maritimus). However, this surplus of water in older leaves is not sufficient to dilute the salt to such an extent that a rise in concentration can be prevented (except chloride in Suaeda maritima and Chenopodium glaucum).Rosette plants (Triglochin maritimum, Plantago maritima, Scorzonera parviflora, Aster tripolium) with the ability to renew their leaves continuously throughout the growth period are characterized by only insignificant changes of saturation water content with increasing leaf age. In these plants, shedding of old salt-saturated leaves is thought to be the main strategy for salt regulation.A modification of Steiner's classical concept of different "salt regulation types" is proposed, based on original findings about salt regulation in Austrian halophytes and on new bibliographical data upon additionally revealed regulatory principles in halophytes and saltaffected nonhalophytes.

The ionic relations in halophytes from the region east of Neusiedler Lake in Austria have been investigated. The study encompasses the following compounds: Na, K, Mg, Ca; Cl, SO, phosphate, nitrate, and organic acids.The ionic composition varies substantially among the species investigated. Frequently a specific pattern of ion content can be found within a specific taxon. a) Dicotyledons: Extraordinary accumulation of sodium, high intake of inorganic ions (mainly Cl, less SO), and regular occurrence of free oxalate, causing low Ca-concentrations, are typical for Chenopodiaceae and Caryophyllaceae (Spergularia media). Lepidium crassifolium shows similar sodium preponderance accompanied by high levels of SO, Cl, and organic anions other than oxalate (mainly citrate and malate). The remaining dicotyledons show rather moderate salt content; Asteraceae and Cichoriaceae prefer Cl, and Plantago maritima accumulates high amounts of SO as well as Cl. Malate and citrate are, without exception, the main organic anions. The K:Na ratios in dicotyledons (esp. Chenopodiaceae and Lepidium-Brassicaceae) lie far below unity. b) Monocotyledons: In marked contrast, Poaceae, Cyperaceae, and Juncaceae are characterized by a general low salt status. With few exceptions, Cl is stored as the main inorganic anion, phosphate reaches higher levels than in dicotyledons and in many cases lies in nearly the same concentration range as SO. The pattern of organic anions with malate and citrate as the main acids, does not basically differ from nonhalophilous species. In any case, K:Na ratio exceeds unity. Triglochin maritimum is the only monocotyle species exhibiting as high salt content and low K:Na ratios as dicotyledons. Nitrate and phosphate are of minor quantitative importance with regard to their osmotic efficiency; their mEq percentage of the total anion concentration range between 0.03 to 2.6 (NO) and 0.5 to 13.6 (phosphate), respectively.The results are discussed from different points of view: on the one hand, the general problems of salt tolerance, on the other hand, the taxonomical and ecological aspects. beneficial to plant growth in view of salt sensitivity of enzymatic reactions. However, low osmotic potential of cell sap, and consequently, the acquisition of water is guaranteed by storing high amounts of sugars: according to our data (Albert and Popp, in preparation) total sugar concentration in halophilous Poaceae, Cyperaceae, and Juncaceae amounts to up to 200 mmol·l fresh water, whereas in salt rich dicotyle species the sugar content is comparatively low (up to 50 mmol).

Aluminum toxicity limits plant growth in acid soils. Because of Many of these studies were conducted with solution their advanced state of weathering, acid soils of the tropics also tend culture rather than with soil to circumvent the problem to be deficient in nutrients. A realistic assessment of plant adaptation to these soils would therefore require Al-toxic conditions under which that several soil properties change simultaneously when growth is simultaneously limited by nutrient deficiency. We developed soil acidity is modified. Initially, solutions contained and tested a nutrient solution for this purpose. We analyzed soil nutrient levels far in excess of those required for maxisolutions of two Oxisols from the Colombian savannas. Nutrient conmum plant growth rates (see critiques by Blamey et al. centrations were extremely low (ionic strength Ͻ1.7 mM). Nitrifica-[1991] and Edmeades et al. [1995]). In such solutions, tion during incubation of soil samples acidified soil solutions, resulting Al toxicity is alleviated as a result of physicochemical in a release of cations from the exchange phase, an increase in the interactions between Al and other ions, including the activity of Al 3؉ , and a decrease in that of H 2 PO Ϫ 4. Predicted ion formation of nontoxic complexes with OH Ϫ , SO 2Ϫ 4 , and activities were taken as guidelines for designing a nutrient solution silicate ions; precipitation of Al as hydroxide or phosthat simulates these soil solutions. Growth of well-adapted signalgrass phate; and high ionic strength per se (Blamey et al., (Brachiaria decumbens cv. Basilisk) and less-adapted ruzigrass (Brachiaria ruziziensis cv. Common) in this solution mirrored the 1983; Blamey et al., 1991; Wheeler and Edmeades, 1995; interspecific difference in forage yield that had previously been ob-Kinraide, 1997; Ma et al., 1997). In addition, high conserved in a field close to where one of the soils originated. This centrations of divalent and, to a lesser extent, monovasuggests that the designed solution may be a realistic approximation lent cations can ameliorate Al toxicity, presumably beto chemical soil properties that limit forage productivity. The different cause they reduce cell-surface negativity (Kinraide and growth response of the two grasses was apparently due to increased

No abstract

Pelagic food web processes with focus on phyto-and bacterioplankton dynamics were followed in a high Arctic lake on Ziegler Island, Franz Joseph Land archipelago, during July and August 1996. The oligotrophlc, permanently ice-covered lake is characterized by a rather short pelagic food web with rotifers representing the highest trophic level. Phytoplankton biomass and net primary production averaging 1.8 pg chl a 1-' and 22 pg C 1-' d-', respectively, decreased dunng the investigation period. Photosynthetic extracellular release (P,,) corrected for bacterial uptake was high and contributed between 31 % (July) and 96% (August) of total primary production. The abundance of bacteria (9.3 to 17.3 X 10' ml-l) and flagellates (7.8 to 17.3 X 10' ml-l) varied within a narrow range. Bacterioplankton production ranging from 1.2 to 3.9 pg C 1-' d-' and bacterial growth rates (0.1 to 0.3 d-') increased with increasing % P,,, indicating that algal exudates are the major carbon source for bacterioplankton. Bacterial carbon demand (assuming a 50% growth efficiency) amounted to 19% of gross pelagic primary production (P,,,, + P,,) and 31 % of P,, during the investigation period. Evidence was found that bacterioplankton metabolism responds quickly to slight increases in temperature (1.2 to 2.0°C) with increased growth. Overall, production rates of phyto-and bacterioplankton in this high Arctic lake are simdar to other Arctic lakes studied thus far, and the food web structure is even simpler than in most lakes at similar latitudes.