Growth and nitrogen fixation of legumes at increased salinity under field conditions: implications for the use of green manures in saline environments.

The use of legumes as green manure can potentially increase crop productivity in saline environments and thus contribute to the sustainability of agricultural systems. Here, we present results from a field experiment conducted in the Netherlands that addressed the efficiency of nitrogen (N) fixation by a legume at varying salinities. We grew Melilotus officinalis in an agricultural field using drip irrigation with water salinity varying in electrical conductivity between 1.7 and 20 dS m(-1). In the experiment, nearly 100 % of total plant N in M. officinalis was derived from symbiotic fixation at all but the highest salinity level (20 dS m(-1)). Our results indicated that this species derived substantial amounts of N via symbiotic fixation, the N becoming available in the soil (and thus available to crops) when cultivated legumes senesce and decompose. Based on the growth performance of M. officinalis and its ability to fix N at moderate soil salinity in our field experiments, we identified this species as a promising source for green manure in saline agriculture in temperate regions.

Comparing salt tolerance of beet cultivars and their halophytic ancestor: consequences of domestication and breeding programmes.

Salt tolerance of higher plants is determined by a complex set of traits, the timing and rate of evolution of which are largely unknown. We compared the salt tolerance of cultivars of sugar beet and their ancestor, sea beet, in hydroponic studies and evaluated whether traditional domestication and more recent breeding have changed salt tolerance of the cultivars relative to their ancestor. Our comparison of salt tolerance of crop cultivars is based on values of the relative growth rate (RGR) of the entire plant at various salinity levels. We found considerable salt tolerance of the sea beet and slightly, but significantly, reduced salt tolerance of the sugar beet cultivars. This indicates that traditional domestication by selection for morphological traits such as leaf size, beet shape and size, enhanced productivity, sugar content and palatability slightly affected salt tolerance of sugar beet cultivars. Salt tolerance among four sugar beet cultivars, three of which have been claimed to be salt tolerant, did not differ. We analysed the components of RGR to understand the mechanism of salt tolerance at the whole-plant level. The growth rate reduction at higher salinity was linked with reduced leaf area at the whole-plant level (leaf area ratio) and at the individual leaf level (specific leaf area). The leaf weight fraction was not affected by increased salinity. On the other hand, succulence and leaf thickness and the net assimilation per unit of leaf area (unit leaf rate) increased in response to salt treatment, thus partially counteracting reduced capture of light by lower leaf area. This compensatory mechanism may form part of the salt tolerance mechanism of sea beet and the four studied sugar beet cultivars. Together, our results indicate that domestication of the halophytic ancestor sea beet slightly reduced salt tolerance and that breeding for improved salt tolerance of sugar beet cultivars has not been effective.

No divergence in Cassiope tetragona: persistence of growth response along a latitudinal temperature gradient and under multi-year experimental warming.

BACKGROUND AND AIMS: The dwarf shrub Cassiope tetragona (Arctic bell-heather) is increasingly used for arctic climate reconstructions, the reliability of which depends on the existence of a linear climate-growth relationship. This relationship was examined over a high-arctic to sub-arctic temperature gradient and under multi-year artificial warming at a high-arctic site. METHODS: Growth chronologies of annual shoot length, as well as total leaf length, number of leaves and average leaf length per year, were constructed for three sites. Cassiope tetragona was sampled near its cold tolerance limit at Ny-Ålesund, Svalbard, at its assumed climatic optimum in Endalen, Svalbard, and near its European southern limit at Abisko, Sweden. Together these sites represent the entire temperature gradient of this species. Leaf life span was also determined. Each growing season from 2004 to 2010, 17 open top chambers (OTCs) were placed near Ny-Ålesund, thus increasing the daily mean temperatures by 1·23°C. At the end of the 2010 growing season, shoots were harvested from OTCs and control plots, and growth parameters were measured. KEY RESULTS: All growth parameters, except average leaf length, exhibited a linear positive response (R(2) between 0·63 and 0·91) to mean July temperature over the temperature gradient. Average leaf life span was 1·4 years shorter in sub-arctic Sweden compared with arctic Svalbard. All growth parameters increased in response to the experimental warming; the leaf life span was, however, not significantly affected by OTC warming. CONCLUSIONS: The linear July temperature-growth relationships, as well as the 7 year effect of experimental warming, confirm that the growth parameters annual shoot length, total leaf length and number of leaves per year can reliably be used for monitoring and reconstructing temperature changes. Furthermore, reconstructing July temperature from these parameters is not hampered by divergence.

Ecophysiological response of Crambe maritima to airborne and soil-borne salinity.

BACKGROUND AND AIMS: There is a need to evaluate the salt tolerance of plant species that can be cultivated as crops under saline conditions. Crambe maritima is a coastal plant, usually occurring on the driftline, with potential use as a vegetable crop. The aim of this experiment was to determine the growth response of Crambe maritima to various levels of airborne and soil-borne salinity and the ecophysiological mechanisms underlying these responses. METHODS: In the greenhouse, plants were exposed to salt spray (400 mM NaCl) as well as to various levels of root-zone salinity (RZS) of 0, 50, 100, 200 and 300 mM NaCl during 40 d. The salt tolerance of Crambe maritima was assessed by the relative growth rate (RGR) and its components. To study possible salinity effects on the tissue and cellular level, the leaf succulence, tissue Na(+) concentrations, Na(+) : K(+) ratio, net K(+)/Na(+) selectivity, N, P, K(+), Ca(2+), Mg(2+), proline, soluble sugar concentrations, osmotic potential, total phenolics and antioxidant capacity were measured. KEY RESULTS: Salt spray did not affect the RGR of Crambe maritima. However, leaf thickness and leaf succulence increased with salt spray. Root zone salinities up to 100 mM NaCl did not affect growth. However, at 200 mM NaCl RZS the RGR was reduced by 41 % compared with the control and by 56 % at 300 mM NaCl RZS. The reduced RGR with increasing RZS was largely due to the reduced specific leaf area, which was caused by increased leaf succulence as well as by increased leaf dry matter content. No changes in unit leaf rate were observed but increased RZS resulted in increased Na(+) and proline concentrations, reduced K(+), Ca(2+) and Mg(2+) concentrations, lower osmotic potential and increased antioxidant capacity. Proline concentrations of the leaves correlated strongly (r = 0.95) with RZS concentrations and not with plant growth. CONCLUSIONS: Based on its growth response, Crambe maritima can be classified as a salt spray tolerant plant that is sensitive to root zone salinities exceeding 100 mM NaCl.

Raising groundwater differentially affects mineralization and plant species abundance in dune slacks.

The experience with restoring high water levels (i.e., rewetting) within restoration ecology is limited, and information on changes in soil nutrient supply is scarce. A reduction in nutrient supply is needed to restore the desired oligotrophic vegetation. We determined the effects of restoration of high water levels on decomposition and net carbon (C), nitrogen (N), and phosphorus (P) mineralization rates in wet dune slacks and its consequences for the relative abundance of eutrophic vs. oligotrophic species in the vegetation. This was done by analyzing these variables for valleys that experienced a large groundwater rise vs. valleys that had a small groundwater rise but the same current water level. In addition, the influences of underlying factors (waterlogging, vegetation dieback, and soil dynamics prior to groundwater rise) were separated in a transplantation experiment. Short-term effects of large groundwater rise were a massive dieback of vegetation, increased thickness of the fermentation layer, increased microbial decomposition activity, increased C mineralization, and decreased net N mineralization. Net P mineralization was not affected. The relative abundance of oligotrophic vs. eutrophic species was greater at large groundwater rise. Changes in decomposition and mineralization by large groundwater rise were, however, not caused by the vegetation dieback, but due to previous soil conditions. Soils experiencing waterlogged conditions for 3-4 years or more prior to large groundwater rise had lower C and higher net N mineralization rates at waterlogged conditions than soils that had experienced aerobic conditions, presumably due to differences in labile soil C contents. Contrary to expectations induced by previously determined nutrient pulses and measured vegetation dieback, large groundwater rise resulted in lower soil nutrient supply rates and more oligotrophic vegetation. If these trends continue on the longer term, restoration of high water levels may be effective in restoration ecology to establish oligotrophic, wet vegetation in dune slacks.

Development of a proxy for past surface UV-B irradiation: a thermally assisted hydrolysis and methylation py-GC/MS method for the analysis of pollen and spores.

A method was developed for the analysis of the UV-absorbing sporopollenin monomers p-coumaric acid and ferulic acid in very low numbers of pollen. This enables the analysis of pollen or spores from cultured plants, from herbarium collections, and from sediment, soil, and peat cores. The method involves thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide combined with gas chromatography and mass spectrometry. Pyrolysis, gas chromatographic, and mass spectrometric conditions were optimized for the analysis of minimal amounts of pollen. The method has a detection limit of approximately 60 fresh pollen of Alnus glutinosa and a relative standard deviation of approximately 10% between 100 and 600 pollen.