Amino acids in dew – origin and seasonal variation
Introduction
In the course of the increased activity of research into atmospheric chemistry in connection with the expected climate change there has been an intensification of research into cycles of substances in the atmosphere. Here particular interest has been shown on the release and uptake of substances from air by plants and soils. There are comparatively few studies on the transport and uptake of nutritionally relevant substances via air. Kesselmeier et al (1998a), Kesselmeier et al (1998b) showed that trees, for example, release into the surrounding air large quantities of formic and acetic acids which are taken up by cultivation. Thus a flow of substances from forest trees to agriculture takes place. Measurements of carbonyl sulphide, an organic sulphur compound (COS) in the air surrounding young oak woods have shown that during the day many plants take in carbonyl sulphide in a relatively close relationship to carbon dioxide uptake and use it as a source of organic sulphur (Kuhn et al., 1999). Carbonyl sulphide is formed from the photo-oxidative decomposition of dimethylsulphoxide and CS2.
The release of isoprene and monoterpenes by plants into the atmosphere also appears to be well researched. In a review article, Kesselmeier and Staudt (1999) commented on a large number of investigations in the last few years on the release of this group of substances by 248 species comprising 52 plant families. Usually a particular species produced either isoprene or monoterpene. Research into other substance groups which are released by plants into the air include alkanes, alkanes, organic acids, carbonyl compounds, formaldehyde, acetaldehyde, alcohols, esters and ethers.
According to Kesselmeier (personal communication) most plants investigated release into the atmosphere 2–5% of their photosynthetic capacity in the form of the substance groups mentioned above, though significantly more under stress conditions. If this release is applied to intensive agricultural systems with ca. 15 t dry matter (dm) ha−1 then it means that ca. 0.3–0.8 t dm ha−1 of organic substances are released into the air.
Plants only take up carbonyl sulphide during the day, whereas the soil absorbs them during both day and night (Kuhn et al., 1999). Microorganisms can synthesise amino acids from CO2, NH4 and water (Schlegel, 1985). Carbonyl sulphide could in this respect be an important source of sulphur for the synthesis of the sulphur amino acids.
It is interesting to note that in carbonyl sulphide free air soils release this substance into the air again. This points to a sensitive equilibrium which is influenced by the air concentration (Kuhn et al., 1999). It means that there is a constant exchange of substances taking place into the surrounding air between plants and ecosystems including the soil about whose processes and functions little as yet is known.
A further group of substances which are transported by air are amino acids, peptides and proteins. Spitzy (1990) reported that Fonselius (1954), Munczak (1960), Degens et al. (1964), Dean (1963) and Sidle (1967) had already detected free and peptide-bound amino acids in coastal and continental rain. Mopper and Zika (1987) found free amino acids and primary amines in coastal rain. They emphasised that the exact sources of amino acids in rain are unknown. Rain flushes out the layers of air close to the earth. One gram of condensed water extracts ca. 2–3 m3 air (Mopper and Zika, 1987). Spitzy (1990) investigated aerosols and rain in the northern Indian Ocean. They found amino acid concentrations of 0.47–1.13 nmol m−3 in air. There are little or no investigations so far into the amino acid and protein content of the layers of air close to the earth's surface and their release or uptake by soils and plants.
In 1996 and 1997 we studied the amino acid content of dew (and rain) as natural extraction media. Furthermore, water evaporated from meadows and unplanted fields were obtained and their amino acid content was investigated. The following questions were raised:
- 1.
What is the amino acid content and composition of dew?
- 2.
How do amino acid content and composition of dew vary at the same site during the time of vegetation growth?
- 3.
What are the sources of the amino acid contents of dew?
This paper concentrates on the results of investigations of dew and vapours from soils covered with vegetation. As hardly any work on this subject could be found in the literature, we conducted a preliminary investigation of the phenomenon. Thus the results of amino acid analysis in dew samples point partly to new phenomena whose temporal variablity was tested at two sites. The possibility of carrying out these investigations arose in the context of another research project.
Section snippets
Materials and methods
Fifty micrometers thick 2×2 m and 2.5×2.5 m plastic foils pre-washed with demineralised water were hung on posts at two sites each evening at 22:00 h to collect the falling dew. The Armenhof House site was in Armenhof village on the western entry to the district in the vicinity of dwellings. Half of the surroundings were meadows, the other half comprised country gardens, gravel tracks and artificial surfaces. The village of Armenhof (ca. 350 m above sea level) is ca. 10 km east of Fulda (255 m above
Results
Free amino acids and the amino acid contents of hydrolysates were measured in dew as well as in the condensation of water vapours rising from meadows at night and in certain instances by day. The hydrolysate analyses of dew samples measured both free and peptide-bound amino acids. In selected samples, free amino acids were detected separately. We could not measure cystine by HPLC in the hydrolysates and the content was too low for the amino acid analyser. The following overview shows the total
Discussion
The results above show what is already easily visible in the yellow colouration of dew, namely that flower pollen from May to the beginning of June is the main cause of the high protein content of dew. From the beginning of June the dew was clear and the protein content was correspondingly reduced. Comparing amino acid distributions in dew and evaporation water shows that the bulk of free protein bound amino acids in the valley meadow derives mainly from transpiration. Plants secrete amino
Acknowledgments
This research received financial support from the Helixor Foundation, Rosenfeld and the tegut. Foundation, Fulda. It was carried out at the Kwalis Quality Research Institute. Fulda, when the author was one of its directors and Project Leader on Protein Quality. The author would like to thank Mrs Barbara Gies for carefully carrying out the analysis. The Society for Goetheanistic Research supported the publication of the results. I would like to thank David Heaf for translating the German
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