Amino acids in dew – origin and seasonal variation

Abstract

At two sites in the Armenhof district, 10 km east of Fulda, Germany, dew samples were collected from June 1996 to June 1997 and investigated for free and protein-bound amino acids. On account of the high pollen content, at the beginning of June 1996 and in May 1997 total amino acid concentrations were 53–390 μmol l⁻¹, in one sample 922 μmol l⁻¹. At other times the concentration in dew was 8–164 μmol l⁻¹. On 4 and 5 June 1996 the diluted free amino acid fraction (DFAA) of the total hydrolysed amino acids (THAA) at both sites amounted to 35–44% and was predominantly arginine, proline and glutamine/glutamate. Likewise on 11 March 1997 the fraction of DFAA was found to be 39.5% with extremely high arginine and proline fractions. At other times the DFAA-fraction was in the range 14–26%. From July 1996 to June 1997 the amino acid concentrations in the vapours rising from a meadow were also measured and it ranged from 8 to 51 μmol l⁻¹. From July to October 1996 the amino acid composition in the hydrolysates of dew samples and meadow vapours collected overnight were almost identical. The DFAA fraction in the condensation water collected overnight from the meadow varied from 18 to 40%. From 4 to 6 June 1996, on 11 and 13 March 1997 and in the period 16–20 May 1997, the amino acid distribution in dew showed much variation. The percentage fraction of arginine and proline in the hydrolysate increased greatly, whereas that of glycine and serine decreased. The large increase in proline and arginine in hydrolysate is attributable solely to the large amounts of free arginine and proline. This effect occurred in both 1996 and 1997 over several days at both sites at any one time and therefore appears confirmed.

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:

• What is the amino acid content and composition of dew?

• How do amino acid content and composition of dew vary at the same site during the time of vegetation growth?

• 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.