Deuterium Content in Organic Material of Hosts and Their Parasites
407
are always significantly richer in deuterium (in their dry mass) than their
hosts.
The exception with the CAM hosts may be due to the fact that mistletoes
obtain their tissue water (used in their own photosynthesis) from the transpiration stream of the host, which is not very much enriched in deuterium,
while the CAM host also uses water from water-storage tissues or vacuoles,
which can be considerably enriched in deuterium (ct. Ziegler 1988).
There is no explanation of the enrichment of deuterium in the biomass in
galls, holoparasites, mistletoes, and fungi (here in contrast to the reduction
of the 13C content) on C 3 hosts, only hypotheses. The following may be
mentioned:
- The tissue water in the galls or parasites could be richer in deuterium
(less negative oD) than the tissue water in the host or substrate, leading
to a deuterium-enriched organic matter. This could happen in the galls
and mistletoes only, since the holoparasites and the fungi perform no
photosynthesis.
- A deuterium enrichment of the tissue water of the galls and parasites
could result in an increased deuterium content of some chemical components with exchangeable (-OH or NH2 bound) hydrogen groups (e.g.,
in cellulose or proteins (cf. Bonhoeffer 1934)).
- The galls and parasites could generally have much lower lipid contents
than the source plants. This hypothesis seems quite improbable, since a
lower lipid content would also mean a relative enrichment in 13C and this
is mostly not the case.
- The galls and parasites may have an especially intensive respiration and
this process preferentially uses deuterium-poor substrates.
Since these hypotheses are experimentally accessible, we may soon have
answers to our questions, even these may be only falsifications of the
proposed hypotheses.
19.4 Conclusions
The differences, or lack of differences, in the 013C values between the hosts
and the parasites can be explained satisfactorily.
Holoparasitic angiosperms and parasitic fungi (both lacking photosynthesis) have the same 013C values as their hosts (substrates) as expected.
The small decrease in 13C content in galls relative to the parent plants may
be due to differences in chemical composition, but this remains to be shown.
The higher 13C content of mistletoes compared with their hosts was convincingly explained earlier.
There is no explanation for higher deuterium content of the organic
material in tissues of parasites than in those of the hosts (exception: mistle-
407
are always significantly richer in deuterium (in their dry mass) than their
hosts.
The exception with the CAM hosts may be due to the fact that mistletoes
obtain their tissue water (used in their own photosynthesis) from the transpiration stream of the host, which is not very much enriched in deuterium,
while the CAM host also uses water from water-storage tissues or vacuoles,
which can be considerably enriched in deuterium (ct. Ziegler 1988).
There is no explanation of the enrichment of deuterium in the biomass in
galls, holoparasites, mistletoes, and fungi (here in contrast to the reduction
of the 13C content) on C 3 hosts, only hypotheses. The following may be
mentioned:
- The tissue water in the galls or parasites could be richer in deuterium
(less negative oD) than the tissue water in the host or substrate, leading
to a deuterium-enriched organic matter. This could happen in the galls
and mistletoes only, since the holoparasites and the fungi perform no
photosynthesis.
- A deuterium enrichment of the tissue water of the galls and parasites
could result in an increased deuterium content of some chemical components with exchangeable (-OH or NH2 bound) hydrogen groups (e.g.,
in cellulose or proteins (cf. Bonhoeffer 1934)).
- The galls and parasites could generally have much lower lipid contents
than the source plants. This hypothesis seems quite improbable, since a
lower lipid content would also mean a relative enrichment in 13C and this
is mostly not the case.
- The galls and parasites may have an especially intensive respiration and
this process preferentially uses deuterium-poor substrates.
Since these hypotheses are experimentally accessible, we may soon have
answers to our questions, even these may be only falsifications of the
proposed hypotheses.
19.4 Conclusions
The differences, or lack of differences, in the 013C values between the hosts
and the parasites can be explained satisfactorily.
Holoparasitic angiosperms and parasitic fungi (both lacking photosynthesis) have the same 013C values as their hosts (substrates) as expected.
The small decrease in 13C content in galls relative to the parent plants may
be due to differences in chemical composition, but this remains to be shown.
The higher 13C content of mistletoes compared with their hosts was convincingly explained earlier.
There is no explanation for higher deuterium content of the organic
material in tissues of parasites than in those of the hosts (exception: mistle-