94
The Radiocarbon Method to Estimate Primary Production
plant specimens are placed into the jars which are then closed and placed for
incubation into a black box (Fig. 2.20) to be kept in complete darkness. After
3 h incubation the experiment is finished, as described before.
The probable losses of 14C assimilated by benthic hydrophytes due
to organic matter exudation in short-term (3-5 h) light incubations were
estimated to be 3-5% (Wetzd 1969; Brylinsky 1977), and are therefore
negligible.
2.5.5 Estimation of Radioactivity in Plant Material
The measurement of radioactivity in plant tissues is among the most critical
problems in the use of the 14C-method for primary production estimation in
benthic hydrophytes. Most of them have massive thalli, leaves, or stems, and
their tissues contain many various pigments which cause chemical and color
quenching and also chemoluminescence. This significantly complicates the
procedure of correct radioassay; but biochemists had the same problems in
counting radioactivity in blood or animal tissues, as is evidenced by the hundreds of publications in this field that appeared in the 1950s and 1960s.
However, now correct ways are known (see Sects. 1.2.2.3 and 4). The problem
is therefore to select the most efficient and least time-consuming technique.
When doing this, it is necessary always to realize that the main goal is not just
to achieve a higher counting efficiency (or less quenching), but rather to
accomplish quasi-equal counting efficiency when measuring radioactivity of
14C-carbonate in the working solution (R;) and in plant tissues (Rc). In this
case, problems of quenching and quench correction are practically avoided
(see Sect. 2.7.4).
Processing the thalli of macrophytes on completion incubation proceeds
as follows. The plants are removed from the incubation vessels and rinsed for
about 1 min directly in the water body. Then they are placed in black plastic
lags and put into a thermoisolated box with dry ice at the bottom. On arrival
at the laboratory, the plant samples are blotted, slightly dried, and from each
of them an average sample is collected by cutting out the pieces of thallus representing its main parts; this should not be more than 5-10 g. This plant material is placed into weighed beakers, dried at - 60°C and weighed again to
establish its exact dry weight. Then the beakers are exposed to of HCI fumes
for 10min to eliminate the remaining 14C-carbonates. Then the dry plant material is placed into the mortar and ground with 5-7 ml of water + glass powder
until it makes a fine homogenate; this can also be done with the help of an
ultrasonic or tissue disintegrator. This homogenate is quantitatively transferred into the weighed beaker and, on a scale, its volume is adjusted with
water to exactly 40ml. For radioassay standard portions of this homogenate
(0.3-0.5 ml) are taken with an automatic pipet with its fine tip cut off to make
the diameter of its opening 1-1.5 mm. These subsamples are taken instantly
The Radiocarbon Method to Estimate Primary Production
plant specimens are placed into the jars which are then closed and placed for
incubation into a black box (Fig. 2.20) to be kept in complete darkness. After
3 h incubation the experiment is finished, as described before.
The probable losses of 14C assimilated by benthic hydrophytes due
to organic matter exudation in short-term (3-5 h) light incubations were
estimated to be 3-5% (Wetzd 1969; Brylinsky 1977), and are therefore
negligible.
2.5.5 Estimation of Radioactivity in Plant Material
The measurement of radioactivity in plant tissues is among the most critical
problems in the use of the 14C-method for primary production estimation in
benthic hydrophytes. Most of them have massive thalli, leaves, or stems, and
their tissues contain many various pigments which cause chemical and color
quenching and also chemoluminescence. This significantly complicates the
procedure of correct radioassay; but biochemists had the same problems in
counting radioactivity in blood or animal tissues, as is evidenced by the hundreds of publications in this field that appeared in the 1950s and 1960s.
However, now correct ways are known (see Sects. 1.2.2.3 and 4). The problem
is therefore to select the most efficient and least time-consuming technique.
When doing this, it is necessary always to realize that the main goal is not just
to achieve a higher counting efficiency (or less quenching), but rather to
accomplish quasi-equal counting efficiency when measuring radioactivity of
14C-carbonate in the working solution (R;) and in plant tissues (Rc). In this
case, problems of quenching and quench correction are practically avoided
(see Sect. 2.7.4).
Processing the thalli of macrophytes on completion incubation proceeds
as follows. The plants are removed from the incubation vessels and rinsed for
about 1 min directly in the water body. Then they are placed in black plastic
lags and put into a thermoisolated box with dry ice at the bottom. On arrival
at the laboratory, the plant samples are blotted, slightly dried, and from each
of them an average sample is collected by cutting out the pieces of thallus representing its main parts; this should not be more than 5-10 g. This plant material is placed into weighed beakers, dried at - 60°C and weighed again to
establish its exact dry weight. Then the beakers are exposed to of HCI fumes
for 10min to eliminate the remaining 14C-carbonates. Then the dry plant material is placed into the mortar and ground with 5-7 ml of water + glass powder
until it makes a fine homogenate; this can also be done with the help of an
ultrasonic or tissue disintegrator. This homogenate is quantitatively transferred into the weighed beaker and, on a scale, its volume is adjusted with
water to exactly 40ml. For radioassay standard portions of this homogenate
(0.3-0.5 ml) are taken with an automatic pipet with its fine tip cut off to make
the diameter of its opening 1-1.5 mm. These subsamples are taken instantly