E. Landolt not only collected duckweed
clones but also taxonomically identified and
classified them, which bestowed his duckweed
collection the “gold standard” for duckweed
research (Lam and Appenroth 2013). From the
very beginning, he was not only in search for
new duckweed species (see Sree et al. 2016) but
also was interested in collecting clones of the
same species from different geographical locations, indicating that evidently, he was very
much interested in biodiversity and adaptation of
plants to specific environmental conditions.
In the 90 s of the twentieth century, E. Landolt
passed a part of the duckweed collection (ca.
1000 clones) to Prof. Anne-Marie Stomp from the
North Carolina State University, USA. In 1997,
Prof. Stomp had transferred this collection to
Biolex Therapeutics, a company that was interested in making use of duckweeds (Cross 2015),
which later transferred a part of the duckweed
collection to Rutgers State University, NJ (Prof.
Dr. Eric Lam) forming the initial stock of the
Rutgers Duckweed Stock Cooperative with presently more than 1000 clones of duckweed.
The collection in Zurich comprised of much
more than 1000 duckweed clones. After the
retirement of E. Landolt in 1996 (Urbanska et al.
2013), gradually it became difficult to maintain
the full stock collection in the cultivation rooms
of the ETH Zurich. The number of clones was
therefore reduced in the collection. Finally, in
2009 it was transferred to private rooms and was
named as Landolt Duckweed Collection in Zurich (Laemmler 2018).
3.4 Requirements of a Duckweed
Stock Collection
Often each stock collection optimizes the standard conditions and requirements according to
the available resources and conducts of that
laboratory. The following is established at the
Duckweed stock collection at Friedrich Schiller
University of Jena being headed by one of the
co-authors, KJA.
The difference between “normal” cultivation
of duckweed, i.e. mainly under optimal growth
conditions for experimental purpose, and stock
cultivation for long-term availability of duckweed clones in the laboratory exists in the point
that the growth of the plants in stock cultivation
is slowed down by regulating different factors.
Maintaining a stock collection not only involves
financial resources but is also time and manpower consuming. The slower growth of the
plants in the stock cultivation optimizes the use
of these resources. Duckweeds in stock cultivation need to be transferred to fresh nutrient
medium from time to time failing which the
plants will die once the nutrient reservoirs are
exhausted. Slower growth of plants in a stock
cultivation means less number of subcultures in a
year, which also helps to avoid introduction of
microbial contaminations to the plant cultures.
The medium for stock cultivation should be
rich of nutrients in order to prevent quick nutrient
shortage. Several media are suitable, e.g.
N-medium with increased phosphate concentration to 1 mM or Steinberg’s medium with
increased Fe-EDTA concentration to 25 µM
(Appenroth 2015).
The growth rate is decreased by regulating
several of the controlled conditions in the laboratory. Use of solidified medium, either by
addition of agar (e.g. 0.9%) or Gelrite (0,45%), is
one of the methods. The light intensity is reduced
from the standard 100 µmole m
−2 s
−1 continuous white light to 30 µmole m
−2 s
−1 white light
often with a photoperiod. All duckweed clones
from around the world, even those from tropical
regions, can survive 17 °C environmental temperature, which further reduces the plant growth
during stock cultivation. Clones of Spirodela
polyrhiza and S. intermedia are often grown on
liquid medium in the stock cultivation because
their relatively large frond size forms different
vertical layers of plants over the solidified medium and after a certain time, the fronds of the
upper layer start to die, as they do not any more
have access to the nutrient medium. There exists
an alternative method for the long-term live
storage of the species S. polyrhiza. This species
forms turions or starch-filled submerged resting
bodies, which can be easily separated from the
floating fronds. These turions under favourable
42
K. S. Sree and K.-J. Appenroth
clones but also taxonomically identified and
classified them, which bestowed his duckweed
collection the “gold standard” for duckweed
research (Lam and Appenroth 2013). From the
very beginning, he was not only in search for
new duckweed species (see Sree et al. 2016) but
also was interested in collecting clones of the
same species from different geographical locations, indicating that evidently, he was very
much interested in biodiversity and adaptation of
plants to specific environmental conditions.
In the 90 s of the twentieth century, E. Landolt
passed a part of the duckweed collection (ca.
1000 clones) to Prof. Anne-Marie Stomp from the
North Carolina State University, USA. In 1997,
Prof. Stomp had transferred this collection to
Biolex Therapeutics, a company that was interested in making use of duckweeds (Cross 2015),
which later transferred a part of the duckweed
collection to Rutgers State University, NJ (Prof.
Dr. Eric Lam) forming the initial stock of the
Rutgers Duckweed Stock Cooperative with presently more than 1000 clones of duckweed.
The collection in Zurich comprised of much
more than 1000 duckweed clones. After the
retirement of E. Landolt in 1996 (Urbanska et al.
2013), gradually it became difficult to maintain
the full stock collection in the cultivation rooms
of the ETH Zurich. The number of clones was
therefore reduced in the collection. Finally, in
2009 it was transferred to private rooms and was
named as Landolt Duckweed Collection in Zurich (Laemmler 2018).
3.4 Requirements of a Duckweed
Stock Collection
Often each stock collection optimizes the standard conditions and requirements according to
the available resources and conducts of that
laboratory. The following is established at the
Duckweed stock collection at Friedrich Schiller
University of Jena being headed by one of the
co-authors, KJA.
The difference between “normal” cultivation
of duckweed, i.e. mainly under optimal growth
conditions for experimental purpose, and stock
cultivation for long-term availability of duckweed clones in the laboratory exists in the point
that the growth of the plants in stock cultivation
is slowed down by regulating different factors.
Maintaining a stock collection not only involves
financial resources but is also time and manpower consuming. The slower growth of the
plants in the stock cultivation optimizes the use
of these resources. Duckweeds in stock cultivation need to be transferred to fresh nutrient
medium from time to time failing which the
plants will die once the nutrient reservoirs are
exhausted. Slower growth of plants in a stock
cultivation means less number of subcultures in a
year, which also helps to avoid introduction of
microbial contaminations to the plant cultures.
The medium for stock cultivation should be
rich of nutrients in order to prevent quick nutrient
shortage. Several media are suitable, e.g.
N-medium with increased phosphate concentration to 1 mM or Steinberg’s medium with
increased Fe-EDTA concentration to 25 µM
(Appenroth 2015).
The growth rate is decreased by regulating
several of the controlled conditions in the laboratory. Use of solidified medium, either by
addition of agar (e.g. 0.9%) or Gelrite (0,45%), is
one of the methods. The light intensity is reduced
from the standard 100 µmole m
−2 s
−1 continuous white light to 30 µmole m
−2 s
−1 white light
often with a photoperiod. All duckweed clones
from around the world, even those from tropical
regions, can survive 17 °C environmental temperature, which further reduces the plant growth
during stock cultivation. Clones of Spirodela
polyrhiza and S. intermedia are often grown on
liquid medium in the stock cultivation because
their relatively large frond size forms different
vertical layers of plants over the solidified medium and after a certain time, the fronds of the
upper layer start to die, as they do not any more
have access to the nutrient medium. There exists
an alternative method for the long-term live
storage of the species S. polyrhiza. This species
forms turions or starch-filled submerged resting
bodies, which can be easily separated from the
floating fronds. These turions under favourable
42
K. S. Sree and K.-J. Appenroth