Aspects regarding the physiology of cuscuta (Cuscuta L

Volume 16(1), 212-215, 2012
JOURNAL of Horticulture, Forestry and Biotechnology
www.journal-hfb.usab-tm.ro
Aspects regarding the physiology of cuscuta (Cuscuta L.
Convolvulaceae)
Tănase Maria1*, Sand Camelia1
1
“Lucian Blaga” University of Sibiu
*Corresponding author. Email: [email protected]
Abstract
Many species of Cuscuta contain a small amount of chlorophyll;
and while both chlorophyll a and b are present in normal proportions, they
only occur in a very low concentration. Several authors have demonstrated
14
the capacity of Cuscuta for photosynthesis, making use of CO2 . Fer has
demonstrated that Cuscuta exposed to strong air illumination, with a CO 2
concentration much higher than the normal atmospheric CO 2 concentration,
fixes more CO2 during the photosynthesis process than the amount produced
by means of respiration in both independently living seedlings and in the stem
excised from the host.
The migration of assimilates is only occurring acropetally, not only in
the case of the plantula, but also in the case of the mature parasite. The
entirely acropetal migration of assimilates can be explained by the attracting
effect exerted by the terminal sprout, as well as by the lateral sprouts situated
on the subterminal part of the branch. The rudimentary root of the plantula
does not have a meristem [19]; [12]; [28]; [23]; and the absence of a
downward migration within the plantula thus appearing to be determined by
the absence of the radicular meristem [9].
Photosynthetic apparatus and chlorophyll
degree of concentration. With the exception of its
roots, the organs of Cuscuta have only a low grade of
chlorophyll concentration, and are ‘equipped’ with
stomates [19];[29];[4]. According to several studies
carried out by means of an electronic microscope, the
chloroplasts of several Cuscuta species have a
rudimentary structure and contain substantial amounts
of starch granulates.
The occurrence of the chlorophyll a and b in
numerous Cuscuta species is certain. The results of
MacLeod [17] and Laudi [15] indicate the chlorophyll
grade of concentration of these parasites; a smaller one
than that of autotrophic plants. With the exception of
Laudi [15], all authors agree that the value of the
proportion chlorophyll a / chlorophyll b is of between
2.2 and 3.5; a value which is analogous to that
occurring with most autotrophic plants.
Highlighting and measuring the intensity of
photosynthesis. One has noted that the plantulae
situated under light/brightness conditions liberate
oxygen [8];[24] and the quantity of oxygen is directly
proportional to the lighting of the plantulae. Moreover,
the CO2 anchorage and its stimulation by brightness
have been noted as well [16] and [18]; [20]; [1] and
[2]; [14]; [13]. Among the authors who acknowledged
the occurrence of a photosynthetic CO2 anchorage
when it comes to Cuscuta, Ismail and Obeid [13]
attempted to establish an accurate balance of the
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Key words
Cuscuta
sp.,
photosynthesis, chlorophyll
a
and
b,
assimilate
transport
phenomenon, based on measuring the intensity of the
photosynthesis, resp. of the respiration. According to
their results, when it comes to the Cuscuta hyalina, in
an atmosphere which is not enriched with CO2, the
intensity of photosynthesis and that of respiration
appear to be equal.
In this context one has to raise the following
question: to what degree is the photosynthesis capable
of allowing the development of the plantula Fer’s
experiments [9] confirmed Pizzolongo’s results [22],
showing that in an atmosphere which is not enriched
with CO2, the photosynthesis cannot compensate for
the respiration. Under these conditions, the growing of
the plant is not possible, and the intensity of
respiration, resp. photosynthesis thus explains the
species’ parasitism.
Therefore, it becomes obvious that – since its
growth is fast – the plant exhausts the resources
contained in its seed rapidly, thus dying unless it
anchors on a host plant. The photosynthesis is not
intense enough in order to allow the growth of the
parasite by itself; the latter thus needing a host plant to
provide itself with carbonated substances.
If the atmosphere has a higher degree of CO2,
the photosynthetic intensity of Cuscuta will be
increased. Under such conditions, despite its low
degree of chlorophyll, Cuscuta is able to provide – if
not all, then at least a significant part of those
substances which are necessary for its growth.
Table 1
Bibliographic data regarding the chlorophyll degree of concentration in Cuscuta
Degree of
Culture conditions
Chlorophyll
development /
Cl a /
Authors
( g/g of fresh
host plant
Cl b
matter)
C. reflexa
Parasite,
on
natural
70 (1*)
2,8
Mac LEOD [17]
Nicotiana
C.campestris
Parasite,
on
natural
37
2,2
Mac LEOD [17]
Bulbine
C. pentagona
Plantulae, 4-7
Continuous
- ( 2*)
2,7
Zimmerman [29]
days
lighting (2000
Lux)
C australis
Plantulae, 4-7
Continuous
- ( 2*)
2,7
Bertossi et al. [3]
days
lighting (1000
Lux)
C. epithymum plantulae
Not mentioned
- ( 2*)
3,3 - 3,5 Sala et al. [24]
C. australis
parasite
(host
natural
- ( 2*)
3,5
Baccarini et al. [2]
not mentioned )
C. australis
parasite
(host
natural
145 (3*)
1
Laudi [15], Tanase [27]
not mentioned )
(1*)- if the host plant, exhausted by the parasite,
any comparison between haustoria and roots; a
is dying, Cuscuta reflexa, ‘greened’ by the high degree
comparison to which several authors tended to refer in
of chlorophyll, reaches its highest values;
their works (Pierce, [21] and Fratianne, [11)].
(2*)- the studied data does not allow the
In the case of Cuscuta, once the vascular
calculation of chlorophyll concentration;
connections between parasite and host plant have been
(3*)- gall stems (produced by the Cuscuta
established, haustoria no longer develop [26]; in
ladybug – Smicronyx sp.) have a very high degree of
conclusion, in a functional haustorium no single spot
chlorophyll concentration.
characterized by a merismatic occurrence persists.
Thus it can be explained why the Cuscuta haustoria do
not exert any attracting effect, and why no downward
The assimilates’ migration within the
plantula. A significant part of assimilates synthesized
migration of assimilates occurs.
by the plantula migrate toward the stem’s apex. This
point of view is confirmed by the results of Fer’s
Material and Method
experiments [10], according to which, there is no
downward migration of assimilates, the only occurring
Several Cuscuta plants have been used, both
migration being acropetal, not only in the case of the
independent and parasitic ones, in order to highlight
plantula, but also in that of the mature parasitic plant.
and measure the photosynthesis intensity under certain
The rudimentary root of the plantula lacks a
conditions; not only normal atmospheric ones, but also
meristem character [19]; [12]; [29];[23], whereas the
carbon dioxide-enriched atmospheric conditions.
absence of any downward migration within the
The migration of assimilates – which has shown
plantula seems to be determined by the absence of the
only an upward tendency - has also been analyzed; the
radicular meristem [9]; [31]. Even if it plays a certain
lack of a downward migration has been highlighted by
role with regard to the absorption of mineral elements,
samples of five-days-aged plantulae.
the rudimentary root of the plantula does not represent
any attraction for the assimilates, due to the meristem’s
Obtained results
absence. The lack of a downward migration explains
the limited duration of the root, which exhausts its
According to the results of our research, the
resources and dies unless it is provided with
migration of assimilates is only occurring acropetally,
assimilates.
not only in the case of the plantula, but also in the case
Similarly, when it comes to the parasite plant,
of the mature parasite. This result can be accompanied
haustoria do not represent any centre of attraction for
by several remarks:
the assimilates, even if they ensure the absorption of
Downward migration can be induced on the
water and dissolved substances. The attracting effect
parasital plant by leaving a single sprout at the basis of
exerted on the assimilates by the roots of the superior
the branch. The entirely acropetal migration of
autotrophic plants [5];[25];[6];[7] occurs neither at the
assimilates can also be explained by the attracting
rudimentary root of the plantula, nor at the adult plant
effect exerted by the terminal sprout, as well as by the
haustoria. This constitutes a decisive argument against
Analyzed
species
213
lateral sprouts situated on the subterminal part of the
branch.
When it comes to the plantula, the only
occurring vegetative spots are the terminal sprout as
well as two or three lateral sprouts, situated quite close
to the terminal one. The rudimentary root of the
plantula does not have a meristem [19]; [12]; [28];
[23]; the absence of a downward migration within the
plantula thus appearing to be determined by the
absence of the radicular meristem [9].
For the sake of comparison, it would be
interesting to present some findings regarding the
migration of assimilates in the case of other parasitic
fanerogams, namely: hemiparasites on the root
(Scrophulariaceae) and hemiparasites on the stem
(Loranthaceae). On the studied Scrophulariaceae, the
assimilates synthesized by a branch migrate to different
parts of the parasite. One has to note the basipetal
migration of the synthesized part towards the root of
the parasite, as it is the case with the Striga
senegalensis, parasite on the Sorghum vulgare and
with the Odontites verna, parasite on the Hordeum
vulgare[10] .
Concerning the hemiparasital Loranthaceae, one
can mention two different cases:
- The first, most often-occuring case refers to a
situation in which marked assimilates, synthesized by a
branch of the parasite, migrate to all directions,
including towards haustoria, but without passing into
the phloem of the host plant. This is the case of the
Viscum album, parasite on the Populus nigra [10], of
the Phoradendron sp, parasite on several gymnosperms
as well as a large number of wood dicotyledon as well
as Tapinanthus bangwensis, parasite on the Theobroma
cacao [10].
- The second case, still controversial, is that of
the Arceuthobium sp, parasite especially on the
species Abies and Pinus. According to Redisske and
Shea, a part of assimilates synthesized in the parasite’s
branches migrate towards the haustoria, thus in a
basipetal direction [10].
Conclusions
Thus, among the studied parasitic fanerogams,
the lack of downward migration, which has been
highlighted in the case of Cuscuta, seems to occur only
in the case of the Arceuthobium species. In order to
explain the occurrence of a downward migration in the
case of parasitic fanerogams, one could formulate the
following hypothesis: Regarding the hemiparasites
belonging to the Scrophulariaceae family, only the
extremities of certain roots come into contact with the
roots of the host plant, thus forming haustoria .
Therefore, there are free roots whose apex can exert an
attracting effect on the assimilates. Similarly, in the
cases of the Loranthaceae family, the Viscum,
Phoradendron and Arceuthobium genera, beside
primary haustoria there are cortical cords, of indefinite
214
growth, which spread around the primary haustoria. It
therefore seems logical to think that the extremities of
these cords have a merismatic character, thus
constituting centres of attraction for the assimilates. In
the case of the Arceuthobium genus, the lack of
downward migration is justified by the absence of the
differentiated phloem [10].
When it comes to Cuscuta, once the vascular
connections between parasite and host plant have been
established, haustoria no longer develop [26]. In
conclusion, in the case of a functional haustorium no
single zone characterized by a merismatic occurrence
persists. Thus it can be explained why the Cuscuta
haustoria do not exert any attracting effect and why no
downward migration of assimilates occurs.
The lack of a sampled downward migration
from the host to the Cuscuta lupuliformis partly
explains the deterioration of the older branches of the
parasite.
In fact, it has already been shown that a
potential Cuscuta haustorium does not have any spot
preserving a merismatic character. This finding can be
explained by the fact that during their ageing process,
absorbing cells progressively lose their ability of
sampling metabolites. Regarding the assumption of
metabolites, a Cuscuta haustorium is functional only
for a very limited time. At the end of this phase, the
branch situated above will no longer be nourished with
metabolites, thus starting to deteriorate.
A
further
assumption
explaining
the
deterioration of the older part of the parasite is the
following one: those leaves of the host plant which are
parasitized by a haustorium for several weeks in a row,
display obvious signs of a progressive deterioration
(peripheral yellowing of the foliar limb, then
desiccation), which then disperse in a centripetal way,
and which eventually has an effect on the entire limb as
well as on that part of the petiole situated over the
haustorium. From that moment the quantity of
metabolites sampled by such a haustorium must be
extremely reduced or even null. The deterioration of a
Cuscuta branch occurs when it is no longer nourished
by the upward flux of the sampled assimilates in the
host. The deterioration of the branches is accelerated
when they have fruit.
Acknowledgement
This work was cofinanced from the European
Social Fund through Sectoral Operational Programme
Human Resources Development 2007-2013, project
number POSDRU/89/1.5/S/63258 ”Postdoctoral school
for zootechnical biodiversity and food biotehnology
based on the eco-economy and the bio-economy
required by eco-san-genesys”.
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http://www.wlu.ca/page.php?grp_id=2147&p=195542007 -Digital Atlas of Cuscuta (Convolvulaceae)
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