Abstract: Microscopic examinations and chemical studies were performed to study the response of two legume crops, hyacinth bean (_Lablab purpureus L._) Sweet and kidney bean (_phaseolus vulgaris L._) to the filed dodder (FD) (_Cuscuta campestris Yuncker_) parasitism. Hyacinth bean, showing no effective resistance mechanisms,was found to be a highly susceptible host to FD. However, kidney bean, displaying resistant reactions towards the parasitism of FD,was found to be an incompatible host.The possible reasons for the resistance of kidney bean to FD were anatomical (hypersensitivity) and chemical (high contents of phenolic acids and lignin) stimulated defence mechanisms, which developed during the actual intrusion of FD haustorial cells inside its tissues

Awad F. Farah

English

Nature Precedings

 1
The response of two legume crops (hyacinth bean and kidney  
bean) to the parasitism of field dodder (Cuscuta campestris) 
AWAD FAGEER FARAH 
Department of Botany, Faculty of Science and Technology, 
Omdurman Islamic University, Omdurman, Sudan 
Awadhfarah2@yahoo.com
Abstract: Microscopic examinations and chemical studies were 
performed to study the response of two legume crops, hyacinth 
bean (Lablab purpureus (L.) Sweet and kidney bean (phaseolus
vulgaris L.) to the filed dodder (FD) (Cuscuta campestris Yuncker)
parasitism. 
 Hyacinth bean, showing no effective resistance mechanisms, 
was found to be a highly susceptible host to FD. However , kidney 
bean, displaying resistant reactions towards the parasitism of FD, 
was found to be an incompatible host. 
 The possible reasons for the resistance of kidney bean to FD 
were anatomical (hypersensitivity) and chemical (high contents of 
phenolic acids and lignin) stimulated defence mechanisms , which 
developed  during the actual intrusion of FD haustorial cells inside 
its tissues. 
Keywords: Filed dodder, Hyacinth bean, hypersensitivity, incompatible 
host, kidney bean , lignin , phenolic acids, susceptible host. 
 
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Introduction:
Field dodder (FD) (Cuscuta campestris Yuncker , family 
Cuscutaceae) is an obligate parasite attacking the shoot system of 
numerous species of dicotylednous plants,  specially legume crops. 
Dawson et al. (1994) reported the genus Cuscuta as a serious 
problem in forage legumes principally alfalfa (Medicago sativa), 
clover (Trifolium spp.) and lespedeza (Lespedezia spp.). However , 
Cuscuta  species may grow poorly or not at all on some hosts. 
These are known as incompatible hosts. Several factors may cause 
incompatibility e.g. hypersensitivity as in tomato, hyperplasia as in 
okra and hypertrophy as in potato (Farah, 2007). 
 The objectives of the present work were to study through 
anatomical and chemical techniques, the response of two legume 
crops (hyacinth bean: Lablab purpureus (L.) Sweet , and  kidney 
bean: Phaseolus vulgaris L.) to FD (Cuscuta campestris Yuncker)
parasitism, and to identify the anatomical and chemical 
mechanisms involved in their response.
Materials and Methods: 
 Two legume crops, hyacinth bean and kidney bean were 
raised in towenty 25 cm diameter plastic pots filled with 1:1 
mixture of sand and peatmoss, at the rate of 10 pots per each crop. 
In one half of the pots, the soil was thoroughly mixed with FD 
seeds at the rate of 150 seeds per pot. The second half of  the pots 
were left free from FD seeds, as untreated control. 
 3
Ten legume seeds per pot were sown in 2004/2005. Ten days after 
emergence, the seedlings were thinned to four per pot. 
 The pots were placed in a greenhouse (280 C / 230C day and 
night temperatures) at the king Faisal University Farm, AL-Ahsa 
(25022' N Latitude, 49034' E Longitude), Saudi Arabia. 
 After one week from the beginning of parasitism, samples 
(one plant / pot) of infected and non infected plants of the two 
legume crops were taken for anatomical studies at the Botany 
laboratory, College of Agricultural and food Sciences, King Faisal 
University , Saudi Arabia. The samples were fixed in F.A.A 
(Formaldehyde, Acetic acid , Alcohol) in the  ratio of 1:1:18 ; 
dehydrated in gradual ethanol solutions ranging from 15% to 100% 
then treated in mixtures of absolute ethanol (99.9%) and xylene in 
three different ratios, 3;1 , 1:1 , 1:3 , respectively (Berlynand and 
Miksche , 1976). 
 The samples were infiltrated with xylene and embedded in 
paraffin wax. Sections (15-20 μm thick) were prepared using 
Bright 5040 type rotary microtome. They were stained with 
safranin  and light green stains and mounted in Canada balsam. 
The stained sections were examined microscopically using an 
Olympus 1-12 Binocular Microscope. Microscopic photographs 
(micrographs) were taken under 10 x and 40 x objective lenses 
using Leitz laborlux 12 microscope supplemented with Leitz 
Vario-Orthomat (Camera system) for automatic photography. 
 4
 The total phenolic acids and lignin contents were determined 
for the two crops (hyacinth bean and kidney bean) at the 
Biochemistry laboratory, Department of Animal Production, 
College of Veterinary Medicine, King Faisal University, Saudi 
Arabia. Samples of the shoot system of the infected and non-
infected crops were allowed to dry at room temperature for 10 
days. The air dried samples were ground into powder and used for 
determination of total phenolic acids contents according to Basden 
and Dalvi (1983), and determination of lignin according to Van 
Soest and Win  (1967). 
Results and Discussion: 
Hyacinth bean was found to be a highly susceptible host, 
while kidney bean was found to be a resistant host to FD. Figure 1 
depicts the susceptibility of hyacinth bean. A clear haustorial 
connections were established between the vascular tissues of the 
host and the parasite (Fig.2). Moreover , the presence of a 
conspicuous xylem – ylem bridge between the two partners (Fig.2) 
revealed the higher susceptibility of hyacinth bean. Thus, in the 
current study, it was evaluated as a true host. This result is in 
agreement with that of Kuijt (1969) who stated that true hosts are 
only those in which hyphal tracheary connections are complete. 
The formation of effective connections between FD searching 
hyphae and hyacinth bean vascular tissues was of paramount 
importance for successful development of the parasite. In this 
 5
respect, Joel et al. (1996) reported that haustorial cells must 
differentiate as soon as the parasite intrusive cells contact the host 
conductive tissues. However, Fineran (1987) claimed the existence 
of an extreme apoplastic pathway suitable for transport between 
the haustorium and the host tissues. 
 It was noticed, during the present study, that the steps of field 
dodder parasitism in hyacinth bean were as follows: 
The attachment of FD shoot to the host surface; the formation of 
superficial protuberances at the contact surface of the parasite with 
the host; the formation of several haustoria and the penetration of 
the functional ones across the host tissues; the formation of 
searching hyphae; the differentiation of the searching hyphae 
reaching hyacinth bean conductive tissues into xylem and phloem 
cells. All these steps are crucial for the survival of the parasite. 
Failure of the parasite to perform one or more of these steps may 
lead to the failure of the desired functional and dynamic host-
parasite union. 
 In the present study, the best example illustrating  the failure 
of the host-parasite union, is the one manifested by kidney bean 
and FD (Fig.3). The resistance of kidney bean to the parasitism of 
FD was attributed to hypersensitive reactions, i.e. the damage of 
the  epidermal and / or cortical cells of kidney bean surrounding 
the FD haustorium , thus isolating the latter from reaching the host 
vascular tissues (Fig.4). The hypersensitivity of kidney bean was 
 6
an stimulated defence mechanism which developed during the 
actual intrusion of FD haustrial cells inside the incompatible host 
(kidney bean) tissues. This finding was in harmony with that of 
Arnaud et al. (1996) in the case of  Phaseolus  vulgaris and 
Cuscuta reflexa.
          Phenolic acids and lignin comprised a second type of 
defence mechanism against FD parasitism. They are chemical 
barriers used by certain incompatible hosts to resist the attack of 
parasitic weeds (Farah,2000). Phenolic compounds probably 
constitue one of the widely spread and diverse group of secondary 
plant metabolites, and their role includes excretion of  waste 
products and defence against invading pathogens (Abercrombie et
al.,1978). Lignin , on the other hand, is formed of phenylpropanoid 
units that encrust the cellulose of the cell walls of the higher plants 
thus contributing to their mechanical strength and rigidity (Walker, 
1975). The results of the current study, indicated a rise in both 
phenolic acids and lignin contents with FD  infestation in hyacinth 
bean and kidney bean. The FD infected plants of kidney bean and 
hyacinth bean, showed 116.2% and 7.4% ; and 48.0% and 1.5% 
increase over the control in phenolic acids and lignin contents, 
respectively (Table1). Thus, these chemicals were found to be  
much higher in the resistant crop (kidney bean) as compared to the 
highly susceptible crop (hyacinth bean). The increase in the levels 
of both chemicals in the infected kidney bean may be attributed to 
 7
the fact that these chemicals were stimulated as part of the defence 
reactions of this crop against the penetration of FD haustorium into 
its tissues. These results are in line with those of Arnaud et
al.(1996) in Cusuta reflexa and Phaseolus vulgaris; Antonove and 
terBorg (1996) in Orobanche cumaua and Helianthus annuus, and 
Goldwasser et al. (1999) in Orobanche aegyptiaca and Vicia
atropurpurea , who attributed the resistance of the host plants to a 
number of factors including phenolic compounds and lignin. On 
the other hand, susceptible crops can also respond to the invasion 
of the parasite. This was demonstrated by the limited increase in 
the levels of both phenolic acids and lignin contents in the infected 
plants of hyacinth bean. Similarly , Joel et al. (1996) claimed that 
the susceptible host plants can also sense the invasion of the 
parasite , but lacking the appropriate genes, they do not display any 
effective resistance mechanism. 
 In conclusion, hypersensitivity (a stimulated anatomical 
defence mechanism) and the higher contents of phenolic acids and 
lignin (stimulated chemical defence mechanisms) were assumed to 
isolate the FD haustorium from the incompatible host (kidney 
bean) vascular tissues , by blocking its intrusion. The susceptibility 
of hyacinth bean may be attributed to lacking such defence 
mechanisms. 
 8
 
 
 
 
 
 
  
 
Fig.1: On the highly  susceptible host (hyacinth bean) field dodder is 
characterized by luxuriant and prolific growth, with many functional 
coils , thick stem, yellow or yellowish orange colour, and high 
capacity to produce flowers and fruits. 
           
 
 
 
 
 
 
 
 
Fig.2: Establishment of connections between filed dodder's 
searching  hyphae and the host (hyacinth bean) vascular 
tissues , and formation of clear xylem– xylem bridge between 
the two partners (100x).   
 9
 
 
 
 
 
 
 
 
Fig.3: Field dodder on the incompatible host (Kidney bean) 
looks pale green, inadequately nourished, with thin stem and 
limited number of coils.  
 
 
    
 
 
 
  
  
  
Fig.4: Isolation of field dodder's haustoria form reaching the 
vascular tissues of kidney bean,(an example of hypersensitive 
reaction)(100x). 
 10
Table 1. Total phenolic acids and lignin contents in kidney bean 
and hyacinth bean 
 
Legume crop Treatment Phenolic acids 
content (%) 
Lignin content
(%) 
Kidney bean Untreated plants 3.7  4.6 
 Treated plants 8.0  6.8 
Hyacinth bean Untreated plants 2.7  6.5 
 Treated plants 2.9 6.6  
 
 Untreated plants  plants free from field dodder. 
 Treated plants  plants infected with field dodder. 
 11
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The response of two legume crops (hyacinth bean and Kidney bean) to the parasitism of field dodder (_Cuscuta campestris_)

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The response of two legume crops (hyacinth bean and Kidney bean) to the parasitism of field dodder (_Cuscuta campestris_)

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