Introduction

The study of host-parasite chemical interactions is a relatively new field of research in allelopathy that is receiving increasing attention for economical and scientific reasons. The existence of parasitic plants has been reported since ancient times. Dioscorides (s. I. a. C.) described plants belonging to the family Orobachaceae, the genera Orobanche being described by Linnaeus in 1793. Up to date, over 4000 species of parasitic plants grouped in 20 families have been described so far.35 There are five families of special importance because of their adverse impact on different crops, namely Schrophulariaceae, Orobanchaceae, Cuscutaceae, Viscaceae and Loranthaceae. Among them, weeds belonging to the Schrophulariaceae and Orobanchaceae phyla are important economical threats on crops such as legumes, several Gramineae, tomato, sunflower, and tobacco (Table 6.1). However, parasitic weed control techniques have not been studied until recently, and proper control methods are not available yet.

Parasitic plants can be broadly divided into hemiparasites and holoparasites, according to the presence or absence of chlorophylls. The holoparasites depend on their hosts to get the nutrients and to complete their life cycle, as they are not able to fix carbon through photosynthesis. The hemiparasites take from their host just minerals and water, and their parasitism can be facultative.

Two well-differentiated phases can be established in the life cycle of most of the parasitic plants; the independent and the parasitic phases. The first one comprises seed dispersion, the latent phase, the seed-conditioning period, and germination. During this period, the plant does not need the presence of any host to survive. The parasitic phase includes the haustorium formation and penetration processes, the connection of the weed to the vascular system of the host, and the development and flowering of the parasite attached to its host. Among all of these different developmental stages, the germination and the formation and establishment of the haustorium are crucial for the survival of the plant.

Table 6.1

Some of the most important parasitic weeds according to the economical losses they cause.

Table 6.1

Family

Genera

Species

Host Crop

Schrophulariaceae

Striga

S. hermonthica

sorghum, maize,

millet

S. asiatica

maize, sorghum

S. gesnerioides

cowpea

Orobanchaceae

Orobanche

O. cernua

sunflower, tomato,

(O. cumana)

tobacco

O. crenata

green pea, lentils,

broadbean, chickpea,

carrot, celery

O. ramosa /

onion, lettuce,

O. aegyptiaca

sunflower,

broadbean,

greenpea, lentils,

chickpea, tomato,

tobacco, potato,

carrot, celery, canola

O. minor

lettuce, broadbean,

tobacco, carrot,

celery, red clover

Agallinis

A. purpurea

Alectra

A. vogelii

cowpea

Convolvulaceae

Cuscutaceae

C. campestris

Loranthaceae

Amyema

A. sanguineum

eucalyptus

Dendropthoe

D. curvata

Tapinantus

T. buchneri

Viscaceae

Arceutobium

A. americanum

pines

A. abietinum

red fir

A. pusillum

white spruce

A. verticilliflorum

pines

Phoradendron

P. bolleanum

western juniper

P. juniperinum

western juniper

Cuscutaceae

Cuscuta

Cuscuta campestris

Cuscutaceae

Cuscuta

Cuscuta campestris

The relationship between a parasite and its host is extremely specific: each species of parasite recognizes only its host(s). Host specificity depends upon such a diverse range of factors as the ability of the parasite to recognize and attack the host plant, to break down the defense responses of the host, and the existence of enough resources in the host to assure the growth development of the parasite. The interaction of the parasite and the host is chemically mediated and represents a clear example of allelopathy: the parasite recognizes certain chemicals exuded by the roots of their potential hosts. These chemical clues serve the parasite to

"know" that there is a potential host in the vicinity to which to get attached. Depending on the parasite, the haustorium development can also be chemically mediated, as it will be noted later. However, the general process is not so easy. The germination conditions to break the dormancy of the seeds require a narrow range of temperatures and humidity before the inductor of the germination becomes effective. During this conditioning phase several changes occur inside the seed. The respiration changes, as does the protein synthesis, but the most important thing is the synthesis of high levels of gibberelins.19 If there is not any germination inductor reaching the seed after this period, the seed can go into a second latent period. However, this second period of dormancy might affect adversely the capacity of the seed to germinate.

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