Orobanche cumana WALLR. is a host-specific root parasite of cultivated sunflowers with increasing economic importance in Europe, North Africa, and parts of Asia. While sesquiterpene lactones (STLs) released from sunflower roots were identified as natural germination stimulants of O. cumana seeds in the soil, the chemical nature of the signals guiding the emerging germ tube toward the host root has remained unknown hitherto. Thus, we designed a bioassay that allowed the observation of broomrape germination and subsequent germ tube development in the presence of substances with putative chemotropic activity. Root exudates and sunflower oil extracts, both containing STLs in micromolar concentrations, caused the positive chemotropic orientation of germ tubes. A similar positive chemotropic effect was achieved with costunolide, one of the four STLs of sunflower present in the exudate and oil extracts. In contrast, GR24, a synthetic strigolactone (SL) with germination-inducing activity on O. cumana seeds, showed no effect on the germ tube orientation. The effect of costunolide was concentration-dependent and within the range of its natural micromolar occurrence in roots. We assume that an STL gradient is responsible for the stronger inhibition of elongation growth on the host-facing flank of the germ tube compared with the far side flank. This would confer a double role of STLs from sunflower root exudates in the sunflower–broomrape interaction, namely, as germination stimulants and as chemotropic signals.

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Introduction Cedrus libani A.Rich. is native to the Lebanon Mountains of Lebanon and to the Taurus Mountains of Syria and southern Turkey. After centuries of overexploitation, today its largest distribution area is in Turkey, where this species still covers around 600,000 ha with pure stands mainly found in the western Taurus Mountains (

Sunflower broomrape Orobanche cumana WALLR. is a rapidly growing threat to the oil crop production in many countries. Fast adaptation to new environments and increasing host resistance suggests that phenotypically distinctive populations of the weed may have evolved. The classification of the species and the differentiation of such populations on the base of seed micromorphological characters were attempted. Morphometric measurements allowed the distinction of O. cumana from several other Orobanche and Phelipanche species. An irregularly thickened cell wall of the anticlinal testa cells differentiated O. cumana and O. cernua from O. caryophyllacea, O. crenata, O. minor, P. aegyptiaca, P. arenaria and P. ramosa. However, populations of sunflower broomrape from five European countries and China could not be separated from each other on the base of micromorphological seed characters. In contrast, length to width measurements indicated that the Asian samples had a slightly different seed shape which was less elongated than the European samples. However, this seemingly geographic effect may as well be a consequence of sampling which comprised a higher rate of the so-called modern races E-H in the European samples.

Orobanche cumana Wallr. is a non-photosynthetic angiosperm naturally distributed in wild flora of some European countries, where parasites different species of Asteraceae, especially Artemisia spp. The broomrape paraziting A. maritima, A. austriaca or Matricaria sp. has been also observed on the territory of the Republic of Moldova.According to some authors broomrape has gradually evolved and extended from wild host plants to cultivated Helianthus annuus. In Moldova, first evidence of the parasite was confirmed in 1866. In 1937, sunflower varieties resistant to race A were found, suggesting the appearance and evolution of race B. Several decades later (in the 1970s), a new broomrape biotype, named race C or the Moldovan race, rapidly spread over all sunflower growing regions. Currently, the most virulent races of O. cumana, G and H, have been confirmed.Considering the fact that O. cumana is present for more than 150 years in Moldova, it is important to analyze and highlight distinctive morphological features of broomrape seeds collected in different periods of time, as well as from different hosts.Thus, 9 samples of broomrape seeds from National Botanical Garden of Moldova herbarium have been studied, including 5 samples collected from H. annuus, 3 from Artemisia sp. and one from Matricaria sp. The morphometric parameters (length (L), width (W) and ratio of L/W) of seeds collected from H. annuus showed very similar mean values. Seed length and width were ranging between 0.352-0.425 mm and 0.144-0.193 mm, with a coefficient of variation of 12.6% and 13.4%, respectively. As for the L/W ratio the minimum values -1.91 (SD±0.452) has been established in the case of broomrape collected from H. annuus in 1921. As for the samples collected in 1938-1979, the values of L/W ratio were more similar, ranging between 2.12-2.52 (SD±0.37), with the coefficient of variation 17.3%. Comparing obtained results with those of the analysis on 39 populations of sunflower broomrape collected in 2014 no significant differences have been found. This shows that seed parameters have not changed substantially during O. cumana evolution.In the case of broomrape seeds collected from wild species of Artemisia (A. maritima, A. austriaca) and Matricaria, even the values of L/W ratio are also around 2.0 (ranged between 2.12-2.44) the seeds were smaller than those from sunflower crop. Thus, seed length and width were ranging between 0.232-0.307 mm and 0.101-0.172 mm.Correlative analysis of studied parameters revealed a high positive correlation (r=0.875) between length and width. This finding isn't confirmed in contemporary populations.The highest dissimilarity (0.627-0.714) of broomrape samples, determined by measurement of Euclidean distance based on seed parameters, has been highlighted in samples collected from Artemisia sp. and those from H. annuus. In the case of Matricaria sp. and H. annuus, as well as Matricaria sp. and Artemisia sp. this distance is lesser (around 0.300-0.406). Thus, the differences between analyzed samples could be prefere...