To the morphophysiological co-evolution of the broomrape (Orobanche cumana) and sunflower

The accelerated morphofunctional variability of Orobanche cumana, a flowering parasitic plant of the family Orobanchaceae, can be traced to the influence of anthropogenic factors. This broomrape species is an obligate parasite of sunflower. Continuous breeding of the crop for immunity to broomrape has been conducted in the Russian Federation for more than 100 years. In the last 30 years, the accelerated return of sunflower in crop rotation to the previous field in 1-3 years (instead of the scientifically justified 8–10 years) resulted in the rapid emergence of new races of broomrape and contamination of fields with its seeds, which provided intense competition between individuals for the nutrition received from the host plant. Under these conditions, interrow cultivation in sunflower sowings infested by the broomrape contributed to permanent injury of developing stems of many individuals of the parasite, which led to the removal of apical dominance of one shoot in some species of the broomrape. There have appeared the bushy forms with equal development of many shoots from a tubercle, which were detected for the first time by us. Strengthening of the haustorial part of the parasitic plant, externally manifested as thickening of the host plant root under the tubercle, was required for feeding the multiple shoots. Such thickening is not a defensive reaction of the host plant but is caused by changes in the haustorial area of the parasite and is observed when sunflower roots are affected by species of the race G of O. cumana that became widely spread and dominant during the last decade. Thus, in the co-evolution of the parasitic plant and its host, we first discovered bushy forms of O. cumana and their characteristic change in the haustorial zone, which is externally manifested by the visible thickening of the sunflower root in this area.