The carbohydrate and water balance of beans (Vicia faba) attacked by broomrape (Orobanche crenata)

Abstract

SUMMARY Orobanche crenata parasitizing beans maintained a slightly higher osmotic pressure than the bean roots, largely because of the higher concentration of sugars in the broomrape tissues. The sugar was withdrawn from the bean mainly as sucrose, which was hydrolysed to glucose and fructose by the Orobanche. These sugars were then rapidly translocated to the developing flower spike. As well as maintaining a high osmotic pressure this hydrolysis ensured a sucrose concentration gradient between host and parasite.In the field, bean plants showed wilt symptoms at about the time that the Orobanche flower spikes emerged. It was found that the higher the level of infection the lower was the water content of the host. This fall in water content was not due to increased water loss by the bean shoots and it seemed unlikely that it was due to water removal by the parasite.It was concluded that the death of the bean was due to desiccation brought about largely by the reduced ability of the carbohydrate‐starved roots to extract water from the soil.