Water stress modifies the influence of powdery mildew on root growth and assimilate import in barley

Abstract

Barley growing in soil columns was inoculated with powdery mildew, Erysiphe graminis f.sp. hordei Marchal, when two leaves had fully expanded. Half the plants were not watered after inoculation so that they suffered water stress as gradients of bulk soil water potential of between 0·6 and 1·8 MPa developed between the uppermost (0 to 12·5 cm deep) and lowest (37·5 to 50·0 cm deep) regions of the profile.Growth of the whole root system, and of different organs of the shoot, was strongly correlated 8 days after inoculation with the amount and distribution of 14C fixed 24 h previously by the whole plant. Assimilates fixed on 2 occasions before inoculation did not significantly contribute to root growth 7 days after inoculation. Infection reduced the amount of 14C fixed in watered and stressed plants, but only in stressed plants did infection reduce the percentage of total assimilates that was translocated to roots. Stress stimulated root growth, particularly in healthy plants, making differences in root growth between healthy and infected plants greater in stressed than in watered plants. Reductions in root growth caused by infection were most marked in the upper drier regions of the soil profile, while reductions in translocation were most marked in the middle regions.The reduction in shoot growth caused by infection was similar in watered and stressed plants. The percentage of total assimilates that was translocated out of healthy third leaves was not affected by infection of the 2 lower leaves, but was reduced by water stress. Infection increased translocation into infected first and second leaves at the expense of the remainder of the shoot, especially in stressed plants.