Plant-fungus water relations affect carbohydrate transport from pea leaf to powdery mildew ( Erysiphe pisi) mycelium

Abstract

The effects that water relations may have on the net uptake of carbohydrate by powdery mildew mycelium were investigated. Strips of pea leaves infected by the powdery mildew fungus (Erysiphe pisi) were fed [ 14 C]sucrose. Net uptake into leaf (plus haustoria) and mycelial fractions were measured separately. An examination of experimental variables governing uptake showed that total uptake was approximately linear over a 3 h pulse period, about 12% of the total being in the mycelium. This increased to 26% during a 3 h chase period on unlabelled medium. Uptake by both leaf and mycelium was generally insensitive to pH in the range 4.8–6.8. Over a range of external sucrose concentrations uptake by both leaf and mycelium was biphasic, the pattern suggesting that uptake to the fungus was predominantly active in the range 0.05–10.0 mmol dm −3 . The fungus apparently had a higher affinity for sucrose, K m 7.65 mmol dm −3 , than did the leaf, K m 23.52 mmol dm −3 , and on a dry-weight basis took up more sucrose than the leaf. Water stress, both of plants from which strips were taken and during incubation, increased the amount of sucrose taken up by the leaf but decreased the percentage of total 14 C taken up that was found in the mycelium. In combination, pre-incubation and incubation stress had additive, negative effects on transport to the mycelium. Uptake by leaf and mycelium were stimulated by air movement and the proportion of total uptake in the mycelium rose to 38% in the most rapidly moving air. It is proposed that air movement stimulated transpiration which, in turn, promoted the development of turgor pressure gradients driving carbon transport by mass flow into the mycelium. Water stress inhibits the development of such turgor pressure gradients and in this way it could exacerbate the stress-induced inhibition of carbohydrate transport to the mycelium that was observed in leaf strips in still air.