Changes in Cell Membrane Permeability in Sunflower Hypocotyls Infected with Sclerotinia sclerotiorum

Influx and efflux of water and urea and electrolyte leakage are less for sunflower (Helianthus annuus) hypocotyl sections above lesions caused by Sclerotinia sclerotiorum than for those from healthy plants. Urea uptake by sections above lesions is reduced (celery, squash, and tomato) or unchanged (bean) in other hosts after Sclerotinia infection. Efflux of urea from sunflower hypocotyls is biphasic, suggesting diffusion in series from two cellular compartments (cytoplasm and vacuole). Efflux during the fast phase was 7 to 20 times greater than that during the slow phase. No difference was noted in urea efflux from healthy and diseased tissues during the slow phase. However, efflux during the fast phase from diseased tissues was slower than from healthy tissues, suggesting that the increased resistance to diffusion of urea in host cells above lesions resides in the plasmalemma. Water movement across cell membranes of healthy and diseased sunflower hypocotyls was reduced when tissues were treated with p-hydroxymercuribenzoate.