VERTICILLIUM WILT OF CHRYSANTHEMUM - QUANTITATIVE RELATIONSHIP BETWEEN INCREASED STOMATAL RESISTANCE AND LOCAL VASCULAR DYSFUNCTION PRECEDING WILT

The relationship between the development of water stress and foliar symptom expression within chrysanthemum (Chrysanthemum morifolium Ramat) cuttings infected with V. dahliae Kleb. was examined using relative water content (RWC), diffusive resistance to water vapor loss (stomatal resistance) and dye distribution along water-transport pathways as indicators of water stress. The RWC remained at a normal level until symptoms appeared, but stomatal resistance increased beginning about 8 days before symptoms. Dye movement along xylem elements was uniform within uninoculated checks and within infected plants until stomatal resistances increased. Veinal dye distribution was incomplete within tissue exhibiting increased stomatal resistances, and the extent of dye interruption was closely aligned to the magnitude of resistance increase. Apparently, stomatal closure effectively prevented tissue desiccation under conditions of high, localized internal water stress, but this mechanism could not prevent tissue from becoming flaccid or wilted when water transport became so limited that water was deficient even within the large veins.