Endophyte-Host Associations in Grasses. XVIII. Moisture Relations and Insect Herbivory of the Emergent Stromal Leaf of Epichloe

Abstract

In order to compare relative transpiration in the stromal leaf of Epichloë typhina to that of nonstromal leaves produced on culms without stromata, translocation of dye in excised culms was monitored. Stromal leaf blades accumulated dye rapidly and became densely stained, while minimal staining occurred in nonstromal leaf blades. This is interpreted to indicate that transpiration is enhanced in stromal leaves. Sections of stromal leaves show that several cell types are hypertrophied. The epidermis contains cells with thin outer walls, where in many instances the cells have collapsed or appear ruptured. It is apparent that the stromal leaf has been anatomically modified so that evaporation of water from its surface is increased and unregulatable by the plant. In one experiment stromal leaf blades were excised from stromata resulting in desiccation of upper parts of stromata. The biological role of the stromal leaf may be to draw water through the stroma to replace evaporative loss from the stromal mycelium. Herbivory studies using fall armyworms also indicate that the stromal leaf is chemically altered to deter herbivory. Enhanced insect resistance and drought tolerance in grasses with nonstroma-forming endophytes, that are believed to have evolved from Epichloë, are characteristics that may have been preadapted by the need for Epichloë to alter the stromal leaf and defend it from herbivory.