Photomorphogenically Defined Light and Resistance of Poa pratensis to Drechslera sorokiniana.

Photomorphogenic light definitions were derived by mathematical determination of the estimated phytochrome photoequilibrium for each light treatment spectrum. A wide range of photomorphogenic light treatments represented by spectra with estimated phytochrome photoequilibria of 0.45, 0.54, 0.60, 0.67, and 0.71 was utilized to determine the influence of photomorphogenically defined light on resistance of Poa pratensis L. to pathogenesis by Drechslera sorokiniana. Accurate resolution of D. sorokiniana leaf spot development required evaluation of separate leaf ages due to the sequential appearance, development, and senescence of P. pratensis leaves. Disease development (all light treatments) was greatest on leaf 4 (oldest, postmature) followed by leaf 1 (youngest, premature). Low levels of disease occurred on leaves 2 and 3 (mature). Photomorphogenic light defined by estimated phytochrome photoequilibria greater than 0.60 (natural light approximately 0.60) was most disease promotive on leaf 1. Conversely, photomorphogenic light defined by estimated phytochrome photoequilibria of less than 0.60 was most disease promotive on leaf 4. These responses indicate that inherent resistance or susceptibility expressed by P. pratensis to pathogenesis by D. sorokiniana is regulated in part by leaf age (developmental senescent stage) and by photomorphogenically defined light quality. A hypothesis is presented and discussed which integrates and speculates on these observations with respect to the literature.