DIFFERENTIAL RESPONSE TO SHADING IN ORTHOTROPIC AND PLAGIOTROPIC SHOOTS OF THE CLONAL HERB GLECHOMA-HIRSUTA

Previous studies have shown that internodes and petioles of closely re lated erect and stoloniferous species show marked differences in their response to shading. Vertical structures show strong elongation respo nses while horizontal structures show significantly smaller elongation responses. This paper reports an experiment designed to test whether internodes and petioles on orthotropic (vertically oriented) and plagi otropic (horizontally oriented) shoots produced by the same plant, als o show such differential responses. The study species, Glechoma hirsut a, produces plagiotropic shoots in its vegetative stage and orthotropi c shoots during its generative stage. Shoots of G. hirsuta were grown either in full daylight or under simulated canopy shade. Internode and petiole elongation, biomass investment patterns and other growth-rela ted parameters were measured on plants in each light treatment. In ort hotropic shoots the length of internodes responded more strongly to sh ading than the length of petioles, while the opposite was true for pla giotropic shoots, confirming the hypothesis that vertical spacers have higher degrees of shade-induced plasticity than horizontally oriented spacers. Growth and development of horizontally oriented shoots was r educed by shading, whereas that of vertically oriented shoots was unaf fected by light treatment. The results are discussed in terms of the d ifferences in function of vertically and horizontally oriented spacers , and the probable benefits of plasticity in enhancing essential activ ities including photosynthesis, habitat exploration and seed dispersal .