ANATOMICAL, PHYSIOLOGICAL, AND BIOCHEMICAL EVIDENCE FOR THE C3 PATHWAY IN VERBASCUM THAPSUS L.

Abstract
SUMMARY: Anatomical, physiological and biochemical characteristics of Verbascum thapsus demonstrate that this species is a C3 carbon fixation type rather than C4 as suggested by Parkhurst (1976). Leaf anatomy, net photosynthesis rate, carbon dioxide compensation point, photorespiration rate, temperature optimum for photosynthesis and ratio of ribulose‐1,5‐diphosphate carboxylase to phosphoenolpyruvate carboxylase activity for V. thapsus are all typical of C3 rather than C4 species. The data presented illustrate the danger of assuming a species is either C3or C4 based on a single characteristic. Parkhurst (1976) predicted that optimum photosynthesis for V. thapsus would occur at high temperature and would increase faster than transpiration with increased temperature. Our data are contrary to these predictions.