Photosynthetic Mechanisms of Weeds in Taiwan

Abstract

One hundred and one species (in 36 families) of weeds on cultivated land in Taiwan were investigated for the occurrence of Kranz leaf anatomy and activities of key enzymes of C₄ photosynthesis to determine their photosynthetic mechanisms. Based on the anatomical and biochemical analyses, 75 species were found to possess the C₃ and 26 species the C₄ pathway of photosynthetic CO₂ fixation. Among the 26 C₄ species, 15 species are in Gramineae, 6 in Cyperaceae, 2 each in Euphorbiaceae and Amaranthaceae, and 1 in Portulacaceae. Two C₄ species in the Gramineae, namely Digitaria radicosa (Presl) Miq. and Sporobolus fertilis (Steud.) Clayton, were recorded as C4 plants for the first time. The biochemical subdivisions of these C₄ weeds were also determined. As in the natural C₄ populations, the NADP-malic enzyme subtype of C₄ photosynthesis dominates the list of C₄ weeds on this island (62%), while the PEP carboxykinase subtype is relatively rare (12%). NAD-malic enzyme subtype has an intermediate representation (26%). The high proportion of weeds in Taiwan being C₃ plants is noteworthy, and it may be accounted for by the high precipitation in this subtropical island.