Photosynthetic Characteristics of the C3-C4 Intermediate Parthenium hysterophorus

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The weedy species Parthenium hysterophorus (Asteraceae) possesses a Kranz-like leaf anatomy. The bundle sheath cells are thick-walled and contain numerous granal chloroplasts, prominent mitochondria, and peroxisomes, all largely arranged in a centripetal position. Both mesophyll and bundle sheath chloroplasts accumulate starch. P. hysterophorus exhibits reduced photorespiration as indicated by a moderately low CO2 compensation concentration (20-25 microliters per liter at 30°C and 21% O2) and by a reduced sensitivity of net photosynthesis to 21% O2. In contrast, the related C3 species P. incanum and P. argentatum (guayule) lack Kranz anatomy, have higher CO2 compensation concentrations (about 55 microliters per liter), and show a greater inhibition of photosynthesis by 21% O2. Furthermore, in P. hysterophorus the CO2 compensation concentration is relatively less sensitive to changes in O2 concentrations and shows a biphasic response to changing O2, with a transition point at about 11% O2. Based on these results, P. hysterophorus is classified as a C3-C4 intermediate. The activities of diagnostic enzymes of C4 photosynthesis in P. hysterophorus were very low, comparable to those observed in the C3 species P. incanum (e.g. phosphoenolpyruvate carboxylase activity of 10-29 micromoles per milligram of chlorophyll per hour). Exposures of leaves of each species to 14CO2 (for 8 seconds) in the light resulted in 3-phosphoglycerate and sugar phosphates being the predominant initial 14C products (77-84%), with ≤4% of the 14C-label in malate plus aspartate. These results indicate that in the C3-C4 intermediate P. hysterophorus, the reduction in leaf photorespiration cannot be attributed to C4 photosynthesis.

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