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CROP PHYSIOLOGY & METABOLISM

Limitation to Photosynthesis in Pratylenchus penetrans- and Verticillium dahliae-Infected Potato

^a Department of Plant Pathology, 1630 Linden Dr., University of Wisconsin, Madison, WI 53706 USA
^b Dep. of Botany, University of Wisconsin, Madison, WI 53706 USA

macguid{at}macc.wisc.edu

Mechanism(s) responsible for decrease in photosynthetic rates of potato (Solanum tuberosum L.) leaves infected by the fungus Verticillium dahliae (Kleb) and the nematode Pratylenchus penetrans (Cobb, Sher, Allen) have not been fully researched. Two growth chamber experiments were undertaken to determine the factors contributing to the decrease in CO₂ exchange rates of young, fully expanded leaflets of potato (cv. Russet Burbank) plants grown in pots infested with P. penetrans and/or V. dahliae. Treatments were P. penetrans-infested soil, V. dahliae-infested soil, soil infested with both the nematode and the fungus, and a noninfested control. Leaf CO₂ response curves were measured at early (16 d after inoculation [DAI]) and late (42 DAI) stages of infection for all treatments at saturating light (1500 µmol m^-2 s^-1 of photosynthetically active radiation [PAR] using a portable photosynthesis system. Carbon dioxide exchange rates were also measured at 1000, 400, and 200 µmol m^-2 s^-1 PAR to determine leaf light response. At ambient CO₂ concentration, concomitant infection by both pathogens significantly reduced C assimilation rate (A) and light use efficiency (µmoles CO₂ fixed per µmol of light used), and increased the intercellular CO₂ (C_i ) of these young leaves at 42 DAI, but not at 16 DAI. Infection by either pathogen alone had little or no effect on the leaf gas exchange parameters. Analysis of the curve relating A and C_i showed that either treatment alone did not change the initial slope of the curve at 16 DAI. A significant reduction in both the initial slope of A vs. C_i curves and A at C_i = 500 µmol mol^-1 in the jointly infected plants were noticeable at 42 DAI indicating that nonstomatal effects could explain the reduction in C assimilation rate at this late stage of disease infection. Leaf patchiness might also be a contributing factor to this phenomena in the leaves of the jointly infected plants.

Abbreviations: ATP, adenosine triphosphate • DAI, days after inoculation • PAR, photosynthetically active radiation • PED, potato early dying disease • Rubisco, carboxylase-oxygenase • RuBP, ribulose bisphosphate

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