Limitation to photosynthesis in Pratylenchus penetrans- and Verticillium dahliae-infected potato

Mechanism(s) responsible for decrease In photosynthetic rates of potato (So lanum tuberosum L.) leaves infected by the fungus Verticillium. dahliae (Kl eb) and the nematode Pratylenchus penetrans (Cobb, Sher, Alien) have not be en fully researched, Two growth chamber experiments were undertaken to dete rmine the factors contributing to the decrease in CO2 exchange rates of you ng, fully expanded leaflets of potato (cv. Russet Burbank) plants grown in pots infested with P. penetrans and/or V. dahliae, Treatments,were P. penet rans-infested soil, V. dahliae-infested soil, soil infested with both the n ematode and the fungus, and a noninfested control. Leaf COL response curves were measured at early (16 d after inoculation [DAI]) and late (42 DAI) st ages of infection for all treatments at saturating light (1500 mu mol m(-2) s(-1) of photosynthetically active radiation [PAR] using a portable photos ynthesis system. Carbon dioxide exchange rates were also measured at 1000, 400, and 200 mu mol m(-2) s(-1) PAR to determine leaf light response. At am bient CO2 concentration, concomitant infection by both pathogens significan tly reduced C assimilation rate (A) and light use efficiency (mu moles CO2 fixed per mu mol 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 pat hogen alone had Little or no effect on the leaf gas exchange parameters. An alysis of the curve relating A and C-i showed that either treatment alone d id not change the initial slope of the curve at 16 DAI. A significant reduc tion in both the initial slope of A vs. C-i curves and A. at C-i = 500 mu m ol mol(-1) in the jointly infected plants were noticeable at 42 DAI indicat ing 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 infec ted plants.