Mechanism of Photosynthesis Decrease by Verticillium dahliae in Potato.

Young, visually symptomless leaves from potato (Solanum tuberosum) plants infected with Verticillium dahliae exhibited reduced carbon assimilation rate, stomatal conductance, and intercellular CO(2), but no increase in dark respiration, no change in the relationship between carbon assimilation rate versus intercellular CO(2), and no change in light use efficiency when intercellular CO(2) was held constant. Therefore, the initial decrease in photosynthesis caused by V. dahliae was caused by stomatal closure. Errors in the intercellular CO(2) calculation caused by uneven distribution of carbon assimilation rate across the leaf were tested by (14)CO(2) autoradiography. Patchiness was found at a low frequency. Low stomatal conductance was correlated with low leaf water potentials. Infection did not affect leaf osmotic potentials.