Mechanism of Photosynthesis Decrease by Verticillium dahliae in Potato

Abstract

Young, visually symptomless leaves from potato (Solanum tuberosum) plants infected with Verticillium dahliae exhibited reduced carbon assimilation rate, stomatal conductance, and intercellular C02, but no increase in dark respiration, no change in the relationship between carbon assimilation rate versus intercellular C02, and no change in light use efficiency when intercellular CO2 was held constant. Therefore, the initial decrease in photosynthesis caused by V. dahliae was caused by stomatal closure.Errors in the intercellular CO2 calculation caused by uneven distribution of carbon assimilation rate across the leaf were tested by "4CO2 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.on the biochemistry of photosynthesis can be ruled out, especially if the relationship is determined at several light levels. If differences are found between control and infected plants, these differences can be interpreted using biochemically based models of photosynthesis (9,20).Using this analysis can be complicated when stomata close unevenly across the leaf (6,23). This patchiness has been observed in water-stressed plants (6, 21). The occurrence of patchy photosynthesis can be checked by autoradiography.We have tested for patchy photosynthesis using autoradiography in control and Verticillium infected potatoes. We determined the relationship between photosynthesis and Ci at three light levels. Then we compared the relationship between stomatal conductance and water potential. MATERIALS AND METHODSThe potato early dying disease causes losses of 30 Inoculum PreparationInoculum was produced from pathogenic cultures of Verticillium dahliae which were isolated from potato. In experiment 1, eight cultures were grown on 10% strength Difco potato dextrose agar for 2 weeks. The agar containing the fungal colonies was homogenized in a blender, then mixed with peat-vermiculite at the rate of 1 mL/L of soil mix. The infested soil was dried and stored 5 months at room temperature prior to use.In experiments 2 through 4, three isolates of V. dahliae from potato were grown on autoclaved rye grain at 20°C for 4 weeks. The infested grain was dried for 2 weeks at room temperature, then ground in a Wiley mill. Dried, ground rye www.plantphysiol.org on May 12, 2018 -Published by Downloaded from

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