Impact of Verticillium albo-atrum and photosynthetic proton flux density on ribulose-1,5-bisphosphate carboxylase/oxygenase in resistant alfalfa

Resistance of an alfalfa clone to Verticillium albo-atrum was attenuated by manipulating photosynthetic photon flux density (PPFD). Effects of PPFD and V. albo-atrum on dark respiration, in vivo ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, in vitro total Rubisco activity, quantity of Rubisco, molar activity of Rubisco, and ribulose-1,5-bisphosphate (RuBP) regeneration rate were examined in a factorial experiment with pathogen (inoculated or noninoculated) and PPFD (40, 70, and 100% PPFD) as the treatments. Pathogen X PPFD interactions were detected in dark respiration, total protein, and amount of Rubisco. Dark respiration was suppressed by V. albo-atrum under 40% PPFD, increased under 70% PPFD, and unaffected by the pathogen under 100% PPFD. Total protein and Rubisco were reduced in inoculated plants only under 40% PPFD. In vitro total Rubisco activity was increased and regeneration rate of RuBP was reduced in all inoculated plants. The photosynthetic properties of the inoculated, resistant plants under 40% PPFD mimicked those reported for susceptible plants, but only dark respiration and amount of Rubisco were reduced by the presence of V. albo-atrum: the other properties were reduced by the PPFD treatment. The increase in in vitro total Rubisco activity and decrease in RuBP regeneration rate in inoculated, resistant alfalfa suggest that photosynthetic acclimation may be optimizing carbon assimilation in alfalfa resistant to V. albo-atrum