IMPACT OF VERTICILLIUM-ALBO-ATRUM AND PHOTOSYNTHETIC PHOTON FLUX-DENSITY ON RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE IN RESISTANT ALFALFA

Resistance of an alfalfa clone to Verticillium albo-atrum was attenuat ed by manipulating photosynthetic photon flux density (PPFD). Effects of PPFD and V. albo-atrum on dark respiration, in vivo ribulose-1,5-bi sphosphate carboxylase/oxygenase (Rubisco) activity, in vitro total Ru bisco activity, quantity of Rubisco, molar activity of Rubisco, and ri bulose-1,5-bisphosphate (RuBP) regeneration rate were examined in a fa ctorial experiment with pathogen (inoculated or noninoculated) and PPF D (40, 70, and 100% PPFD) as the treatments. Pathogen X PPFD interacti ons were detected in dark respiration, total protein, and amount of Ru bisco. 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 reg eneration rate of RuBP was reduced in all inoculated plants. The photo synthetic properties of the inoculated, resistant plants under 40% PPF D mimicked those reported for susceptible plants, but only dark respir ation and amount of Rubisco were reduced by the presence of V. albo-at rum; the other properties were reduced by the PPFD treatment. The incr ease in in vitro total Rubisco activity and decrease in RuBP regenerat ion rate in inoculated, resistant alfalfa suggest that photosynthetic acclimation may be optimizing carbon assimilation in alfalfa resistant to V. albo-atrum.