Document: DAN-3-30-2

Timing and mechanism of impact of the Swiss needle cast fungus, Phaeocryptopus gaeumannii, on Douglas-fir needle gas exchange and rubisco activation.

MANTER, D.* ¹, B.BOND ¹, K.KAVANAGH ², P.ROSSO ¹ and G.FILIP ¹

Oregon State University, Corvallis, OR 97331 ¹
University of Idaho, Moscow, ID 83844 ²

Abstract:
Gas exchange of two-year-old Douglas-fir seedlings, artificially inoculated with the Swiss needle cast (SNC) fungus, Phaeocryptopus gaeumannii, was monitored on a monthly basis using A/C_i curve analysis. Gas exchange (CO₂ and H₂O) showed a significant reduction during the winter months; however, no impact of fungal presence on gas exchange was noted until the emergence of pseudothecia from needle stomata. Once present, maximum stomatal conductance was inversely proportional to pseudothecia presence. Impacts of SNC on CO₂ assimilation rates were also observed at the onset of pseudothecia development, and when ca. 19% of the stomata were occluded with pseudothecia, net CO₂ assimilation declined by ca. 50%. A/C_i curve analysis showed that declines in CO₂ assimilation were due to both stomatal and non-stomatal limitations. Stomatal limitations were the direct result of reduced CO₂ diffusion through blocked stomata. Non-stomatal limitations arose, in part, from pseudothecia development and the associated decline in stomatal conductance reducing rubisco activation. For example, in both SNC infected foliage and foliage with artificially blocked stomata (i.e., external application of petroleum jelly), the amount of rubisco activation showed a strong, positive relationship with maximum stomatal conductance. However, the total amount of fungal-derived non-stomatal limitations to CO₂ assimilation could not be attributed to this effect only.

Keywords: gas exchange, rubisco, Pseudotsuga menziesii, Phaeocryptopus gaeumannii

This abstract is being presented at: 3:45 PM in session:
Oral Session #50: Plant Gas Exchange.