Energy Dissipation and Photoinhibition in Douglas-Fir Needles with a Fungal-Mediated Reduction in Photosynthetic Rates

The dissipation of absorbed light and potential for photooxidative damage was explored in Douglas-fir (Pseudotsuga menziesii ) seedlings with and without Phaeocryptopus gaeumannii infection. The presence of P. gaeumannii significantly reduced net CO2 assimilation rates from ca. 6 lmol/m/s to 1.5 lmol/m/s, without any significant impact on chloroplast pigments. The partitioning of absorbed light-energy to photochemistry or thermal dissipation was determined from chlorophyll fluorescence measurements. Maximum thermal dissipation for both control and infected needles was ca. 80%, consistent with the similar xanthophyll pool sizes in the two treatments. At high photosynthetic photon flux density (PPFD), when thermal dissipation was maximized, the lower photochemical utilization in infected needles resulted in greater amounts of excess absorbed light (ca. 20 and 10% for the infected and control needles, respectively). A second experiment, monitoring changes in photosystem II (PSII) efficiency (Fv=Fm) in response to a 1 h high light treatment (PPFD 1⁄4 2000 lmol/m/s) also suggests that infected needles absorb greater amounts of excess light. In this experiment, declines in Fv=Fm were 1.5 times greater in infected needles, despite the similar xanthophyll pool sizes. Furthermore, increases in minimum fluorescence (178 and 122% of initial values for the infected and control needles, respectively) suggest that the reduction in PSII efficiency is largely attributable to photooxidative damage. Finally, reductions in PSII efficiency under high light conditions provide a plausible explanation for the greater pathogenicity (e.g. premature needle abscission) of P. gaeumannii in sun-exposed foliage. Introduction In healthy plants light absorption may exceed photosynthetic utilization resulting in an excess of absorbed light (Demmig-Adams and Adams, 1996; DemmigAdams et al., 1996), which unless thermally dissipated can lead to photooxidative damage. Thermal dissipation results in a reduced efficiency of PSII reaction centres, i.e. reduction in Fv=Fm, of which, a major component is related to the presence of the de-epoxidized xanthophyll pigments (zeaxanthin and antheraxanthin, Z and A). The exact mechanism of Z and A energy dissipation from PSII centers is unclear (cf. Demmig-Adams and Adams, 1992; Horton and Ruban, 1992); however, a close correlation between thermal dissipation and the levels of Z and A has been reported for several plant species (Demmig-Adams and Adams, 1996). Furthermore, the maximum thermal dissipation capacity has an upper limitation based on the size of the xanthophyll pool [i.e. xanthophylls per unit chlorophyll (Chl), Demmig-Adams and Adams, 1994]. At high light, when thermal dissipation may be maximized (Demmig-Adams and Adams, 1996), any reduction in photochemical quenching – without a corresponding decline in total light absorption – will result in greater amounts of excess absorbed light. One situation where this may occur is at low temperatures, as the Calvin cycle is reduced to a greater extent than electron transport (Baker, 1994). Higher antioxidant pools during winter (Logan et al., 1998) suggest the presence of photooxidative conditions (triplet exited Chl, singlet oxygen formation, etc.) at low temperatures; however, because of an increased thermal dissipation capacity, photooxidation is less than expected based on summer measurements. For example, the size of the xanthophyll pool increases during winter due to both an increase in xanthophylls and a decline in chlorophyll (Ottander et al., 1995; Logan et al., 1998). The alteration of needle physiology by pathogenic fungi may represent another scenario where the total U. S. Copyright Clearance Centre Code Statement: 0931–1785/2002/1512–0674 $ 15.00/0 www.blackwell.de/synergy J. Phytopathology 150, 674–679 (2002) 2002 Blackwell Verlag, Berlin ISSN 0931-1785 dissipation of absorbed light-energy by thermal dissipation and photochemical quenching is reduced, resulting in greater quantities of excess absorbed light and photooxidative conditions. For example, the biotrophic foliar pathogen of Douglas-fir [Psuedotsuga menziesii (Mirb.) Franco], Phaeocryptopus gaeumannii (Rhode) Petrak, causes a significant decline in photosynthetic rates by an apparent occlusion of needle stomata (i.e. fungal fruiting bodies emerge from stomatal cavities reducing gas exchange) without any apparent changes in intrinsic PSII efficiency (Manter et al., 2000). Assuming that light absorption and thermal dissipation capacity are unchanged, this decline in photosynthesis (i.e. photochemical quenching) would result in excess absorbed light and potential photooxidative damage in infected needles. The presence of photooxidative damage in P. gaeumanniiinfected needles is unknown; however, symptom development (i.e. needle abscission) is exacerbated in sun-exposed foliage (Manter, 2001), or the conditions were the balance between light absorption and dissipation may be most critical, suggesting that photooxidation may influence P. gaeumannii pathogenicity and symptom development. The purpose of this study was to determine if photooxidative damage occurs in P. gaeumannii-infected Douglas-fir needles. The three primary objectives were to (i) determine the relative rates of photochemical and thermal dissipation, (ii) quantify the chloroplast pigments and (iii) determine changes in PSII efficiency under high light exposures in seedlings with and without P. gaeumannii infection. Materials and Methods