Physiological Effects of Salinity Stress and a Stem Canker Pathogen On Red Maple (Acer rubrum var. October Glory)

In the typical urban or planned landscape, tree decline is often linked to a variety of abiotic and biotic stress factors as opposed to a single highly virulent pathogen or one damaging abiotic stress event.  The purpose of this ongoing study is to determine the physiological effects on red maple of salinity stress, inoculation with the stem canker pathogen Phytophthora cactorum, and combination of the two.  Three year old maple trees were obtained in April as bare-root whips (2-3 meter height from soil, 15 mm caliper) and replanted in 5-gallon containers in a washed sand media.  Trees were grown out-doors (Seattle, WA) for one season to recover from transplant shock and maintained under well-irrigated (drip) conditions with weekly fertilization with full-strength Hoagland’s #2.  In the fall prior to leaf senescence half of the trees were stem-inoculated with agar plugs taken from the margin of actively growing colonies of P. cactorum, which was isolated from the margin of a bleeding canker on red maple, while the other half were mock-inoculated with a sterile agar plug.  Inoculations were made by slicing an inverted U-shaped flap of bark and inserting the plug upper-surface facing inwards between the bark and phloem tissue.  Wounds were immediately wrapped in sterile gauze, moistened with sterile water, wrapped in parafilm, and covered in foil.  The following spring upon full leaf expansion, salinity treatments were imposed on half of the inoculated and half of the mock-inoculated plants through weekly flooding of the container with a 200-mM NaCl solution.  Plant health and physiological responses including growth, chlorophyll content, chlorophyll fluorescence, carbon assimilation, and stress metabolites were determined.  Differences in size of the necrotic lesion expanding from the inoculation point are indicative of relative disease resistance and are compared between treatments.  In addition, we report the internal conductance to carbon dioxide g_i under all treatments using two independent methods. Recent research indicates that g_i may be altered under stress conditions and may act as an important limitation to carbon assimilation under abiotic or biotic stress.  To our knowledge this parameter of photosynthetic performance has not been reported for red maple under stress or disease conditions.