MORPHOLOGICAL-CHANGES AND PHOTOSYNTHATE ALLOCATION IN AGING HEBELOMA CRUSTULINIFORME (BULL) QUEL AND LACCARIA BICOLOR (MAIRE) ORTON MYCORRHIZAS OF PINUS-PONDEROSA DOUGL EX LAWS

Seedlings of Pinus ponderosa Dougl. ex. Laws were grown in root observation containers. They were inoculated with either Hebeloma crustuliniforme (Bull.) Quel. or Laccaria bicolor (Maire) Orton or left uninoculated (control). On a monthly basis starting at the eighth month of a 12-month growing period, roots (mycorrhizal root tips from inoculated plants, non-mycorrhizal from control plants) were traced onto acetate sheets. Each root tip was classified as 'light', 'intermediate' or 'dark' in colour. Roots initiated between months eight and nine were monitored for the next 90 d. All root tips progressed from light-brown to dark-brown to black for all three treatments. Hebeloma crustuliniforme, and to a lesser extent L. bicolor, retarded this progression relative to the control. At the end of 12 months, seedlings were labelled with (CO2)-C-14 to determine the effects of ectomycorrhizal inoculation on carbon supply to roots of the different morphological categories. The amount of C-14 in 'light' L. bicolor and H. crustuliniforme mycorrhizas was 2.3 and 1.8 times greater, respectively, than that in 'light' control root tips. The amount of C-14 in mycorrhizas of the inoculated treatments and, to a lesser extent, roots of the control seedlings decreased as they progressed from 'light' to 'dark'. It is concluded that ponderosa pine seedlings continue to allocate photosynthate to morphologically older roots, perhaps to meet maintenance requirements or to supply carbon for growth and metabolism of extra-matrical hyphae. Such allocation map enhance root longevity, which would have an important influence on tree, forest and soil carbon budgets.