WATER DISTRIBUTION IN FUNGAL LESIONS IN THE WOOD OF SYCAMORE, ACER-PSEUDOPLATANUS, DETERMINED GRAVIMETRICALLY AND USING NUCLEAR-MAGNETIC-RESONANCE IMAGING

Water contents in healthy xylem, reaction zones (column boundary layer s) and decayed wood were determined in sycamore (Acer pseudoplatanus L .) naturally infected by sooty bark disease (Cryptostroma corticale (E llis & Everh.) Gregory & Waller) or wound-inoculated with Ustulina deu sta (Fr.) Petrak. or Ganoderma adspersum (Schulz) Donk. Water contents determined gravimetrically were greater in reaction zones than in hea lthy wood by factors of 1.1-2.1 times. Maps of imageable proton densit y (M-0), essentially equivalent to relative water contents, were also calculated from two independent nuclear magnetic resonance (NMR) imagi ng (MRI) data sets, acquired from T-1 or T-2-weighted experiments. M-0 values derived from these, corrected for T-2 and T-1 respectively, we re in good agreement, and paralleled the results obtained when total w ater was determined gravimetrically, although the proportional increas e in M-0 in reaction zones (1.3-3.7 times) was greater than the corres ponding increase in total water. Living wood saturated with water by v acuum infiltration contained approx. 1.3 times more water, determined gravimetrically, than uninfiltrated wood. The proportional increase in M-0 was greater (1.8-2.4 times). This was attributable to the allevia tion of magnetic susceptibility effects by elimination of the gas-wate r interfaces normally present in the uninfiltrated wood. The similarit ies between infiltrated wood and reaction zone tissues in both total g ravimetric water content and NMR-visible water suggest that the biophy sical environment of the water might be similar in bath. This is compa tible with the hypothesis that in reaction zones the gases normally pr esent in the lumens of empty (dead) xylem fibres are replaced by an aq ueous solution of phytoalexin-like compounds. This has been confirmed by microscopy. The implications of this in relation to antimicrobial d efence and xylem function are discussed. Although contrast in NMR imag es of lesions was predominantly attributable to changes in M-0, a decr ease in the spin-spin relaxation time (T-2) was seen in the vicinity o f some lesions. Spin-lattice relaxation times (T-1) were not altered t o the same extent. Results obtained from both early and late stages of decay were much more variable than those from healthy wood or from re action zones, and reflected both the low water content of decayed wood (and hence poor signal-to-noise ratios for NMR data) and the changing physical environment as the wood was degraded.