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Rug ^05 99 01:35p IVathaneil Sperry & Teresa (541) 461 -0091 p.2 inside the root, and there is a certain amount of dissolved air in water. Although waterlogging can be deadly, periodic saturation of pore space by water is desirable, for it forces gases out of the soil pores. As the soil dries to field capacity, air re- enters the pore system, ventilating the soil. Gases also move through the soil by diffusion, and by mass movement due to diurnal temperature change. Wounding and exposure of roots often goes along with soil compaction. When roots are mechanically wounded, infections by facultative parasites are facilitated. Defects resulting from wounds can extend into the root crown, and columns of discoloration and decay from various wounds can coalesce in the stem. Although root tissue might continue to generate, roots gradually become hollow and weakened. Wound fungi cause the rooting zone to shrink and anchorage to reduce, although much more slowly than the aggressive pathogens [Sharon, 1980 #72]. The growth vs. differentiation model The growth vs. differentiation model of plant resource allocation is that the production of defensive compounds occurs when growth (cell division and enlargement) slows or ceases. Whenever all factors favor growth, it predominates within the plant. When moderate environmental stresses limit growth, but are not severe enough to stop photosynthesis entirely, • carbohydrates from photosynthesis accumulate in excess of growth requirements, and carbon compounds are allocated to the defensive infrastructure. In the Willamette Valley, Douglas -fir and similar species have adapted to summer drought, but irrigation and other common cultural practices, including mulching and nitrogen fertilization, can force the growth cycle to continue through the dry summer. This growth is at the expense of the chemical differentiation phase. An imbalance between the growth and differentiation stages reduces the ability of the tree to defend against microbial attack, and therefore predisposes the root system to decline. A tree which is predisposed in this way would be expected to produce a larger ratio of early wood (growth) to late wood (differentiation) than a tree in its normal rhythm. Such a tree has not shifted its metabolism from the growth processes of cell division and enlargement to differentiation, where the build up and maintenance of the constitutive and induced defense reactions occur. Because the primary (growth) and secondary (differentiation) metabolic pathways share common chemical precursors and intermediates, the cost of switching back and forth between growth and differentiation is low. The chemical and morphological changes include lignification and thickening of cell walls, and the production of substances such as gums and resins [Lorio, 1988 #126]. A tree which is forced to maintain the growth processes becomes more susceptible to disease and insect attack, because preformed resin, tannins, pisatin, and other allelochemicals are not being produced [Herms, 1992 #29j. 2