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Rings of collapsed cells in Pinus radiata stemwood from lysimeter-grown trees subjected to drought

Rings of collapsed or crushed tracheids, whose thin walls have secondary thickening but lack lignin, have been found in the stems of Pinus radiata trees grown in containers and subjected to periodic drought. These rings appear to have arisen as a result of minimal waterings, sufficient to prevent mortality. Ring shake in forest-grown trees may be caused similarly. INTRODUCTION The formation of false rings is a well-known effect of drought in conifers (e.g., Glerum, 1970). The cells of these rings, as seen in transverse section, have dimensions similar to those of latewood tracheids, although their walls are not so thick and the tangential walls may have some concavity, particularly near the base of the terminal leader. The presence of such false rings was expected in the stemwood when Pinus radiata trees, growing in lysimeters, were subjected to drought. Preliminary microscopic examination showed, however, that what appeared to the unaided eye to be false rings consisted partly of rings of crushed or collapsed tracheids. This note describes the phenomenon in detail, and its possible causes are discussed. MATERIALS A N D METHODS The wood examined was from trees which were part of an experiment dealing with soil water deficits, described by Jackson et al. (191G). Rooted cuttings of P. radiata (FRI clones 450, 451 and 460), 27 months old, were planted in evapotranspirometer units of 2.72 m capacity, spaced at 4 X 5 m. After allowing a year for the trees to stabilise, periodic water deficits were applied by excluding rainfall from the surfaces of the units, but not from the tree crowns, at intervals for 5 years. During these periods, soil moisture was maintained at 15% by volume to prevent mortality. Over the 5 years of the experiment, all trees were submitted at various times to year-round deficit, June to November deficit, December to May deficit, or no deficit throughout a year (June to May). During periods of nil deficit, the containers received rainfall, supplemented by watering during natural droughts to keep soil moisture content above 40% by volume. At the end of the experiment the trees were harvested and stemwood discs were removed for examination. From this material transverse sections were cut for light microscopy, using a sledge microtome. These sections were left unstained, or were N.Z. J. For . Sci. 6(2): 461-5 (1976). 462 New Zealand Journal of .Forestry Science Vol. 6 stained with safranin and fast green. Samples of the cambium and adjacent xylem and phloem were removed at the same time, and prepared for light microscopy as described elsewhere (Barnett, 1971). The sections thus obtained were examined by bright field, polarisation, and UV fluorescence microscopy.