Radial gradients in nitrogen content across stems of three hardwoods and two softwoods followed similar patterns. Sapwood and inner bark contained a higher proportion of the total nitrogen in cross sections than heartwood and outer bark, respectively. Nitrogen content of individual annual increments decreased progressively from the cambium to the transition zone between sapwood and heartwood, or, in species without heartwood, to the increments nearest the pith. In some cases, an abrupt decrease in nitrogen content was observed across the sapwood–heartwood boundary. The diminution of nitrogen content across the sapwood was associated with death of parenchyma cells. As the cells die, nitrogen in their cytoplasm apparently is retrieved for possible reuse elsewhere in the tree. Pith contained more nitrogen by weight than heartwood and in some cases more than sapwood. Nitrogen content was correlated directly with parenchyma volume for sapwood but not for heartwood of 16 species of angiosperms.Nitrogen extractable from sapwood with neutral solvents consisted in part of free amino acids but only traces of these acids were detected in heartwood. Hydrolysates of sapwood and heartwood prepared with 6 N HCl contained many typical protein amino acids.

Based on present knowledge of the origin, amounts, chemical form, and distribution of nitrogen ( N ) in wood, hypotheses are proposed to explain radial gradients in N content that exist across the xylem cylinder of tree stems: (1) N in the cytoplasn~ of developing wood cells is diluted by apposition of cell wall substances; (2) after maturation of wood fiber cells, N in their cytoplasm is removed by elution into the transpiration stream; (3) death of xylem parenchyma cells during aging of sapwood and forination of heartwood is accompanied by reinoval of much of the X in their cytoplasm. Hypotheses 2 and 3 above suggest strongly that trees possess an internal recycling mechanism for conservation and reuse of the N in the cytoplasm of xylary cells.Although the supply of N in wood is meager, wood-destroying fungi readily inetabolize the carbon-rich coilstituents of wood and produce large fruiting structures that release vast numbers of spores in nature. To account for these capacities, we postulate that these fungi employ one or more of the following three mechanisms: (1) preferential allocation of N obtainable from wood to substances and pathways highly efficient in the use of wood constituents; (2) reuse of N obtainable from wood by a dynamic and continuous process of autolysis and reuse without significant loss of N ; (3) utilization of N sources outside the wood itself, for example, by fixation of atlnospheric N.

Research of nonspecific defense processes in woody plants has focused on xylem. From these studies the concept of compartmentalization sensu Shigo was developed. Responses of bark to injury and infection, however, are understood poorly. This discussion summarizes evidence for the occurrence of processes similar in function to xylem compartmentalization in bark. We suggest that a developmental approach to research on woody plant responses to injury and infection could result in conceptual unity regarding structural and functional changes in both xylem and bark.

Cuttings of poplar hybrid NE-388 (P. maximowiczii Henry × trichocarpa Torr. & Gray) were inoculated with mycelium of Cytospora chrysosperma (Pers.) Fr. Bark samples for histological study were taken at varying times after inoculation from slowly enlarging cankers, blackstem, control wounds, and uninoculated healthy plants. Histological examination revealed that slowly enlarging cankers are characterized by hyphae in the xylem and necrosis of the cambium ahead of visible bark symptoms, by large- and small-diameter hyphae both between and within host cells, and by hyphal aggregations associated with the leading edge of bark colonization. The blackstem symptom is characterized by rapid colonization of the cortex and phloem by large-diameter intercellular hyphae followed by the digestion of cell contents by small-diameter intracellular hyphae. Histochemical tests indicate degradation of pectic substances, loss of cellulose and starch, and deposition of wound gums. Lignification is not involved. Host response mechanisms associated with wounding and pathogen attack are similar. The underlying common denominators are the formation of a nonsuberized impervious tissue, followed by formation of necrophylactic periderm and redifferentiation of new phellogen and cambium.

Camavi~elk baafieME gen. md sp.nov. (Ascomycetes: Wytismaaes) is a previously undeseribed needlecast fungus attackicing Phw strobm throughout the eastern U.S.A. from North Carolina to Maine. Pinus peuce was severely infected in m d e m Vernon%. Hysterofiecia average 3.2 cm long and are epipbylHous, subhypodemal, and dematiaesns, with clavate asei bearing short rod-sha* ascospores. Qcnidia are epiphyllous , subhypdemd, camalorous, blisterlike with one or more pores, md produce minute bacd.lar conidia. These c h r n c t e~s~c s separate this fungus from existing genera as presendy defined. R k m 4 : Le C a n a v i~e l k bmfieMii$en. et sp.nov. (Ascsmycktes : m y~s m d e s ) est rn chmpignsn du rouge des aiguilks, won-dtc~t jmqu'ici, qaai s9a$t%que au Pinu sfmbus dans I'ensemble de 19est des f h t s -~n i s , du nard de la Caroline aaa Maine. I 2 Pkus peace est se$rement infect& dam le word du Vermont. k s hysdro&&es mesurent environ 3.2 crn de long et sent kpiphyfles, subhypdedques et demtiCs, avec des asques clavifomes portant de c m~a ascospres en fame de batornet. Les pycnides sont Cpiphylles, s u b h y e e~q u e s , concoEores, en fome de pustule avec un ou p i i u~&~~s pores; eIles grduisent de petites csnidies m fome de bciles. Ces caract6ristiques distinpent e champignoa des genres existants, tels que prb=ntement dtfinis. Mots clds : Carnvirgella banfieuii, Pinm s~robus, Pinus peuce, rouge des d p a e s . [Traaduit par $a raction] An emly-season blighting of cument-year needles of Pisus strobw L. was first repod& by Dma (19688) as occurring koughout the nodeastem U. S .A. Since hen, several individuals studied the pmblem without elucidating a cause (see review by Dreisbach 1989). Bmfield (1963) attributed the problem to a nmdecast hngus, desc,bing the sequenw of development of the signs md sympmms, including the gross ces sf the sexual and as Ihby (=Me was o v a l m M or ignored far nmly th-ee decades9 md more recendy the blighthg was widely a&butd to ozone injury (e.g., Bennett 1986).