Comparison of lignin and polysaccharide sugar contents for slash, longleaf, and loblolly pine growth rings formed during periods of soil moisture extremes
Lignin contents of mature wood growth rings formed under soil moisture extremes were determined to investigate the possibility of drought-caused reductions in lignin deposition for the southern pines. A well-defined set of slash (Pinus elliottii Engelm.), longleaf (Pinus palustris Mill.) and loblolly pine (Pinus taeda L.) increment cores were processed to excise growth rings formed during multiyear periods of above- (wet) or below-normal (drought) soil moisture. The average acid-insoluble (Klason) lignin content of the resultant drought-formed slash pine wood samples was signifcantly lower (31.4% vs. 34.1%; P = 0.0005) than that for the corresponding wood samples from growth rings formed under conditions of ample soil moisture; respectively higher glucan (40.6% vs. 36.9%; P = 0.0010) and mannan (11.7 vs. 10.4%; P = 0.0010) values were also observed. Small, but significant differences for other hemicellulose derived sugars (xylan, arabinan) were determined for the longleaf and loblolly pine wood samples. Altogether, these data represent the first wet chemical results showing differences in lignin and polysaccharide sugar contents for mature wood formed in pine trees during annual droughts. Among the southern pines evaluated here, slash pine is the least drought tolerant; the lower lignin content observed for the slash pine drought-formed wood is particularly intriguing since it occurred with the species most susceptible to water stress.