Turgor regulation via cell wall adjustment in white spruce

Turgor regulation at reduced water contents was closely associated with cha nges in the elastic quality of the cell walls of individual needles and sho ots of naturally drought-resistant seedlings of white spruce (Picea glauca [Moench] Voss.) and of seedlings of intermediate resistance that had been p retreated with paclobutrazol, a stress-protecting, synthetic plant-growth r egulator. Paclobutrazol-treated seedlings showed marked increases in drough t resistance, and pressure-volume analysis combined with Chardakov measurem ents confirmed observations that water stress was ameliorated during prolon ged drought. Turgor was maintained in the paclobutrazol-treated and in the naturally resistant drought-stressed seedlings despite water contents near or below the turgor-loss volumes of well-watered controls. The maintenance of turgor in these seedlings was in large part a function of the dynamic pr ocess of cell wall adjustment, as reflected by marked reductions in both th e saturated and turgor-loss volumes and by large increases in the elastic c oefficients of the tissues. Shear and Young's moduli, calculated from press ure-volume curves and the radii and wall thicknesses of mesophyll cells, al so confirmed observed changes in the elastic qualities of the cell walls. E lastic coefficients of well-watered, paclobutrazol-treated seedlings were c onsistently larger than those in well-watered controls and several times la rger than the values in untreated plants, which succumbed rapidly to drough t. In contrast, untreated seedlings that withstood prolonged drought withou t wilting displayed elastic coefficients similar to those in seedlings that had been treated with paclobutrazol but that had not been exposed to droug ht.