Alterations in Chloroplast Thylakoids during Cold Acclimation.

Freeze-fracture electron microscopy reveals a decreased particle concentration on the inner fracture face of acclimated thylakoids, suggestive of some alteration(s) in the hydrophobic region. Sonic oscillation causes a reversal of the altered particle concentration in acclimated thylakoids and suggests that increases in unsaturation of fatty acids can, at most, account for only part of the altered particle concentration. The particles on the inner fracture face of acclimated thylakoids are of one size group (+/- 140 A) as compared to two size groups (+/- 100 A and +/- 165 A) for nonacclimated thylakoids. The paracrystalline array might be associated with the acclimated state of thylakoids. Nonacclimated thylakoids require 50 mm sucrose for maximum protection of light-dependent proton uptake, while acclimated thylakoids require 25 mm sucrose, and the protection afforded acclimated thylakoids during a freeze-thaw cycle is greater. Sucrose is required for alterations in acclimated thylakoids to be manifested. Apparently increased hardiness is not only associated with changes in cellular environment but also alterations in membranes.