Alterations in Chloroplast Thylakoids during Cold Acclimation

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Abstract
Freeze-fracture EM revealed a decreased particle concentration on the inner fracture face of acclimated thylakoids [from Spinacia oleracea], suggestive of some alteration(s) in the hydrophobic region. Sonic oscillation caused a reversal of the altered particle concentration in acclimated thylakoids and suggested 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 were of one size group (.+-. 140 .ANG.) as compared to 2 size groups (.+-. 100 .ANG. and .+-. 165 .ANG.) for nonacclimated thylakoids. The paracrystalline array might be associated with the acclimated state of thylakoids. Nonacclimated thylakoids required 50 mM sucrose for maximum protection of light-dependent proton uptake, while acclimated thylakoids required 25 mM sucrose, and the protection afforded acclimated thylakoids during a freeze-thaw cycle was greater. Sucrose was required for alterations in acclimated thylakoids to be manifested. Apparently increased hardiness was not only associated with changes in cellular environment but also alterations in membranes.