Loss of Adenosine triphosphate synthesis caused by freezing and its relationship to frost hardiness problems

Freezing inactivates the phosphorylating systems of isolated chloroplasts and mitochondria. Since electron transport is not affected by freezing of isolated spinach chloroplasts, breakdown of A TP synthesis is considered as an uncoupling effect. Uncoupling can be prevented by the addition of small amounts of glurnse, sucrose and raffinose to the chloroplast system. Oxidative phosphorylation of mitochondria can also be preserved by sugars. The protective action of sugars on chloroplasts can be overcome by NaCI, KCI, MgCl$_{2}$ and Na$_{2}SO_{4}$. lt is possible to establish a balance between the protective effects of sugars and the deleterious effects of salts over a wide range of concentrations. However, although salts promote uncoupling of photophosphorylation by freezing, washed chloroplasts are uncoupled also. Bovine albumin is also capable of protecting photophosphorylation, although to a much smaller extent than sugars. Cysteine and glutathione fail to protect the chloroplast system. For this reason, and since no oxidation of protein SH-groups due to freezing can be detected, the sulfhydryl-disulficle theory of frost injury is rejected. In order to examine whether the same events take place in isolated cell organelles and in whole cells, tracer experiments were performed with intact tissue. These experiments provide evidence that ATP synthesis is primarily damaged by freezing. On the basis of the observed results, a biochemical mechanismof frost injury and frost hardiness is discussecl