A cyanide-insensitive superoxide dismutase was purified from tomato leaves (Lycopersicon esculentum, Mill., var. Venture) to apparent homogeneity. The enzyme had twofold higher specific activity (about 4000 standard units) than ferric superoxide dismutases purified from Brassica campestris [Salin, M. L. and Bridges.

The chilling response of plants is complex and based on the interplay of two important metabolic processes--lipolytic degradation of membrane lipids and a set of oxidative reactions leading to lipid peroxidation and membrane damage evoked in chilling-sensitive (CS) plants subjected to low temperature and light. The effects of chilling of detached leaves and intact plants differ and are often neglected during experiments. In closely-related species, the activity of several constitutive enzymes (i.e. superoxide dismutase, ascorbate peroxidase and glutathione reductase) appears to be higher in chilling-tolerant (CT) than in CS species; while in several native, closely-related CS species, lipid acyl hydrolase (galactolipase) activity is higher than in CT species. Moreover, in chilling-insensitive (CI) plants, galactolipase activity is very low and is neither activated by detachment of leaves nor under stress conditions in growing plants. Dark and low-temperature treatments of detached leaves of CS species and post-chilling recovery of growing plants in the light activate galactolipase, which is responsible for the release of free fatty acids (FFA), the main substrates of peroxidation by lipoxygenase and free radicals. In several CS species, increased galactolipase activity is an important factor contributing to chilling susceptibility. Thus, it seems likely that enhancement of chilling tolerance may be achieved by genetically suppressing galactolipase in order to reduce both the degradation of chloroplast lipids and the level of released FFA, and thereby avoiding the deleterious action of their peroxidation products on plant tissues.

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Galactolipase activity in chloroplasts of several chill-resistant plants was found to be very low [0.02 -0.13 pmol free fatty acid (FFA) liberated min-' mg protein-'] or not detected. The same phenomenon was observed for soybean and members of the Cucurbitaceae such as cucumber, pumpkin, melon and squash. Since, following cold storage of cucumber leaves, the levels of monogalactosyl-diacylglycerol and digalactosyl-diacylglycerol in chloroplasts decrease while those of FFA accumulate it seems likely that in these typical chill-sensitive plants galactolipase is present but inactivated during isolation procedure. The low galactolipase activity in chloroplasts was accompanied by a relatively low FFA content ranging from 0.05 pmol to 0.30 pmol FFA mg chlorophyll (Chi)-'. However, both pea and horse bean chloroplasts (with low galactolipase activity) exhibit about 0.45 pmol FFA mg Chl-l.Elevated galactolipase activity was observed in chloroplasts of most chill-sensitive species (ranging from 0.31 pmol to 1.32 pmol FFA liberated min-' mg protein-') as well as in chloroplasts from broad bean, a member of a chill-resistant species (1.26 pmol FFA liberated min-' mg protein-'). In addition in this latter group of plants FFA level in chloroplasts often did not fit the galactolipase activity.The results suggest that there exists a tendency for chilling tolerance of plants to decrease both galactolipase activity and FFA level. However, in some plant species with elevated galactolipase activity the chloroplast FFA level does not correlate with enzyme activity.The primary loci of chilling injury are widely believed to be the membranes of subcellular organelles such as the mitochondria and chloroplast. The so-called 'membrane hypothesis', elaborated by Lyons [l] and Raison [2], is based on three key postulates as summarized by Quinn and Williams [3]: (a) the existence of an apparent correlation between chilling sensitivity and a gel-to-liquid-crystal phase transition in the membrane lipids of isolated chloroplasts and mitochondria, (b) the occurrence of a similar correlation between chilling sensitivity and discontinuities at about 10 -12 "C in the temperature dependences of key enzymes (or enzyme systems) located in these organelles, (c) the existence of a correlation between chilling sensitivity and the fatty acid composition of chloroplast and mitochondria membranes. The fact that all three relations are observed in most chillsensitive and chill-resistant species alike casts some doubts on their relevance to chilling injury [3].Since the lipids are important factors of membrane properties, cf.[3], changes in lipid composition and enzyme activities involved in their metabolism should be expected. It is interesting that chloroplast thylakoid and envelope membranes are extremely rich in galactolipids, which constitute of 70 -80% of total lipids [4]. Whereas the lipid composition of chloroplast membrane is well documented [4] our knowledge about galactolipase activity and the level of free fatty acids in chloroplasts is rather p...