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The effect of aging of isolated chloroplasts of two chilling-sensitive (CS) and three chilling-resistant (CR) plants on the inactivation of oxygen evolution and accu­mulation of free fatty acids (FFA) was studied at 30°C, pH 5.5 or 7.0, in the absence or presence of either sorbitol or NaCl. Considerable accumulation of FFA in aged chloroplasts of CS plants: bean and maize line F7-RpIII was accompanied by a marked inactivation of oxygen evolution. This relation was not, however, found in chloroplasts of CR species: pea, wheat and maize line EPl-RpI, in which the accumulation of FFA upon aging was very low whereas the decline of the rate of oxygen evolution was pronounced. In contrast to changes observed at pH 5.5, the inactivation of oxygen evolution in chloroplasts of CR species aged at pH 7.0 was dependent on the composition of the medium, especially in wheat chloroplasts. Thus, for the evaluation of chilling sensitivity based on the measurements of oxygen evolution activity solely, either aging of chloroplasts at pH 5.5 or possibly at pH 7.0 with NaCl included into the incubation medium may be recommended. It is concluded that determination of both the extent of FFA accumulation and inactivation of oxygen evolution in aged chloroplasts might be applied as chilling tolerance indexes.
Degradation of leaf polar lipids [monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG)] and chlorophyll (Chl) were studied in four Zea mays genotypes differing in chilling susceptibility following dark chilling and post-chilling rewarming at original growth conditions. Assessment of visual chilling injury symptoms during post-chilling rewarming differentiated maize inbred lines into chiling-sensitive (CS) CM7 and Co151 lines and chillingtolerant (CT) S215 and EP1 lines. Severity of chilling injury in CS and CT inbreeds were correlated with the extent of Chl and polar lipids degradation. Chilling for either 4 or 6 days followed by 4 days of rewarming caused more extensive degradation of total polar lipids content in CS than in CT lines. MGDG decreased mostly during chilling whereas DGDG dropped during rewarming only. Chl content was not affected during chilling but its large decrease, greater in CS than in CT lines, was observed upon rewarming. Extent of polar lipids breakdown in CS and CT inbreeds during chilling and post-chilling rewarming is correlated with galactolipase activity in chloroplasts (Kaniuga et al., 1998) and visual assessment of chilling injury. In view of the data it is likely that contribution of galactolipase activity induced during low-temperature stress of CS plants is an important factor responsible for thylakoid lipid degradation and development of chilling injury as postulated previously (Kaniuga 1997). It is suggested that genetically engineered reduction of galactolipase activity or elimination of the factors(s) involved in induction/stimulation of its activity during chilling might increase tolerance of CS species to chilling stress.
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