Chilling susceptibility of Cucumis sativus species

Susceptibility to chilling of leaves of Cucumis sativus was studied using two relatively tolerant (CT) species: Borszczagowski and line 303 and two sensitive (CS) species: Skierniewicki and Skierniewicki X Borszczagowski. The response to chilling was followed during chilling at 4.5 degrees for three days by measurement of the rate of inactivation of O2 evolution, deacylation of monogalactosyldiglyceride (MGDG) and digalactosyldiglyceride (DGDG) and accumulation of non-esterified fatty acids (NEFA) in thylakoids. The most characteristic feature of CS species was rapid accumulation of NEFA after the first day of chilling followed by a 75% decrease of O2 evolution activity after the second day of chilling in comparison to CT species. The original level of NEFA in thylakoids is a characteristic feature of each species and it does not affect O2 evolution. Both parameters were higher in control samples and after chilling stress in CT in comparison with those determined in CS species. However, net accumulation of NEFA after chilling, in relation to the original level, was faster and greater by a factor of 2.45 in CS plants in comparison with 1.90 estimated for CT ones. Thus, a fast deacylation of membrane lipids, mainly MGDG, at the beginning of chilling seems to be the characteristic and prime feature of chilling sensitive plants. Total NEFA content and inactivation of O2 evolution upon chilling are rather secondary events. During chilling the content of MGDG decreased by ca 40% whereas that of DGDG dropped only by ca 5-10% of the control. Partially hydrolysed MGDG seems to be the main source of free 18:3. The data suggest that when acting in situ endogenous lipid acyl hydrolase (EC 3.1.1.26)-galactolipase (LAH) in cucumber thylakoids results in release of only 1 mol of fatty acids per mol galactolipid degraded.