Abstract

Physiological changes on leaves of young sunflower plants (genotypes C120, C124, C148 and PAC2) were evaluated in a growth chamber after exposure to night frost of -3 and -5°C for a period of 12 h. Net CO2 assimilation rate (PN), potential photochemical efficiency of electron transport PS2 (FV/FM), relative electrolyte leakage (REL%) and osmotic potential (ψπ) were monitored in the plants. Stressed plants were evaluated immediately (2 h) after the effect of the freezing temperature (var. S1), after 2-day recovery (var. S2) and after 6-day recovery (var. S3). The PN was determined as the most sensitive of the evaluated parameters of the sunflower. In genotypes C120, C148 and PAC2, a statistically evident decrease of PN in all variants S1-S3 was found during a temperature of -3°C. To the contrary, the results have proved minor sensitivity or, more precisely, a high stability of FV/FM, ψπ a REL% at the freezing temperature of -3°C. The results showed that physiological parameters were more sensitive to a freeze temperature at −5°C. It was also possible to determine the differences between genotypes. Statistically evident changes were found there in comparison to control plants: decrease of PN in genotypes C120 and C148 in all variants S1-S3, in genotype C124 in variant S1 and in genotype PAC2 in variant S3. There was also the decrease of FV/FM in genotypes C120, C124 and C148 in variant S1; increased REL% in genotype C120 in variants S1 and S3 and in genotypes C124 and C148 in variant S1; decreased ψπ in genotype C124 in variants S1-S3 and in genotypes C120 and C148 in variant S2. As apparent from the above, the PAC2 genotype may be considered relatively resistant to freeze stress, while genotypes C120, C124 and C148 rather sensitive to freeze stress.