Somaclonal variation and improvement of chilling tolerance in rice: Changes in chilling-induced chlorophyll fluorescence

Chilling is one of the major constraints for rice (Oryza sativa L.) culture at high altitude. The use of chlorophyll fluorescence to screen genotypes for chilling tolerance has been widely documented. Within a program dealing with improvement of chilling tolerance in rice, somaclonal variants were obtained previously and chilling tolerance was selected for during four successive generations. To evaluate the implication of the efficiency of photosystem II (PSII) photochemistry (Fv/Fm) at low temperature in the increased chilling tolerance of these somaclones, the chlorophyll fluorescence of the somaclones and their original parental cultivars was measured with a portable fluorometer (PEA Analyser, Hansatech, England). First, chilling tolerance of four rice cultivars was investigated in phytotron trials by subjecting excised leaves to at 10/5 degrees C day/night for a 72-h period before returning to 28/22 degrees C. Fv/Fm was determined after 0, 3, 6, 12, 24, 36, 48, 60, 72, 75, 144, and 147 h and compared with measurements performed on control leaves maintained throughout at 28/22 degrees C. The chilling treatment induced a marked decrease in Fv/Fm during the day, which was only partially compensated for during night. The extent of this decrease was related to cultivar chilling sensitivity. Second, leaves of somaclonal families improved for chilling tolerance and arising from three of these cultivars were chilled during 6 d under the same conditions as described above and then returned to control conditions. Large differences were found between somaclonal families and their cultivar of origin. Fv/Fm was higher in the Kirundo 3 somaclonal families, but lower in the somaclones originating from Facagro 76 and Kirundo 9. Chlorophyll fluorescence allowed detection of changes in chilling tolerance in several, but not all, somaclonal families. Different mechanisms are implicated in chilling-tolerance improvement.